Lister Petter warning light problem

[Peugeot 106]

9 hours ago, Eeyore said:

Nothing to do with the lifters. Its the difference in compression ratio between the indirect injection LPWS at about 22:1 and the direct injection LPW at about 16:1. The higher compression simply creates more blow by until the piston and rings get up to full working temperature; the best seal being when they have expanded fully into the bore. The pistons and rings will of course only be fully expand if the engine is worked hard enough for it to maintain full working temperature, so more blow by on the LPWS engine is just a fact. Don't forget that the coolant temperature is no indication that the internal components are at full working temperature.

 

The consequences can have everything to do with the lifters as I understand it. I suggest you read the thread "Lister Petter LPSW3 exhaust spitting" where you will read both Tony Brooks and Steve 56 (a Listers Engineer) saying the same thing. Also I know someone (not on the forum) who was an engineer with Listers developing the Alpha engines telling me the same thing

 

The LPWS starts dirtier than the LPW. This could well be due to higher compression ratio and blowback as well as time for the precombustion chamber to get up to temperature

This contaminates the oil quicker for a LPWS than a LPW leading to the following (if you don't change the oil at recommended 100 hour intervals)

 

"The hydraulic lifters then jack open and the valves can hit the piston" (quote Steve56) 

 

"The book says 100 hrs lube and filter change for the LPWS. 

The LPWS is indirect injection so that means it runs "dirty " for a while. Mostly when its warming up. 

It takes a while for the pre combustion chamber to get up to temperature. That can put pollutants into the oil. Hence the early oil change. 

The LPW is direct injection." (quote my Listers development engineer) (He was talking about sticky lifters)

 

I thought the lifter (hydraulic tappet) problem was well understood. Especially when you replace the cylinder head and talked about on this forum. You can find many other forum members and others saying the same thing and I don't think this is an Urban Myth

 

Just as confusing and maybe meriting a new thread. Half the world seems to say it is OK to skim the LPWS cylinder head. The other half seems to say that you should replace it with another as the LPWS head has a rim where the combustion chamber is pressed in and this rim is vital for proper sealing though not apparently on the LPW. I think that Lister Petter has a lot to answer for in terms of information with the Alpha.  The parts lists are totally baffling given all the build no.s and  changes over relatively few years.  Not easy for an amateur!

 

 

 

 

[Eeyore]

13 hours ago, pt2583 said:

Been to the boat tonight and although it was dark took a picture of the relay and did a bit of prodding around the wires. Too dark to do any proper tracing will leave that for day light hours. Discovered that the relay has in fact already been changed to a simple 4 contact (85,86,87,30) heavy-duty relay. The power in and out to glow plug wires (6mm brown) and +ve and - ve wires have been cut from the original 6 way connector plug and attached to the new relay. This means I don't have pre heat function and more of a worry don't know if the temperature switch wires are even connected to anything anymore. Will now have to trace them out to confirm they are. Perhaps should have done all this first, but have to say its been worthwhile for me as I've learnt a lot from what's been said.

I was lucky enough to have quite a lot of slack (extra cable) in the loom between the 11way plugs on the engine and panel. Everything on the engine, except the main battery connection to the starter and alternator was removed. The loom from the panel was then trimmed to suit and wired directly to the engine. Mine was a LPW3 so you would need to retain or rewire the glow plug wiring on your LPWS; you could upgrade the size of the +ve going to, and the heater supply coming from the ignition switch, or use a simple energise to close relay as currently installed. The panel can be the simplest diagram you can find.

[Eeyore]

20 minutes ago, Peugeot 106 said:

The consequences can have everything to do with the lifters as I understand it. I suggest you read the thread "Lister Petter LPSW3 exhaust spitting" where you will read both Tony Brooks and Steve 56 (a Listers Engineer) saying the same thing. Also I know someone (not on the forum) who was an engineer with Listers developing the Alpha engines telling me the same thing

 

The LPWS starts dirtier than the LPW. This could well be due to higher compression ratio and blowback as well as time for the precombustion chamber to get up to temperature

This contaminates the oil quicker for a LPWS than a LPW leading to the following (if you don't change the oil at recommended 100 hour intervals)

 

"The hydraulic lifters then jack open and the valves can hit the piston" (quote Steve56) 

 

"The book says 100 hrs lube and filter change for the LPWS. 

The LPWS is indirect injection so that means it runs "dirty " for a while. Mostly when its warming up. 

It takes a while for the pre combustion chamber to get up to temperature. That can put pollutants into the oil. Hence the early oil change. 

The LPW is direct injection." (quote my Listers development engineer) (He was talking about sticky lifters)

 

I thought the lifter (hydraulic tappet) problem was well understood. Especially when you replace the cylinder head and talked about on this forum. You can find many other forum members and others saying the same thing and I don't think this is an Urban Myth

 

Just as confusing and maybe meriting a new thread. Half the world seems to say it is OK to skim the LPWS cylinder head. The other half seems to say that you should replace it with another as the LPWS head has a rim where the combustion chamber is pressed in and this rim is vital for proper sealing though not apparently on the LPW. I think that Lister Petter has a lot to answer for in terms of information with the Alpha.  The parts lists are totally baffling given all the build no.s and  changes over relatively few years.  Not easy for an amateur!

 

 

 

 

So we agree, its not an issue on a properly maintained engine 😎. I was probably looking from the other perspective. Interesting to note that the filtration system wasn't up the the task of maintaining a supply of clean oil beyond 100 hours, even during development. Cann't remember off hand if the LPWS engine uses the largest of the many genuine oil filters; but clearly a consideration when servicing. 

Edited February 18, 2023 by Eeyore

[Peugeot 106]

48 minutes ago, Eeyore said:

So we agree, its not an issue on a properly maintained engine 😎. I was probably looking from the other perspective. Interesting to note that the filtration system wasn't up the the task of maintaining a supply of clean oil beyond 100 hours, even during development. Cann't remember off hand if the LPWS engine uses the largest of the many genuine oil filters; but clearly a consideration when servicing. 

Again filters are confusing!

I use 201-55370 and have been told it is bigger than the original filter fitted and more than adequate. My engine is 2004.

 

[Eeyore]

3 hours ago, Peugeot 106 said:

Again filters are confusing!

I use 201-55370 and have been told it is bigger than the original filter fitted and more than adequate. My engine is 2004.

 

Off topic, but still useful to the curious. Most filter manufacturers have cross reference tools on their web sites, and one is probably supplying Listers. They all have one thing in common, and that is the size of the screw thread, and thats about it. The range of internal bypass settings are probably the most curious. It would of course be irresponsible to suggest anything other than genuine parts; but it certainly gives the impression that anything that both screws on and seals would do the job!!  

[Psychalist]

Oil filters is going a bit off topic. The three cylinder uses a wider, but not longer can, but most equivalents come in as the narrower can - meaning they're not really equivalent - caveat emptor and all that. The difference seems only to be volume and all have the same bypass. I have used the smaller can and not noticed any difference. Getting to around 100 hours the oil seems to get darker more quickly. Maybe the filter is becoming blocked at that stage. I've not had a problem with the lifters. I had a Cavalier that used to get hydraulic lock at high revs and would wear through hardening on the camshaft. I had three replacement camshafts in that car.

 

Interesting the two cylinder seems to have the relays hanging down the side of the engine where moisture, dirt and oil can get in the connections.

Mine (1997 vintage) has them enclosed in a box:

 

 

Despite that, as Tony said, probably an over complication and another source of problems. E.g. Fuel cut-off solenoid jamming due to incorrect alignment, going over-current and tripping. A pull cable is simpler and easier to diagnose.

 

As for the cylinder head having a lip....none here:

 

[Eeyore]

3 minutes ago, Psychalist said:

 

Interesting the two cylinder seems to have the relays hanging down the side of the engine where moisture, dirt and oil can get in the connections.

Mine (1997 vintage) has them enclosed in a box:

 

 

Despite that, as Tony said, probably an over complication and another source of problems. E.g. Fuel cut-off solenoid jamming due to incorrect alignment, going over-current and tripping. A pull cable is simpler and easier to diagnose.

Interesting, the last one I was in had the relays mounted facing downwards; and yes one of them had vibrated out!

[Peugeot 106]

2 hours ago, Psychalist said:

Oil filters is going a bit off topic. The three cylinder uses a wider, but not longer can, but most equivalents come in as the narrower can - meaning they're not really equivalent - caveat emptor and all that. The difference seems only to be volume and all have the same bypass. I have used the smaller can and not noticed any difference. Getting to around 100 hours the oil seems to get darker more quickly. Maybe the filter is becoming blocked at that stage. I've not had a problem with the lifters. I had a Cavalier that used to get hydraulic lock at high revs and would wear through hardening on the camshaft. I had three replacement camshafts in that car.

 

Interesting the two cylinder seems to have the relays hanging down the side of the engine where moisture, dirt and oil can get in the connections.

Mine (1997 vintage) has them enclosed in a box:

 

 

Despite that, as Tony said, probably an over complication and another source of problems. E.g. Fuel cut-off solenoid jamming due to incorrect alignment, going over-current and tripping. A pull cable is simpler and easier to diagnose.

 

As for the cylinder head having a lip....none here:

 

My oil seems to stay pretty clean. I often look at it and wonder why I am changing it!

 

My engine was bought as a core engine so a lot of the bits and bobs aren’t original. I agree that the relays are not in a good position and I managed to stand on one and break it! I need to fabricate a protective cover but the wires are a bit short!

 

your head has obviously been skimmed. Is it working ok? I got photos of mine from the top only not the cylinder side and can’t remember what it looked like underneath.I had it checked by an engine restorer and they said it was fine, no need to skim so I just put a new gasket in. It’s been fine. 

 

I will put out a few few enquiries regarding skimming a LPWS head. The info re replacing rather than skimming the LPWS head was from a Lister engineer in USA. It might be bollocks but i’ve asked him why not to skim! Another ex Lister engineer told me skimmed hundreds of them. I do wonder if people get confused as there seem to be a lot of LPW engines compared with LPWS engines

[Tracy D'arth]

13 hours ago, Peugeot 106 said:

My oil seems to stay pretty clean. I often look at it and wonder why I am changing it!

 

My engine was bought as a core engine so a lot of the bits and bobs aren’t original. I agree that the relays are not in a good position and I managed to stand on one and break it! I need to fabricate a protective cover but the wires are a bit short!

 

your head has obviously been skimmed. Is it working ok? I got photos of mine from the top only not the cylinder side and can’t remember what it looked like underneath.I had it checked by an engine restorer and they said it was fine, no need to skim so I just put a new gasket in. It’s been fine. 

 

I will put out a few few enquiries regarding skimming a LPWS head. The info re replacing rather than skimming the LPWS head was from a Lister engineer in USA. It might be bollocks but i’ve asked him why not to skim! Another ex Lister engineer told me skimmed hundreds of them. I do wonder if people get confused as there seem to be a lot of LPW engines compared with LPWS engines

The same situation with skimming the pre combustion chamber inserts level with the head face exists with the BMC small diesels and there are thousands of those running that have been skimmed flush. Not a concern in my book.

Oil filters on an engine where the oil is changed every 100 hours are never going to collect enough muck to bring in the bypass valve. Indeed I reckon that on any engine that is serviced at even twice the recommended interval you will never get the bypass opening.

Cut the next oil filter in half. I bet that there is practically nothing in it of any consequence. Lots of plant fitters only change filters at every other oil change, a partly contaminated filter is likely to be more efficient than a new one.

[Psychalist]

15 hours ago, Peugeot 106 said:

My oil seems to stay pretty clean. I often look at it and wonder why I am changing it!

Same here, but it noticeably changes around the 100 hrs mark. Same can't be said for the car though - even the manual says it's expected to be discoloured from the off.

2 hours ago, Tracy D'arth said:

Cut the next oil filter in half. I bet that there is practically nothing in it of any consequence. Lots of plant fitters only change filters at every other oil change, a partly contaminated filter is likely to be more efficient than a new one.

Interesting thought and reassuring (especially as the narrow can is widely available but the wider one takes a bit of finding).

[Steve56]

22 hours ago, Psychalist said:

Oil filters is going a bit off topic. The three cylinder uses a wider, but not longer can, but most equivalents come in as the narrower can - meaning they're not really equivalent - caveat emptor and all that. The difference seems only to be volume and all have the same bypass. I have used the smaller can and not noticed any difference. Getting to around 100 hours the oil seems to get darker more quickly. Maybe the filter is becoming blocked at that stage. I've not had a problem with the lifters. I had a Cavalier that used to get hydraulic lock at high revs and would wear through hardening on the camshaft. I had three replacement camshafts in that car.

 

Interesting the two cylinder seems to have the relays hanging down the side of the engine where moisture, dirt and oil can get in the connections.

Mine (1997 vintage) has them enclosed in a box:

 

 

Despite that, as Tony said, probably an over complication and another source of problems. E.g. Fuel cut-off solenoid jamming due to incorrect alignment, going over-current and tripping. A pull cable is simpler and easier to diagnose.

 

As for the cylinder head having a lip....none here:

 

The earlier engines had all relays mounted to the engine. As the vibration from the engine used to cause the relays to fai prematurely Lister came up with the remote mounted relay box on the later engines.

As for the fuel shut off solenoid, if anything caused it to jam or stopped it from reaching its full travel then the circuit breaker would trip on over current. 

Interesting how we have got to this point from the original question about control panels.

[Peugeot 106]

4 minutes ago, Steve56 said:

The earlier engines had all relays mounted to the engine. As the vibration from the engine used to cause the relays to fai prematurely Lister came up with the remote mounted relay box on the later engines.

As for the fuel shut off solenoid, if anything caused it to jam or stopped it from reaching its full travel then the circuit breaker would trip on over current. 

Interesting how we have got to this point from the original question about control panels.

Steve that’s interesting. It does look a not very sensible place to mount them. I overordered so have a spare! I’d be very interested in your opinion of the LPWS head skim. So many different opinions. I have recently heard of an overheating LPWS3 on the Lancaster. It’s had 2 replacement head gaskets including skims and they have given up and are looking for a replacement engine( a bukh was mentioned!). I think the head gasket diagnosis may be suspect - who knows- but is it possible that the skimming of the head has contributed to the problem? Note this is the LPWS not the LPW. Have you skimmed anyLPWS heads? My engine is a 2004 400 series. Psychalists’ Head looks older than the 400 series.

[Tony Brooks]

5 minutes ago, Peugeot 106 said:

Steve that’s interesting. It does look a not very sensible place to mount them. I overordered so have a spare! I’d be very interested in your opinion of the LPWS head skim. So many different opinions. I have recently heard of an overheating LPWS3 on the Lancaster. It’s had 2 replacement head gaskets including skims and they have given up and are looking for a replacement engine( a bukh was mentioned!). I think the head gasket diagnosis may be suspect - who knows- but is it possible that the skimming of the head has contributed to the problem? Note this is the LPWS not the LPW. Have you skimmed anyLPWS heads? My engine is a 2004 400 series. Psychalists’ Head looks older than the 400 series.

 

Not answering for Steve, but my first question would be was the block face(s) checked for truth, and do these Listers need liner protrusions. I can't see a proper head skim causing that sort of record. Thne we are into do these Lister use stretch head bolts/studs.

[Steve56]

2 minutes ago, Tony Brooks said:

 

Not answering for Steve, but my first question would be was the block face(s) checked for truth, and do these Listers need liner protrusions. I can't see a proper head skim causing that sort of record. Thne we are into do these Lister use stretch head bolts/studs.

I would agree that the block face should be checked. These engines are not fitted with liners so no issue there. I agree that I can't see any issues with skimming the head. These engine used stretch bolts to hold the head in place. Lister always said they could only be used twice and then should be replaced. I suppose the other thing to take into account is the head gasket. The original Lister gaskets had a sealant on the faces of the gasket. If people are using copies, how good are they.

[Tony Brooks]

31 minutes ago, Steve56 said:

I would agree that the block face should be checked. These engines are not fitted with liners so no issue there. I agree that I can't see any issues with skimming the head. These engine used stretch bolts to hold the head in place. Lister always said they could only be used twice and then should be replaced. I suppose the other thing to take into account is the head gasket. The original Lister gaskets had a sealant on the faces of the gasket. If people are using copies, how good are they.

 

It would not surprise me one bit if it turned out the stretch bolts had been overtightened by a person who did not grasp stretch bolts or understand just how close to their elastic limit  the proper tightening would get them.

[Psychalist]

Maybe issues with heads should be in a separate thread (I think there was one?). I have pictures of my gasket before and after resolving an overheating problem (traced to re-use of stretch bolts that loosened.

[LadyG]

3 minutes ago, Psychalist said:

Maybe issues with heads should be in a separate thread (I think there was one?). I have pictures of my gasket before and after resolving an overheating problem (traced to re-use of stretch bolts that loosened.

Confusing.

[Peugeot 106]

1 hour ago, Steve56 said:

I would agree that the block face should be checked. These engines are not fitted with liners so no issue there. I agree that I can't see any issues with skimming the head. These engine used stretch bolts to hold the head in place. Lister always said they could only be used twice and then should be replaced. I suppose the other thing to take into account is the head gasket. The original Lister gaskets had a sealant on the faces of the gasket. If people are using copies, how good are they.

Very interesting comments about the bolts Tony and Steve. If I ever do mine again it will have new bolts and a Lister Petter gasket though I was very careful to tighten them in the correct sequence to the correct torque. Also thanks for clarifying the no liner therefore no rim. I didn’t photo mine when I had it off a couple of years ago so couldn’t remember what the surface looked like. Another Lister engineer suggested using the bolts again and a cloned gasket was fine but I won’t take the risk again. 

46 minutes ago, LadyG said:

Confusing.

Not sure why this is confusing but I would like to see the photos of the gasket. If I am chatting again to the guy with the faulty LPWS3 he might well be very interested to see them too and may not need to buy a Bukh.

 

1 hour ago, Tony Brooks said:

 

It would not surprise me one bit if it turned out the stretch bolts had been overtightened by a person who did not grasp stretch bolts or understand just how close to their elastic limit  the proper tightening would get them.

8 off LPWS2 head bolts delivered £91.47 from Sleeman. Presumably nearer £150 for LPWS3! Maybe not all that surprising that people try to get away with reusing them but probably cheaper to use them in the long run!

[pt2583]

Spent day at boat again today. Further investigation in daylight found that the original combined relay had two connectors (see photo). Colours of all cables matched cct diagrams, however apart from the two 6mm brown cables and red wire from ignition none of the wires were connected. Whoever fitted the new relay ran a new -ve from the relay bracket mounted on engine. Would appear one of the black 6mm cables was ground somewhere but this was also disconnected. This means the alternator has no direct -ve connection, presumably its connection to -ve is via connection bolts to engine.

So on to the temperature switch shown on diagram connected to relay via two purple wires. My one and only switch is indeed fed by two purple wires and test with multimeter found they are the two purple wires originally connected to relays. However as these connectors are no longer connected to any thing my temperature switch is in fact disconnected. I did a resistance test this morning when it was about 6 degrees and the resistance across two contacts of switch was 5.3 kohms. Some Google ING seems to suggest this is about right for this temp. Neither of the switch contacts seems to be connected to -ve through the engine so what I plan to do is run a -ve to one purple wire and then connect the other purple wire to temperature warning light contact on the 11 way plug. So if the switch works correctly (100 ohms resistance at about 100 degrees) the bulb should light. In one of the pictures showing the two purple wires you can also see the two wires from the loom which would be used for a temperature gauge, only installed on deluxe module. Not sure if these would be connected to a second switch where there is currently a plug bolt.

[pt2583]

This hopefully, will be the new cct diagram

[Eeyore]

3 hours ago, pt2583 said:

Spent day at boat again today. Further investigation in daylight found that the original combined relay had two connectors (see photo). Colours of all cables matched cct diagrams, however apart from the two 6mm brown cables and red wire from ignition none of the wires were connected. Whoever fitted the new relay ran a new -ve from the relay bracket mounted on engine. Would appear one of the black 6mm cables was ground somewhere but this was also disconnected. This means the alternator has no direct -ve connection, presumably its connection to -ve is via connection bolts to engine.

So on to the temperature switch shown on diagram connected to relay via two purple wires. My one and only switch is indeed fed by two purple wires and test with multimeter found they are the two purple wires originally connected to relays. However as these connectors are no longer connected to any thing my temperature switch is in fact disconnected. I did a resistance test this morning when it was about 6 degrees and the resistance across two contacts of switch was 5.3 kohms. Some Google ING seems to suggest this is about right for this temp. Neither of the switch contacts seems to be connected to -ve through the engine so what I plan to do is run a -ve to one purple wire and then connect the other purple wire to temperature warning light contact on the 11 way plug. So if the switch works correctly (100 ohms resistance at about 100 degrees) the bulb should light. In one of the pictures showing the two purple wires you can also see the two wires from the loom which would be used for a temperature gauge, only installed on deluxe module. Not sure if these would be connected to a second switch where there is currently a plug bolt.

Are you confusing a temperature switch with a temperature sender? A switch would be either open or closed circuit. Perhaps that particular unit on your existing panel used a temperature sender?

 

Edited February 19, 2023 by Eeyore

[Peugeot 106]

3 hours ago, pt2583 said:

Spent day at boat again today. Further investigation in daylight found that the original combined relay had two connectors (see photo). Colours of all cables matched cct diagrams, however apart from the two 6mm brown cables and red wire from ignition none of the wires were connected. Whoever fitted the new relay ran a new -ve from the relay bracket mounted on engine. Would appear one of the black 6mm cables was ground somewhere but this was also disconnected. This means the alternator has no direct -ve connection, presumably its connection to -ve is via connection bolts to engine.

So on to the temperature switch shown on diagram connected to relay via two purple wires. My one and only switch is indeed fed by two purple wires and test with multimeter found they are the two purple wires originally connected to relays. However as these connectors are no longer connected to any thing my temperature switch is in fact disconnected. I did a resistance test this morning when it was about 6 degrees and the resistance across two contacts of switch was 5.3 kohms. Some Google ING seems to suggest this is about right for this temp. Neither of the switch contacts seems to be connected to -ve through the engine so what I plan to do is run a -ve to one purple wire and then connect the other purple wire to temperature warning light contact on the 11 way plug. So if the switch works correctly (100 ohms resistance at about 100 degrees) the bulb should light. In one of the pictures showing the two purple wires you can also see the two wires from the loom which would be used for a temperature gauge, only installed on deluxe module. Not sure if these would be connected to a second switch where there is currently a plug bolt.

Looks like you are getting somewhere!

 

My original drawings I did before I added the panel and rewired. 2 purple wires from the temp switch to the relay. White and Pink via the loom to the temp switch alarm light on the panel. So I guess the temp switch acts purely to excite the alarm lamp. My heaters were I think wired direct to the ignition switch i’m not aware there was any relay or timers involved. I subsequently added the controller after Peter Thompson was concerned about the lack of timing etc. But it was expensive and maybe over the top. I’m not sure how important the “post heat” which I imagine means the heaters stay on for a while after starting is ?  Maybe Steve can enlighten us. The Lucas 33RA was/is used as a split charger for the aux battery.

Anyway good luck. I’ve learned quite a lot from this post. Cheers!

[pt2583]

5 hours ago, Eeyore said:

Are you confusing a temperature switch with a temperature sender? A switch would be either open or closed circuit. Perhaps that particular unit on your existing panel used a temperature sender?

 

Yes I have to honest, I'm not sure what exactly it is. Another test I did was to rig up a 12v bulb cct and ran it to +ve and -ve on the battery, but through the switch and at its current state (5kohms) the bulb doesn't light. My thinking being that once the resistance reaches a low enough value the bulb will glow. I was making this assumption because on the combined relay the L terminal appears to have been the temperature warning light wire. That said never got the bottom of how that relay worked. I ran out time yesterday but my next plan was to take the switch/sender out of the engine and test it with boiling water.

[pt2583]

5 hours ago, Peugeot 106 said:

Looks like you are getting somewhere!

 

My original drawings I did before I added the panel and rewired. 2 purple wires from the temp switch to the relay. White and Pink via the loom to the temp switch alarm light on the panel. So I guess the temp switch acts purely to excite the alarm lamp. My heaters were I think wired direct to the ignition switch i’m not aware there was any relay or timers involved. I subsequently added the controller after Peter Thompson was concerned about the lack of timing etc. But it was expensive and maybe over the top. I’m not sure how important the “post heat” which I imagine means the heaters stay on for a while after starting is ?  Maybe Steve can enlighten us. The Lucas 33RA was/is used as a split charger for the aux battery.

Anyway good luck. I’ve learned quite a lot from this post. Cheers!

Yes diagrams look very similar except for your glow plug set up. Your wiring loom exactly same colours with same non used wires for oil and temperature senders 5 and 8 and Tachometer wire on 7. Did you ever do any tests on temp switch.

[Tracy D'arth]

If it reads 5k ohm then its not a switch but a thermistor sensor whose resistance should alter with temperature to work a gauge. The temperature light switch will be open circuit, infinite resistance, when cold and short circuit, zero resistance when too hot.