Current troubles?

[MtB]

15 minutes ago, Tony Brooks said:

 

and AI seems to not know that with modern permanent magnet field starters giving the starter a "firm" tap can shatter the field magnets, One persons "firm" is another persons heavy clout. I am surprised MHS did not know that (considering his qualification list) and did not censor that part.

 

Arguably, he might have scripted AI to make posts here on his behalf with no human review. Certainly looks that way to me given he never engages in conversation. 

[Tony Brooks]

18 minutes ago, nicknorman said:

Do modern starters tend to be permanent magnet field? Genuine question because I would have thought an electromagnet field starter would be lighter and cheaper. But quite possibly I’m wrong!

 

Yes, they are starters with a gearbox (often epicyclic) between the armature and the pinion, but the design varies. One example here:

 

The armature spins a lot faster and likely with less torque, but the reduction gear boosts the torque. Electromagnet starter fields have a very low resistance so draw a lot of current when starting - especially the ones in series parallel - and use a lot of copper.

 

Probably easier and faster to assemble because the field magnets tend to be clipped n place whereas  electromagnet ones need pole pieces screwed into place.

 

The manufacturer's claim they are lighter as well

 

Edited to add: you can get replacements for conventional starters that are geared etc.

Edited May 3 by Tony Brooks

[MtB]

FWIW I happen to have a BMC1.8 starter motor about my person at the moment, and a set of decent digital scales.

 

It weighs a whopping 11.22 kg! 

 

The starter motor, not the scales. 

 

 

[Andyaero]

Battery shows 12.6v with a multimeter, when an attempt is made to turn it on there is no drop in voltage, but I only get a clunk, engine does not turn. All wires appear fine, cant see anything that's come lose or detached.

Gave the starter motor a tap, no help.

It seems the solenoid relay is a thing that goes on a Vetus m4.45...where is it, what does it look like, little black relay box, of which there are 2 at rear of engine, or something more substantial on the starter motor?

Am I correct in thinking to check current on the battery I need to disconnect it? In which case I may need to find someone more dextrous with tiny hands.

 

 

 

[Tony Brooks]

2 minutes ago, Andyaero said:

Battery shows 12.6v with a multimeter, when an attempt is made to turn it on there is no drop in voltage, but I only get a clunk, engine does not turn. All wires appear fine, cant see anything that's come lose or detached.

Gave the starter motor a tap, no help.

It seems the solenoid relay is a thing that goes on a Vetus m4.45...where is it, what does it look like, little black relay box, of which there are 2 at rear of engine, or something more substantial on the starter motor?

Am I correct in thinking to check current on the battery I need to disconnect it? In which case I may need to find someone more dextrous with tiny hands.

 

 

 

 

That relay (yes one of those mounted on the engine), usually a plastic cube about 25 x 25 x 25mm. It should click, not clunk. Clunk suggests the really is working but the starter solenoid not making proper contact.

 

To find out which relay is the start relay lightly hold one while someone operates the key to the start position, you should feel it click. I suspect one will click as the key goes through the glow plug position and then the start relay will click.

 

The starter solenoid should have a stud connection with the battery lead and possibly others on it, plus (usually) a 6mm blade. That blade is the one that energise the solenoid. So using an old stout screwdriver short between the stud connection and the blade (don't touch any other metal!). If you still get the clunk then either the solenoid or starter motor is likely to be faulty (not a 100% test). If the motor spins up then the solenoid and motor is probably OK.

 

If you only got the clunk, steel yourself, take a firm grip, and short between the two stud connections. The motor should spin up. If it does then the fault is likely to be in the circuit somewhere or that relay has burned contacts. If the motor does not then get the motor to a specialist for overhaul.

 

 

 

 

18 minutes ago, Andyaero said:

Am I correct in thinking to check current on the battery I need to disconnect it? In which case I may need to find someone more dextrous with tiny hands.

 

I don't understand what you mean by that. Current flows to or from the battery, not on it. I suspect that you may mean check the battery voltage.

 

No, you need a clamp type DC ammeter around one of the main leads, but it will read all but zero unless something is drawing current form the battery.

 

What are you trying to find out?

[Andyaero]

I know the battery has high enough resting voltage, but wanted to test it was providing current, as it could still be a battery issue?

[Tony Brooks]

5 minutes ago, Andyaero said:

I know the battery has high enough resting voltage, but wanted to test it was providing current, as it could still be a battery issue?

 

I agree - that is known as the cranking voltage. The minimum cranking voltage should be 10V or more measured across the battery. Less suggests the battery is faulty.

[Andyaero]

Both relays seem to click/vibrate.

Starter makes a rattle.....

 

 

https://youtube.com/shorts/W_NqBCT0U30?si=eOs7crgL24YVaGAY

 

 

7 minutes ago, Tony Brooks said:

 

I agree - that is known as the cranking voltage. The minimum cranking voltage should be 10V or more measured across the battery. Less suggests the battery is faulty.

isn't cranking voltage the dip when key is turned, as I've said, I get no drop in voltage when key is turned. Was trying to find the amps?

Edited May 5 by Andyaero

[ditchcrawler]

10 minutes ago, Andyaero said:

 

isn't cranking voltage the dip when key is turned, as I've said, I get no drop in voltage when key is turned. Was trying to find the amps?

That is because the battery has no load connected to it, Bad connection on the starter, solenoid, earth return, solenoid contacts faulty starter motor . If you start checking voltage at different points somewhere it will probably disappear. You may even have lost the earth to the engine 

[Tony Brooks]

10 hours ago, Andyaero said:

Starter makes a rattle.....

 

 

https://youtube.com/shorts/W_NqBCT0U30?si=eOs7crgL24YVaGAY

 

 

That is known as machine-gunning and is typically caused by:

 

discharged battery - with 12.6 open circuit voltage unlikely

dirty/loose battery terminals - take them off and clean the mating surfaces.

lose/dirty connection anywhere in the main (thick cable) pos. or neg. circuit. See * below

Faulty master switch (put both cables on one terminal as a test)

Detached solenoid hold in winding (not that common)

 

10 hours ago, Andyaero said:

isn't cranking voltage the dip when key is turned, as I've said, I get no drop in voltage when key is turned. Was trying to find the amps?

 

No, it is the voltage registered while the engine is cranking.

 

The fact you have no significant voltage fall while trying to crank indicates there is not much current flowing because the solenoid is not making proper contact for some reason - see above).

 

* Connect your voltmeter between the start battery positive (lead post or stud pad) and the second stud connection on the starter but IGNORE the reading. Operate the starter and the meter should read a fraction of a volts (say less than 0.5). If it reads more then the problem is in the main positive circuit. You can start stepping back towards the battery with the voltmeter and repeat, so the main stud terminal, the master switch output terminal, ditto input terminal, the outside of the battery clamp/bolt. When you find the reading drops to less than 0.5V then you just passed the fault.

 

Then repeat the procedure on the negative circuit.

 

ONLY NOTE THE READING WHILE TRYING TO START

 

Edited to add - take the negative battery terminal off first and put it back on last. Do not let your spanner bridge between the terminal and metal, especially the positive one.

Edited May 6 by Tony Brooks

[Jen-in-Wellies]

On 03/05/2026 at 21:16, nicknorman said:

Do modern starters tend to be permanent magnet field? Genuine question because I would have thought an electromagnet field starter would be lighter and cheaper. But quite possibly I’m wrong!

I've no idea, but the price of Nd magnets has dropped considerably, so it wouldn't surprise me if the cost graph lines have crossed. It would be an easy swap for car manufacturers to fit a different, cheaper starter motor when assembling the engine package. It would make no difference when dropping the engine in to the car assembly. 

[IanD]

5 hours ago, Jen-in-Wellies said:

I've no idea, but the price of Nd magnets has dropped considerably, so it wouldn't surprise me if the cost graph lines have crossed. It would be an easy swap for car manufacturers to fit a different, cheaper starter motor when assembling the engine package. It would make no difference when dropping the engine in to the car assembly. 

The price of Nd has doubled in the last year, and heavy-duty motors/starters need quite a lot of it. There are intensive efforts going on to find ways of reducing or removing Nd from EV motors to try and reduce costs...

Edited Wednesday at 11:18 by IanD

[Creaking Gate]

Check the earth. 

[Tony Brooks]

39 minutes ago, Creaking Gate said:

Check the earth. 

 

That is exactly what he will be doing if he follows my procedure above. 

 

The slight problem is where in the earth circuit he needs to check. It could be the negative battery post connection, any terminal, the negative to the engine, or the negative hull bond, even then it could still be in the positive so he needs to follow some logical procedure to locate the position of the fault.

[MtB]

1 hour ago, Creaking Gate said:

Check the earth. 

 

Point of Order...

 

Boats don't have an "Earth" connection.

 

The negative battery terminal is often bonded to the hull and engine though. 

 

(I'm feeling particularly pedantic today!)

[Tony Brooks]

55 minutes ago, MtB said:

 

Point of Order...

 

Boats don't have an "Earth" connection.

 

The negative battery terminal is often bonded to the hull and engine though. 

 

(I'm feeling particularly pedantic today!)

 

Well, to be equally pedantic the majority of canal boats and probably a lot of others do use earth return for the electrics on the engine, then it goes insulated return from engine metal back to battery negative.

[wakey_wake]

5 hours ago, Tony Brooks said:

The slight problem is where in the earth circuit he needs to check.

 

Again a DMM is helpful.

1. Select volts range, initially suitable for 12V but you can go lower if that is too high.
2. Set up solid hands-free connections to the starter negative terminal (croc clip, ring under a nut, etc.)
   and also to the starter body.
3. Voltage should be zero when everything sits idle.
4. Drawing any current (such as glow plugs or starter solenoid) should show a modest voltage, well under 0.5 volt and maybe only millivolts,unless the starter has full power.
5. If the voltage is higher, start walking the DMM probe back towards the battery and repeating until the voltage is low again.

Corroded connections should be discoverable systematically by this walking process.