Odd Glow Plug Relay Issue 1985 300TD - PeachParts Mercedes-Benz Forum

Murdock · #1 12-09-2013, 12:20 AM

I know this topic has been covered in myriad forms but I have not seen this particular issue documented before. I am real savvy on searching for content so I did a lot of this prior to posting.

I am also well versed in electrical wiring, laws, and troubleshooting. I was a bench repair tech for audio equipment for many years.

I have an 85 300TD-T. I have the glow plug light issue that many have had. Confirmed it was accurate as my cold starts are rough. The symptom is my glow plug light is intermittent, most times the relay & light does not turn on. When it does work, the light goes out quickly(within 1-2 seconds). I reamed and changed all glow plugs, same result.

I did an extensive electrical test on the glow plug system. The entire system checks out:

Battery: 12.7 v when engine is off, 14.2v when running
Battery voltage present at the input to the relay as well as at both sides of the fuse
Average .3Ω from the glow plug relay connector to the glow plug on all 5 wires
Average .75Ω from battery ground to each glow plug wire connection

I pulled the smaller connector off of the relay and did a full test, it also checks out:

Ignition on (not start): 12v at ignition wire (red/blk)
Applied 12v to the indicator light pin, dash light came on
Wire going to the starter has 12v during cranking.
Measured resistance from the ground pin (brown wire) to battery ground, .12Ω which is plenty acceptable.

I then took clip leads, grounded the ground pin on the relay then applied 12v to the ignition pin, relay turned on and after ~30sec timeout happened as it should. I repeated this 4 times, relay worked every time. This tells me the relay is fine and the issue is outside of it.

Here is the interesting part, where I found the issue. I set up to measure the ground pin (brown wire) again, .12Ω again. However, I turned the key to run and the resistance went up to 1.1Ω, off back to .12Ω. 1980 and up the coolant temp sensor was taken out of this circuit so I know that isn't it

I got under the dash to try and find the ground wire not realizing there are about 30 other brown ground wires. Before I start ripping it apart, wondering if anyone has run into anything like this before. I do have an oil leak on the back of the gauge panel but shouldn't affect ground, perhaps it is.

Diesel911 · #2 12-09-2013, 01:17 AM

After you reamed and replaced the Glow Plugs the starts are still rough?

I might have miss this in the reading but after the Glow Plugs were replaced what was the Voltage at each of the Glow Plugs when the Key was in the Pre-glow position?

If you have new working Glow Plugs and each one is getting enough Voltage the Glow Plugs would have to be working reguardless what the Glow Plug light is doing.

If that is the case after you start the idle is rough I would expect the rought idle is being cause by something else.

The Glow Plug Relay timing out too soon could also cause the rough idle depending on how short of a time the Realy satys on.
As long asthe Key is in the Pre-glow positon you should get voltage at the Glow Plugs for an anpoximate total time of 30 seconds. So you might want to also check if your Glow Plug relay is timing out too soon.
You should here the Relay click on and later click off.

cooljjay · #3 12-09-2013, 01:39 AM

A leaking oil gauge can cause a lot of electrical issues with connections.....I would fix that and clean everything with brake cleaner and electrical cleaner.....remove the two bundles of grounds behind the cluster and clean/polish the metal....

You also have a ground wire that runs from the chassis to the transmission bell on the drivers side of the car....clean that and the areas it touches....

Also confirm your battery cables are good, they are known to swell with corrosion under the insulation.....the hood hinge pocket is also know to rot and that is where the negative side of the battery connects to the chassis...

Also confirm you are using Bosch plugs...

Valve adjustment, filter changes and no fuel leaks are all confirmed....

Murdock · #4 12-09-2013, 01:47 AM

I am using brand new Bosch plugs. The ground checks out at other parts of the vehicle with no issues and I have no other electrical issues that I know of. Lights, signals, radio, gauges all working fine.

It is due for a valve adjustment for sure. All filters (oil, power steering, and both fuel) have been changed. These items do not pertain to the relay not working though.

TnBob · #5 12-09-2013, 03:06 AM

So if I am reading correctly:
Using the existing wiring the GP relay frequently fails proper operation
Adding a ground wire to the GP relay and all is well.

How about adding a wire to ground under the hood to the existing brown wire ?

tiger64 · #6 12-09-2013, 10:53 AM

Recheck the glow plug part number on your old box. I had the same issue and found I was given wrong glow plugs part by the parts guy. They screwed in fine, but were too long. They did not glow and starts in cold impossible. Did job again with right part number and changed nothing else. Resulted in perfect cold start every time. Has been very cold and all good for 30 days.

Murdock · #7 12-09-2013, 11:12 AM

Quote:

Originally Posted by tiger64

Recheck the glow plug part number on your old box. I had the same issue and found I was given wrong glow plugs part by the parts guy. They screwed in fine, but were too long. They did not glow and starts in cold impossible. Did job again with right part number and changed nothing else. Resulted in perfect cold start every time. Has been very cold and all good for 30 days.

This solution doesn't seem to fit the symptoms. It doesn't explain that I can bypass the wiring harness and make it work.

Quote:

Originally Posted by TnBob

So if I am reading correctly:
Using the existing wiring the GP relay frequently fails proper operation
Adding a ground wire to the GP relay and all is well.

How about adding a wire to ground under the hood to the existing brown wire ?

It seems like the easy solution except for a couple of things:

Splicing on to the brown wire in the harness is not easy, I would have to cut the sheath open
This might mask an underlying issue

I wonder if I just need to pull the gauge cluster apart and trace that brown wire. I am puzzled why a ground wire increases resistance with the ignition on. pre-1980 would make sense with the coolant temp sensor.

SD Blue · #8 12-09-2013, 11:44 AM

It is not uncommon for grounds to "increase resistance" with the ignition on. Sometimes this will change by just reversing the leads. Remember, meter resistance is derived from a voltage and regulated current ratio.

This also points out how even the most minor faults in the ground connections can cause elecrical glitches.

I've noticed this same issue with mine and did a little research in the FSM. There was mention of cycling the glow plugs several times, to recharge a capacitor, if the vehicle has sat for a period of time. I haven't delved into it but my thinking is if there is a large electrolytic capacitor involved, these will eventually degrade with time and cycles. It may be the reason we are seeing more glow plug relays with intermittent problems. I've done a little searching but haven't found anyone that has opened one up a glow plug relay to dispay it's inner workings.

cooljjay · #9 12-09-2013, 04:00 PM

When you review the wiring diagram at the link below.....it says the ground for the glow plug relay is grounded to the washer pump and then to ground....there is a ground bundle behind the headlight on the passengerside.....I would start cleaning the grounds....

24 Electrical Troubleshooting

yvairguy · #10 12-09-2013, 05:24 PM

i changed the caps inside my GP relay as well as a diode that was bad, the board has a mask on it and makes it difficult to clean and solder but i did manage to make the repairs. I then added 2 leads to bring on the factory GP with a switch and be able to hold it on.

I probably should have just left well enough alone as it worked fine before i added the wires to be able to operate it manually. after i added the wires the relay would hold end and not even time out after 1 min. this is the reason i started the DIY GP relay thread.

if the spot where the wire harness is grounded shares the same grounding screw and it's lose i could see you "back feeding" through something else on that same ground, but with that little of a change i don't see how it would be effecting the GP cycle.

Murdock · #11 12-10-2013, 03:02 AM

Loads in series act as a voltage divider. I am not sure of the resistance across the relay input but if low enough a 1.1Ω resistance to ground could cause enough voltage drop that it falls below what will actuate the relay. If the resistance of the relay was ~3.5Ω (Seems reasonable as the current at this resistance would be ~3.4A) the voltage would drop to about 9v.

Cooljay is probably on the right track, except for the brown ground wire enters the wiring harness and is run through the firewall. It might be tied on to an instrument ground which may be faulty.

I will be working on it again tomorrow. I will be taking voltage readings with the circuit intact and most importantly I will be tracing the wire behind the instrument cluster. I will detail my results.

cooljjay · #12 12-10-2013, 05:02 PM

I am not going to try and argue the point, but if you click that link and download your wiring diagram....it clearly shows the drown wire to the relay runs to the washer pump then to the ground and that the indicator bulb is grounded on its own wire from the cluster....granted this is a 1983 diagram but I have a hard time believing it has changed...

connerm · #13 12-10-2013, 06:36 PM

What happens if you do two glow cycles before the start?

cooljjay · #14 12-10-2013, 10:33 PM

Nothing besides wasting time waiting in the cold or draining a battery more possible.....its just that amount of glowing time isn't required unless you are attempting to start below 25°......it says in the manual for the car, when it is recommended I believe...

Murdock · #15 01-06-2014, 12:35 PM

I know it has been a few weeks, had to break for work and holidays. I went back at it this past weekend and I am still stumped on it. I did follow the diagrams as coolljay suggested and it shows them going to the washer pump and the headlight as you indicated. I cleaned the grounds behind the cluster as suggested but no avail. I verified with a DMM that the ground is coming back to the cluster, unless there is another wire I do not know of. As I pulled the grounds I saw the resistance increase between the relay harness and battery ground. While I was in there I fixed the oil leak and cleaned up the oil residue.

I also tried running a ground wire direct to the battery, still no dice and still the resistance increases with ignition set to on (with battery connected only). One day, though, the relay began working on its own for a bit. It was a dry sunny day, unusually for this time of year in this region.

I will update as I dig further. Something is malfunctioning somewhere and the relay is just picky enough to not actuate. It's a long shot am thinking it has something to do with my leaky window gasket so I will be addressing that right away. One thing at a time, right? I feel accomplished on the items I have tackled so far on this neglected stepchild but sometimes get discouraged when some items get this puzzling.