What current does a starter motor use?

[Gibbo]

Yes.

 

Why?

 

Nick

 

Well you said the starter motor was on one side and the batteries were on the other. But the meter was well away from the starter motor.

 

So presumably it was near the batteries. Could he have measured the current in the engine battery lead with the switch on the "both" position?

 

Gibbo

[Theo]

Well you said the starter motor was on one side and the batteries were on the other. But the meter was well away from the starter motor.

 

So presumably it was near the batteries. Could he have measured the current in the engine battery lead with the switch on the "both" position?

 

Gibbo

 

He may well have measured the current in the thick negative lead that goes from the starter battery to the earth point on the engine mount. That one's easier to get at than the +ve.

 

Ah! I wonder if there is an alternative path to earth. Come to think about it all the negatives ore commoned at the battery connections so there is likely to be at least one other parallel path through the negative busbar, depending upon the precise point to which the busbar is connected. I will wait until they get back and then measure the current in the +ve lead direct to the starter solenoid. I will post new readings next week.

 

I'll be back...

 

Nick

[Gibbo]

I measured two boat engines this morning.

 

A beta 38. 190 amps

 

A thornycroft summat. He didn't know what it was and it had no plate on it. He "thinks" it's about 30 horse power. 165 amps.

 

I also measured my car. 1.8 litre petrol. 140 amps.

 

Gibbo

[Theo]

I measured two boat engines this morning.

 

A beta 38. 190 amps

 

A thornycroft summat. He didn't know what it was and it had no plate on it. He "thinks" it's about 30 horse power. 165 amps.

 

I also measured my car. 1.8 litre petrol. 140 amps.

 

Gibbo

 

Now I can't wait to get Theodora back so that I can measure the current properly...

 

Nick

[Theo]

No 1 son phoned with some new readings which are a bit more sensible, but still confusing. I can see that I will have to do a bit of careful tracing of wires.

 

Cranking current measured on the big red wire that passes under the engine in the direction of the starter motor: 102A

Cranking current measured on big black wire coming from the battery negative: 198A

 

I will have to possess my soul in patience and wait until they are back.

 

Nick

[Sir Nibble]

Not 50 then.

[Gibbo]

There is always some exaggeration about this, one of those things that people simply make assumptions based on what others have said, I have read reports of 250 and even 300 amps for narrowboat engines, I was always a bit sceptical.

 

I did the test last year with a clamp meter. On what was about the coldest day of the year, early in the morning, my engine is a heavy 2.1 litre 3 cylinder diesel and had not been run for a couple of weeks, I manually held the 'stop' solenoid closed whilst engaging the starter to prevent it from starting.

 

I tried a few times, the highest current reading I could measure was 150 amps, of course after the initial acceleration the current dropped back well below 100 A

 

[Sir Nibble]

Now now Gibbo, don't be like that. We have a nice set of measurements taken to demonstrate the point which John always prefers over mathematical predictions of what those readings might be, so I am sure he will be along shortly to concede the point.

[justintime]

No 1 son phoned with some new readings which are a bit more sensible, but still confusing. I can see that I will have to do a bit of careful tracing of wires.

 

Cranking current measured on the big red wire that passes under the engine in the direction of the starter motor: 102A

Cranking current measured on big black wire coming from the battery negative: 198A

 

I will have to possess my soul in patience and wait until they are back.

 

Nick

198 amps seems more accurate .

Part of Computerised bench test result from a new small diesel engine starter motor recently supplied is as follows:

Rated Power : 1.20 kW

Rated Voltage: 12.00 v

Free Run Speed : 3352 Rpm

Free Run Current : 41 amps

Max of Power : 1.25kW

Max of Torque : 14.07 Nm

Max of Current : 566 Amp ( Presume the starter was stalled at this point as would happen if engine locked for some reason)

Ripple : 18 Amp

Hold Current : 11 Amp ( solenoid)

The above figures were from a small japanese starter motor used on a two cylinder diesel engine.

Edited October 14, 2007 by justintime

[RobinJ]

Free Run Current : 41 amps

Max of Current : 566 Amp

Think this answers the question, somewhere between 40 and 550!

It will depend on whether you are measuring peak, continuous, whether the engine is warm or cold. This not just affects the 'tightness' of the engine but also whether there is still oil on all the surfaces!

Pre engaged starters will probably have a higher 'peak', possibly 500A as they start to turn a stiff engine in winter, but once spinning, that could drop as low as about 70A depending on the compression, no. of cylinders, wear etc.

I'll have to measure mine, but judging by the amount of times the battery turns the thing over when its being bled (battery lasts about 20 minutes in 30 second bursts) and the amount that has to be put back after starting (high charge for no more than 2 minutes), it doesn't take an excessive amount.

I have seen different fugures for the 1.5 BMC, but I think the accepted is 38hp.

[FORTUNATA]

Speaking of current, one thing I was pondering over was wire guage for a starter motor. I was hot-wiring my starter motor with a direct feed from the battery but I now figure my wire may have been too think, thus causing voltage drop and resistance. I figure if I use a thicker guage I'd get more current and more power on cranking. That and bleeding the fuel lines more diligently.

I did try measuring current on my multimeter once but burnt it out. It was too cheap a meter.

 

 

What current would you expect the starter motor to take? My son measured it this morning using a clamp meter and found it to be a little under 50A. I would have thought that this was very low but can't find any data in other threads.

 

I could hear the engine spinning as he measured the current. (He had the stop pull pulled so that the engine didn't start.) It was spinning freely with the batteries fully charged.

 

Edited to say that it is on a BMC 1.5 engine

 

TIA

Nick

[dor]

Just noted in my vetus technical information that the starter motor is rated at 2KW.

 

At 12v that's 166 amps assuming a one to one between watts and volt-amps. Pretty well fits with my observed current of 180 amps when cranking.

[Sir Nibble]

Just noted in my vetus technical information that the starter motor is rated at 2KW.

 

At 12v that's 166 amps assuming a one to one between watts and volt-amps. Pretty well fits with my observed current of 180 amps when cranking.

assuming a one to one between watts and volt-amps.

Always will be on pure dc, watt you see is watt you get.

Coat?

[DHutch]

Indeed, becuase powerfactor is always one, or doesnt exist, or whatever. P = p(sin#). Or somthing like that!!

 

 

Daniel

[John Orentas]

Just noted in my vetus technical information that the starter motor is rated at 2KW.

 

At 12v that's 166 amps assuming a one to one between watts and volt-amps. Pretty well fits with my observed current of 180 amps when cranking.

 

 

 

Comes close to my measurements too..

 

2.2 Litre 3 Cylinder diesel...150 amps.. Starter documented as 1.9 Kw

 

My car, 1800 Merc petrol draws 130 / 140 amps.

[Big Steve]

Speaking of current, one thing I was pondering over was wire guage for a starter motor. I was hot-wiring my starter motor with a direct feed from the battery but I now figure my wire may have been too think, thus causing voltage drop and resistance. I figure if I use a thicker guage I'd get more current and more power on cranking. That and bleeding the fuel lines more diligently.

I did try measuring current on my multimeter once but burnt it out. It was too cheap a meter.

 

Most meters will burn out or overload measuring starter current. You need a clamp on job or a purpose made "shunt" Shunts are readily available for the more expensive meters but cost lots. If you can do the arithmatic they aren't hard to make.

 

In fact, you could use the starter cable as a shunt. if it's 35mm2, which it should be, then it'll drop 52mV per metre per 100amps, and even cheap meters can read mVs.

 

So if your starter cable is 1 m long measure the voltage between the cable ends while cranking, 52mV =100Amps. the calculation is :

 

meter reading in millivolts/.52 x cable length in metres

 

eg meter reads 75mV, and cable is 80cms long, then 75/.52 x .8= 115 Amps.

 

Of course this turns to a complete crock if the battery/starter terminals aren't squeaky clean, and I'm sure the whizz kids will find a few things wrong with the idea, but then it wouldn't be such an entertaining site if they didn't, would it?

 

Steve

[Gibbo]

Most meters will burn out or overload measuring starter current. You need a clamp on job or a purpose made "shunt" Shunts are readily available for the more expensive meters but cost lots. If you can do the arithmatic they aren't hard to make.In fact, you could use the starter cable as a shunt. if it's 35mm2, which it should be, then it'll drop 52mV per metre per 100amps, and even cheap meters can read mVs.So if your starter cable is 1 m long measure the voltage between the cable ends while cranking, 52mV =100Amps. the calculation is : meter reading in millivolts/.52 x cable length in metreseg meter reads 75mV, and cable is 80cms long, then 75/.52 x .8= 115 Amps.Of course this turns to a complete crock if the battery/starter terminals aren't squeaky clean, and I'm sure the whizz kids will find a few things wrong with the idea, but then it wouldn't be such an entertaining site if they didn't, would it? Steve

It's good enough for measuring starter current. Heck even 10% or possibly even 15% accuracy is ok for that and you should get within that using the above method. It'll give a close enough idea.To get round the problem of the connectors just stick the meter probes right through the insulation and into the cable about an inch away from the connector. Problem solved.Gibbo

Indeed, becuase powerfactor is always one, or doesnt exist, or whatever. P = p(sin#). Or somthing like that!!Daniel

Close.PF=Cos of the phase angle between voltage and current. Which there isn't on DC. Hopefully :)There is another type of PF which I won't bore you with.Gibbpo

[chris w]

Most meters will burn out or overload measuring starter current. You need a clamp on job or a purpose made "shunt" Shunts are readily available for the more expensive meters but cost lots. If you can do the arithmatic they aren't hard to make.

 

In fact, you could use the starter cable as a shunt. if it's 35mm2, which it should be, then it'll drop 52mV per metre per 100amps, and even cheap meters can read mVs.

 

So if your starter cable is 1 m long measure the voltage between the cable ends while cranking, 52mV =100Amps. the calculation is :

 

meter reading in millivolts/.52 x cable length in metres

 

eg meter reads 75mV, and cable is 80cms long, then 75/.52 x .8= 115 Amps.

 

Of course this turns to a complete crock if the battery/starter terminals aren't squeaky clean, and I'm sure the whizz kids will find a few things wrong with the idea, but then it wouldn't be such an entertaining site if they didn't, would it?

 

Steve

 

Just extending Steve's great idea a little further....... if you measure off exactly half a metre of the start cable and insert probes there, the readout on the multimeter in mV corresponds exactly to the cranking current in amps.

 

Note though that this (and Steve's equation) only works for 35mm² cable. For other cable thicknesses, use the following equation:

 

17/A (where A = the cross-sectional area of the cable in mm²)

 

This number will give the length in metres over which to measure the cable to get a direct readout of cranking current when the multimeter is set on mV.

 

So, for example, if you have 60mm² cable the length over which to measure will be:

 

17/60 = 0.28m

 

If the reading on the multimeter is then say 0.197 volts (ie: 197mV), the cranking current is 197 amps

 

Chris

Edited October 17, 2007 by chris w

[onthecut]

Hi Snibble.

 

Just moving aside from the main topic slightly, can I pick your brains as a starter pro ? CAV CA45. I have one which has ceased to push the pinion out, but otherwise works fine. Everyone I've spoken to shudders when I mention this motor and they all say they are a complete ****** to work on, particularly to reassemble. I have withdrawn the main body, leaving the armature and head section intact and it appears that the solenoid (which is working fine) simply doesn't have any purchase on the pinion. Looking at the parts list, it appears it engages a shoulder of some sort. Is this something that wears away, or is a replaceable part (snap ring, etc.) ? Any thoughts most appreciated.

 

Mike.

[Big Steve]

Just extending Steve's great idea a little further....... if you measure off exactly half a metre of the start cable and insert probes there, the readout on the multimeter in mV corresponds exactly to the cranking current in amps.

 

So, for example, if you have 60mm² cable the length over which to measure will be:

 

17/60 = 0.28m

 

If the reading on the multimeter is then say 0.197 volts (ie: 197mV), the cranking current is 197 amps

 

Chris

 

Brilliant, wish I'd thought of that.

[Sir Nibble]

Hi Snibble.

 

Just moving aside from the main topic slightly, can I pick your brains as a starter pro ? CAV CA45. I have one which has ceased to push the pinion out, but otherwise works fine. Everyone I've spoken to shudders when I mention this motor and they all say they are a complete ****** to work on, particularly to reassemble. I have withdrawn the main body, leaving the armature and head section intact and it appears that the solenoid (which is working fine) simply doesn't have any purchase on the pinion. Looking at the parts list, it appears it engages a shoulder of some sort. Is this something that wears away, or is a replaceable part (snap ring, etc.) ? Any thoughts most appreciated.

 

Mike.

These starters are a right bitch to work on IF YOU DON'T KNOW THEM WELL. The solenoid "plunger" contains a garter of 4 segments held together with a spring. They sit in a groove inside the plunger and push against a shoulder on the pinion. If you were to strip it enough to get them out, you will spend all day trying to get them back in and eventually launch them over the side, takes me about 30 seconds! but then after the first 500 or so it gets easier.

Now, you say it will not push the pinion out, but otherwise works fine. This type of atarter the solenoid pushes the pinion out only a very short distance whilst powering the starter in series with a sodding big resistor. The idea is that the pinion only has to touch the flywheel and the rotation of the starter will wind it into engagement with the spiral splines on the armature, then the internal contacts go to their second stage position, byepass the resistor and the engine cranks. Without the flywheel the pinion will move only slightly and rotate on reduced power, DON'T DO THIS FOR LONG OR THE RESISTOR WILL COOK. To test this unit in the vice, Connect the -ve but not +ve, power the solenoid and the pinion will move slightly forward, manually pull and twist it fully forward and you should feel and hear the second stage contact making. Then and only then, connect the main +ve and the starter should go like the clappers.

I am happy to talk any forum member through starter or alternator repairs EXCEPT FOR A CA45, they really are too complex to open without having someone at hand whose hands have done this bafore. I will help if I can.

[onthecut]

Hi. Snibble.

 

Many thanks for explaining the sequence, especially the bit about cooking the resistor. Unfortunately, despite the solenoid clattering in at a rate of knots I'm not getting any forward movement whatsoever on the pinion, so I wonder if the tricky pieces to which you refer have given up, one way or the other. As you mention, if the pinion is manually wound right out, the motor works fine; indeed when I removed the main case, the brushes and segments look pretty much like new. I'm sure I read somewhere that a secondary solenoid should be used in conjunction with this type of motor. Is that correct and, if so, why ? If I do get brave (or stupid !), is the nut on the very end of the shaft threaded L or R hand depending on the rotation ? Thanks again for the info.

 

Mike.

[Sir Nibble]

Hi. Snibble.

 

Many thanks for explaining the sequence, especially the bit about cooking the resistor. Unfortunately, despite the solenoid clattering in at a rate of knots I'm not getting any forward movement whatsoever on the pinion, so I wonder if the tricky pieces to which you refer have given up, one way or the other. As you mention, if the pinion is manually wound right out, the motor works fine; indeed when I removed the main case, the brushes and segments look pretty much like new. I'm sure I read somewhere that a secondary solenoid should be used in conjunction with this type of motor. Is that correct and, if so, why ? If I do get brave (or stupid !), is the nut on the very end of the shaft threaded L or R hand depending on the rotation ? Thanks again for the info.

 

Mike.

Right, this starter has a complex system of steel balls, sliding collars and them there segments intended to disengage the pinion from the flywheel in the event of overspeed. The armature end float is crucial, this is adjusted by means of the shims you will have already removed at the commutator end. I hope you took note of which ones came from outside and which inside! The segments in the solenoid plunger I mentioned must engage a shoulder on the pinion, but only long enough to get it into mesh. As the starter cranks the engine they withdraw from the pinion by magnetic force, if they do not return then that would give the problem you describe. Similarly, if the pinion is too far back (those shims) in the at rest position then the segments will not re engage. Just gonna hafta look!

That nut is a left hand thread, remove it, position the unit comm up in the vice holding the pinion fully forward and lift the armature clear. Inside the pinion there are 10 little steel balls, the sliding collar will push these out as you allow the pinion to retract. Recover them. Undo the circlip on the end of the pinion, remove the "trip plate" and the spring and collar beneath it. Now remove the pinion and the segments inside the plunger will be visible, as will the shoulder on the pinion. Feed the pinion back in and see if you can feel the shoulder meet the segments.

DO NOT ENERGISE THE SOLENOID WITH THE PINION OUT OR ALL THOSE SEGMENTS WILL FLY OUT AND IT'S PARTY TIME GETTING THEM BACK IN.

At this point, get back with your findings.

Edited October 19, 2007 by snibble

[chris w]

I finally got round to measuring my starter current on an Isuzu 35. The result was 160amps on the first crank (ie: stone cold engine) plus another 38A for the heaters giving a total draw of 198A. (measured with a shunt ammeter).

 

Chris

[tomsk]

I finally got round to measuring my starter current on an Isuzu 35. The result was 160amps on the first crank (ie: stone cold engine) plus another 38A for the heaters giving a total draw of 198A. (measured with a shunt ammeter).

 

Chris

 

 

I concur