Likely failed glow plug relay '83 300CD: Check my diagnosis - PeachParts Mercedes-Benz Forum

dogguy · #1 07-08-2023, 01:59 PM

I *think* that I've determined that the original glow plug relay in my '83 300 CD has finally failed after so many years and miles. I'd appreciate anyone reading this to take a look at my diagnosis below to check if my testing is solid or not.
---
Battery: 12.8 volts
GP relay strip fuse: 0 ohms
---
Symptom: Dashboard GP bulb not illuminating in ignition pos. 2 (preglowing).
Test: Applied voltage from battery to female pin 3 of the 4-pin plug in the GP relay and the bulb illuminated.
---
Just for fun: Probing each female pin of the 5-pin plug in the GP relay, checked the resistance of each GP and the wire to each and got 0.6 ohms.
---
With the ignition in pos. 2, measured 12.6 volts at female pin 1 of the 4-pin plug in the GP relay.
---
With the ignition in pos. 2 and the 4-pin female-male plug attached, measured *0* volts at male pins 1-5 inside the GP relay (these power each GP).

So...when voltage is being activated from the ignition (preglowing in pos. 2) and reaches the GP relay, the relay is sending out 0 volts to each glow plug.

Lastly, I can hear the relay click off after 20-30 seconds so the timing part of the relay seems to be functioning but again, the relay does not appear to be passing 12 volts to any of the glow plugs.

Anything that i'm not catching with this diagnosis?
Thanks-

ah-kay · #2 07-08-2023, 02:10 PM

Without knowing whether your meter is good or calibrated. I would think you have done everything right.

Extra tests:

1) Does the car start with no GP light?
2) repeat with the big plug removed. Does it start?
3) could be the light.

If in doubt change the GP relay. I have a few in my junk bin that I want to get rid of. PM me.

dogguy · #3 07-08-2023, 07:54 PM

Quote:

Originally Posted by ah-kay

1) Does the car start with no GP light?

Yes.

Quote:

Originally Posted by ah-kay

2) repeat with the big plug removed. Does it start?

Yes.

ah-kay · #4 07-08-2023, 09:37 PM

Quote:

Originally Posted by dogguy

Yes.
Yes.

If your car starts ( cold ) with the big 6 female pins plug removed then it must have incredible compression. May be you don't need GP relay at all.

Try it in the morning when the car is cold.

ykobayashi · #5 07-08-2023, 10:56 PM

Sounds right but did you check the big metal fuse?

Oops, just read that you did. Sorry.

If it is clicking after 30 seconds I wonder what is wrong. A burned contact in the relay? I mean it sounds like the relay is for the most part working. I would check your high current connections just for kicks before condemning the thing.

I recall it’s really easy to pry the top off the relay and look inside.

vstech · #6 07-09-2023, 08:52 PM

The relay can and will fail as you describe, but one thing to check is with the key on, measure voltage to ground on both fuse screws…
The fuse can pass an ohms test, but fail to pass current to the relay.

dogguy · #7 07-09-2023, 09:41 PM

Quote:

Originally Posted by vstech

one thing to check is with the key on, measure voltage to ground on both fuse screws

With the ignition in pos. 2 (preglowing), tested and measured *0* volts to ground for each fuse screw (I'm not certain which one is the input side and which one is the output side).

Perhaps it is time to open the GP relay unit and look for excessively pitted relay contacts or a broken solder joint or component lead that is in the path of the voltage from female pin 1 of the 4-pin plug to the fuse screw that is the input side.
Using the VOM, I wouldn't think that it would be all that difficult to trace the voltage path and identify where the break is.
Thank you-

BillGrissom · #8 07-09-2023, 10:27 PM

My 1985 300D has excellent compression (>400 psig all cyl) and very little blow-by (see post, >15 sec recall to shutdown if I plug the valve cover vent), and starts right away on the coldest mornings after the glowplug dash lamp goes out. But, with the GP relay box out once (working on crash damage), I tried to start the engine to relocate the car and it wouldn't start after 20 sec cranking, on several tries, on a 70 F day. I had to hot-wire 12 VDC to 3 of the glowplugs to get it to start. Why we are surprised your's started right away without glowplugs powered. Just to be sure, try measuring again with red lead right on a glowplug terminal and the blk wire on BATT-. Also insure your DMM reads correctly by touching red lead to BATT+.

Otherwise, sounds like the contacts in the GP relay have pitted to where they no longer touch. That is common in a starter contactor ("solenoid") and why just the contacts are sold in a kit to repair a common Denso starter motor contactor (not our cars). I've had some starters where you could flip the brass contacts over to get more life, or put a washer behind them to move them out to touch.

I assume you know that the GP relay is always actuated for only ~20 sec after key-on, which is the clunk OFF you here. The dash lamp is just a prediction, based on current temperature (measured inside GP Relay) for when you have glowed long enough to start a spec-compression engine.