Glowplug help please

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Mickyfinn

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I have a Volvo MD2040 (probably irrelevant to the problem), and the glow plugs are not working.

I have checked the glowplugs (several ways) and relays, and all are ok. The only issue I can find is that the battery terminal voltage is 12.6v, but the voltage at the glow plugs is only just over 11v. I have swopped the engine starter motor relay (solenoid?) with the glowplug relay, and the same result, so presumably the components are ok. The relays click when the glowplugs are switched on.

That leaves a poor connection somewhere, but I can't find one.

All suggestions gratefully received !!

Mike Forbes
 
Mickyfinn said:
I have a Volvo MD2040 (probably irrelevant to the problem), and the glow plugs are not working.

I have checked the glowplugs (several ways) and relays, and all are ok. The only issue I can find is that the battery terminal voltage is 12.6v, but the voltage at the glow plugs is only just over 11v. I have swopped the engine starter motor relay (solenoid?) with the glowplug relay, and the same result, so presumably the components are ok. The relays click when the glowplugs are switched on.

That leaves a poor connection somewhere, but I can't find one.

All suggestions gratefully received !!

Mike Forbes
Click to expand...

There may be no poor connection at all. The glow plugs draw a hefty current and 1.5 volts drop might be the resistance of the wiring. You could put your multimeter on the battery and check how much the internal resistance of the battery is dropping the.voltage when ‘on load’.

What makes you think there’s a problem? I can check what the voltage is on our VP2040 glow plug rail when they’re ‘on’ if you like?
 
Mickyfinn said:
I have a Volvo MD2040 (probably irrelevant to the problem), and the glow plugs are not working.

I have checked the glowplugs (several ways) and relays, and all are ok. The only issue I can find is that the battery terminal voltage is 12.6v, but the voltage at the glow plugs is only just over 11v.
Click to expand...

When are you taking the readings ? You should measure at the battery and at the glowplugs when they are "on" to have any meaning.

How have you checked that they work ?

Why do you think they are not working ?
 
Thankyou both for ur replies.
The engine is difficult to start when cold, and never has been before. That is why I suspect a glowplug issue. The engine battery terminal V is 12.6, and hardly drops when the glow plugs are on. Voltage measured between the top of the glow plug rail and battery neg is only 11.2 when glowplugs on.
I saw a YouTube video which said test the glow plugs by measuring V from batt + to top of each plug with rail disconnected, glowplug switch OFF.... 12.6v same as terminal V. Not sure what this measurement actually is.

John Morris offered to measure his glowplug rail V when plugs are on...... that would be very helpful John.

How can I test the plugs are working ? Presumably remove them and ....?

I have also made the assumption that the plugs are connected to the engine start batt, not the domestic batt.

Any further help gratefully received.

Mike. , Scotland.
 
A quick test woild be to use a car jump lead from the battery +ve to the rail connecting the plugs. Then try and start it. Also, if the relay is energised and you get a bit of a spark when touching the jump lead to the rail, then look harder for a connection prob.
Used the jump lead on a 4 pot Yanmar. The start switch was duff in the part that energised the plugs. No spare available, so fitted a hefty relay and push button.
 
DownWest said:
A quick test woild be to use a car jump lead from the battery +ve to the rail connecting the plugs. Then try and start it. Also, if the relay is energised and you get a bit of a spark when touching the jump lead to the rail, then look harder for a connection prob.
Used the jump lead on a 4 pot Yanmar. The start switch was duff in the part that energised the plugs. No spare available, so fitted a hefty relay and push button.
Click to expand...

Curiously we recently had a similar problem with a start switch. In our case it was the start element of the switch that failed. The start relay just "buzzed" and did not energise the starter, using jump leads the relay and starter were fine. Voltage to the relay from the start switch was only 4.3V, thought, poor connection, but no, voltage at the start switch starter terminal was also 4.3V. Been running with a by-pass starter button, have got a replacement switch but not got round to fitting it yet.
 
Glowplugs can go short circuit or open circuit when they fail. If the engtine started OK, then suddenly became difficult on cold start then 1 or more of the glowplugs has failed. If its difficult to start usually a second glowplug has gone, as many engtines start fine with just one down.

A simple test of the plugs is to take them out, then connect it across a 12v battery. Be very careful as the tip gets red hot, so make sure the body is properly supported - dont risk trying to hold it in your hand! When connected the tip should glow an even red. If there are hot or cold spots (darker or lighter shade of red) the the plug is faulty. If you get a big spark on connection but no heat, then the plug is shorted. More usually, nothing happoens and the plug remains cold due to a broken element. Only connect for long enough to check there is an even red glow. Out of the cylinder block there is no heat sink to take the heat away.
 
Testing removed plugs would be a definitive test of the plugs, as above. They're removed a bit like a sparkplug would be, but be wary of getting any muck down the holes!
 
I'm very cautious about removing or refitting glow plugs as they shear easily. Once sheared the remaining stub will be a nightmare to remove.
When they have failed in my car, I can work out which one is broken by checking the resistance between the top of the plug and an earth point on the car. If infinite or zero resistance is seen then it’s broken, a good plug will give about 0.5 to 1 ohm.
The cold starting could be a leak in the fuel line allowing air in. Have you checked if fuel is getting to the injectors?
 
There are other reasons why diesel engine don't start, fuel being one (as mentioned), choked (or is it coked up) exhaust being another.

Even without glow plugs a diesel engine can be persuaded to start, usually and eventually.

Don't stay so focussed.

Jonathan
 
Thanks again all. I'm taking all advice "on board".

The engine isn't really problem to start. It has always started instantly after using the glow plugs, just like a car. The current symptom is that it is starting on a second button press, not the first. So no big deal, but it is definitely a symptom of something not right.

I think the next thing is to remove the plugs and definitively test with jump leads and a battery to check if they are def hot. I am still concerned that the rail voltage is only 11.2v with the plugs ON.

John Morris (first reply) has kindly offered to measure his rail voltage on his MD2040 for comparison to my 11.2v. Hoping you can do it this weekend John.

Thanks to all of you for advice.
 
Simple test for these glow plugs, remove the wiring/rail from all glow plugs. Get a length of wire ( nothing too industrial, just some 1.5mm ish wire/flex will do), connect one end to the battery positive, strip about 10mm of insulation from the other end. Briefly "stroke" the wire across the positive connection to the glow plug. If you get a flash, the glow plug is working, if you don't it's knackered.

Before some smart pedant tell me that glow plugs can fail closed circuit, yes they can. It's very rare and the OPs symptoms would be different.
 
A good idea. Thanks. I'll try that.

Would using a multimeter on the diode test selector switch do the same without high amps? One lead on the top of the GP the other on earth?
 
Last edited:
Thanks a lot. Very helpful as that means my connection to the rail is prob ok. Cant understand why the voltage drop tho?
Plugs tested ok with my meter, but will repeat tests, as it looks most likely the plug(s) are faulty. I have tested them with a continuity test and all were ok (with the rail disconnected).. If they still seem ok, I'll remove them and test with jump leads on the battery.
Thanks a lot for taking time to help. Really appreciated.

Mike
 
Mickyfinn said:
Thanks a lot. Very helpful as that means my connection to the rail is prob ok. Cant understand why the voltage drop tho?
Plugs tested ok with my meter, but will repeat tests, as it looks most likely the plug(s) are faulty. I have tested them with a continuity test and all were ok (with the rail disconnected).. If they still seem ok, I'll remove them and test with jump leads on the battery.
Thanks a lot for taking time to help. Really appreciated.

Mike
Click to expand...

The voltage drop is caused by a bad connection somewhere in the glow plug circuit.

Connect your multimeter , set to the 20 volts DC range, between the glowplug rail and the main battery positive terminal on the starter motor solenoid. You should see the normal no load battery voltage ( 12.6 volts or thereabouts) unless you have an early series engine with an earthing relay

Energise the glowplugs. The meter reading will now show you the volts drop. (Measure it more precisely if you wish by briefly switching to the 2 volts range .)

This will confirm, or otherwise, that the 1.6 volts difference you observed (12.6 - 11) is the result of a bad connection somewhere between these two points.

Main suspects would be the relay contacts or connections to the relay.

FWIW I usually check glow plugs with a car head lamp bulb between battery positive and each glowpug in turn (but if you have an earthing relay that will have to be energised for this to work.)
 
Thanks for that. I'll do that tomorrow.

In a perfect world, should I expect to see the battery terminal voltage between batt - and the GP rail (+) when energised?
 
Mickyfinn said:
Thanks for that. I'll do that tomorrow.

In a perfect world, should I expect to see the battery terminal voltage between batt - and the GP rail (+) when energised?
Click to expand...

Ideally there will be zero volts difference between battery + and glow plug rail while they are energised.

Ohms law says
VOLTS = AMPS x RESISTANCE

If the resistance is zero then Volts = amps x zero, which is zero

In the real world there will be some resistance . Even the wire has some . Crimps can be bad and any other connections can make poor contact.

The difficult situation is when they all contribute a bit and although individually not bad the total resisatance is enough to cause a significant volts drop.

In a "difficult to solve" situation extending the preheating time may help
 
Ways to test glow plugs:
Find the expected resistance of a cold plug, with everything turned off and the engine cold measure the resistance from the glow pug rail to 0v (4 cyl engine it should read 1/4 of a single plug, 6 cyl engine will be 1/6th). Great as there is no need to disconnect anything.

If you have a clamp dc current meter, test the current at all part of the glow plug rail. It should drop evenly between each plug. Again, no need to disconnect anything, but dc current clamp meters are not normal kit.

Fit a permanent current meter in the glow plug supply and monitor it each engine start. Getting a bit aircraft cockpit with instrumentation here.
 
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