glowplug light intermittent issue

[Verve376]

Hi All

Recently changed all glow plugs as ohms readings of 3 were way off, so 5 new went in. Retested new ones as well. Plugs harness tested too and with plugs connected readings are 1.1ohms on all.

Read voltage on all 6 socket pins in the GP Relay and all read the battery. Theres another socket with two pins in the relay, both read battery voltage, however on the plug side one of the wire reads a cinsistent 1.7ohms whereas the second wire the ohms are all over the show and never constant.

1. When the car has done a 50+ motorway miles, and when switched off and back on the glow plug light disappears after the usual few seconds and when running, no matter how many times the car is started.

2. Where are the 2 wires leading to that plug in to the 2 pin socket on the GP Relay.

I'm pulling my hair. Why does a long drive resolve the issue? Is the F5 125Amp glow plug fuse playing tricks. If someone can tell me where those two wires lead to I could possible figure out the issue.

Many thanks.
Verve

 

[Verve376]

Anyone?

 

[mersum1es]

Nowadays glow plugs are used in emission controls too, when engine is not fully at operation temperature. Perhaps your relay control has something wrong? Sorry can't help with wiring...

 

[Diesel Benz]

Sounds like you are measuring the PWM cotrol line.

 

[Verve376]

Sounds like you are measuring the PWM cotrol line.

Yes I am measuring that and the one next to it marked 31.

Both are pins are giving battery voltage but resistance on PMW (if that is the lower one) is all over the place.

Also, when the car has had a good motorway 60mph run, and when it is restarted the light never comes on. Next day the light is one during city driving. This is odd, had a 530 and 535 before and never had such issues with them. And this issue was not there before,

Any idea where the PMW and 31 wires go? The problem started after the engine bay was steam cleaned (used for buses) by cousin's. I have used contact cleaners all over the sockets and plugs. I just can't seen to trace where these 2 wires are going ...

 

[Ted]

Pwm is pulse width modulation.
That is why your measurements seem inconsistent.

 

[Ted]

OK, taken a look at this on a bigger screen, and this is my interpretation.
First of all the obvious.
N14/2 is the glowplug relay, fed by a high current permanent live on pin 3 via fuse F32. Simples.
This is driven via a pwm signal on pin 2 from N3/9. I think that N3/9 will take various sensor inputs - probably including water temp, air temp etc, and this will modulate the 'on' time of the pulse - i.e. cold air +cold engine = long 'on' pulse.
Bit of a waste of time checking this with a DMM - you would really need a scope, however an analogue meter might give a fair indication - i.e. all cold, higher voltage on that terminal, all warm, lower voltage.
Pin 1 is identified as 31. I would expect 31 to go straight to earth via a brown wire - clearly not in this case as the wire is brown/white via W16/5(6). No idea what this is but suspect it is just an earth connector.
My guess is that there is a (high impedance) air or water temperature sensor on the engine and water has got into the connection - these need to be identified and dried out.

JUst my thoughts. I might be wrong. I was once. (1977 I think it was)

 

[Verve376]

OK, taken a look at this on a bigger screen, and this is my interpretation.
First of all the obvious.
N14/2 is the glowplug relay, fed by a high current permanent live on pin 3 via fuse F32. Simples.
This is driven via a pwm signal on pin 2 from N3/9. I think that N3/9 will take various sensor inputs - probably including water temp, air temp etc, and this will modulate the 'on' time of the pulse - i.e. cold air +cold engine = long 'on' pulse.
Bit of a waste of time checking this with a DMM - you would really need a scope, however an analogue meter might give a fair indication - i.e. all cold, higher voltage on that terminal, all warm, lower voltage.
Pin 1 is identified as 31. I would expect 31 to go straight to earth via a brown wire - clearly not in this case as the wire is brown/white via W16/5(6). No idea what this is but suspect it is just an earth connector.
My guess is that there is a (high impedance) air or water temperature sensor on the engine and water has got into the connection - these need to be identified and dried out.

JUst my thoughts. I might be wrong. I was once. (1977 I think it was)

Thanks. I'll look in to all this.

New MAF went in as well, new temp sensor as well that comes in the new thermostat housing - both new from MB. I'll unplug the temp sensor and give it a good spray with contact cleaner again. All these plugs and sockets were given a good amount of contact cleaner spray when the fault first showed up before plugs were changed. Maybe moisture is getting trapped in one or two plugs when car is off.