91 350 sdl,

Car has started like a champ for some time now, however I would notice the glow plug light coming on while I was driving 2 to 3 minutes after starting the car. Went out the other morning and car would not start at all, and now glow plug light will not illuminate on ignition. Having researched other posts I know it is not glow plugs. I have recently replaced these and a ohm meter reads .8 on all, good their. Relay is also getting good power, not a battery as this is new as well. Checked all fuses including one in ovp relay and all are good. I have perhaps narrowed it to the OVP relay or a coolant sensor from reading other posts. Is their a way to test a OVP relay and for that matter the coolant sensor. I assume the coolant sensor is the one on the thermostat housing that also trips the coolant fan?? Any suggestions, am I on the right track??

Did you check the big 80AMP fuse in the GP relay? They can have a minute crack and not work. Unscrew the fuse and examine it.

John

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John
1986 300sdl
1985 380se

your bulb on the instrument cluster may have gone. I would check the circuitry for the glow plugs and the relay itself

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

Okay, this is my glow plug relay, unlike older relay's that I assume you are talking about it does not have a large 80 amp fuse with two screws on the top, It does have a circular type looking doohickie right underneath the red incoming power wire their. I assume this must be some sort of fuse as well. Any ideas on how to test this?, thanks for your help.

also output at the 6 terminals that run to the glow plugs registers at 12.2 to 12.1 throughout the 6. Is that a bit low?

I'm sorry but I am not familiar with that later style glow plug relay. I do have some old books on that style of GP system in my attic. I will look and see what I can find. I also will look at the w126 repair CD when I get home.

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John
1986 300sdl
1985 380se

The light won't come on when 2 or more plugs are burnt out. Does it come on while you are driving?

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

If it does not have the old sacraficial fuse then it must have a "breaker" with a reset system. Does the red circular "doohickie" you discribe compress or push inward? If not there must be a fuseable link wire some where near the relay to protect the system........otherwise if there was a short things would go up in flames.

When do you get the output at the 6 terminals on the relay. Does it stip after 4-5 seconds?

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FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K

I just looked at the FSM, the afterglow systems have a self resetting breaker instead of a one time use fuse.

If you have defective plugs the light should not come on before starting then come on and remain on for about 1 minute after the engine is running. If the light never comes on the FSM suggests looking at the bulb itself.

-J

It used to come on when I was driving, perhaps 2 to 3 minutes after I had pulled out. I just don't think it is the plugs, again I ran the ohm meter on them and they all check out at .8. I replaced them all about 2000 miles ago. car started up great right up until it didn't start.

Does the light in the system close the circuit so it works?
The first day it did not start the glow light came on, but since then nothing so I didn't check the glow light first as it was working while the car would not start. Now not at all. I will check that light out though.

So then I need to find the resetting breaker? is that on the relay because I do not recall seeing anything of the sort

It resets every time you key off.

I get the output of 12.1-12.2 the first 20 to thirty seconds of ignition, then the box clicks and it goes down. That seems to work fine... I mean if their is power out of the sixoutput plug at 12.2, and the glow plugs and wires ohm's read correct should that power nor make them glow? regardless of the light coming on?

thanks, because I was about to dig around