Odd Glow Plug Relay Issue 1985 300TD - Page 2 - PeachParts Mercedes-Benz Forum

funola · #16 01-06-2014, 01:19 PM

"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 am puzzled why a ground wire increases resistance with the ignition on. "

"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). "

You should know this but incase you don't: You cannot make accurate resistance measurements with key on, that is, there cannot be any voltages across what you are measuring. Power has to be off.

Have you tried another known good glow plug relay? I have the same symptoms as you in my 85 and have tried numerous tests and cleaning all connections and grounds to no avail. I am attributing it to a flaky glow plug relay at the moment but have not tried another relay yet since it doesn't happen all the time so the problem is not that serious and I can always get the car started.

Murdock · #17 01-06-2014, 01:41 PM

Quote:

You should know this but incase you don't: You cannot make accurate resistance measurements with key on, that is, there cannot be any voltages across what you are measuring. Power has to be off.

Have you tried another known good glow plug relay? I have the same symptoms as you in my 85 and have tried numerous tests and cleaning all connections and grounds to no avail. I am attributing it to a flaky glow plug relay at the moment but have not tried another relay yet since it doesn't happen all the time so the problem is not that serious and I can always get the car started.

You are correct on the resistance so I moved to voltage tests, all seem to checkout. I am also with you on the relay itself. Since it is intermittant at this point and all wiring has been verifited I am thinking it is on its way out. I do not have another to test with, but I am thinking of getting the upgraded version from Mercedes Source.
Eventhough the temps here don't go much below 40 most of the time, it still makes a big difference in starting.

EDIT: Moderators, why are you blocking m e r c e d e s s o u r c e.com??????

barry12345 · #18 01-06-2014, 02:07 PM

Get a relay from a salvage yard. Then if there is no problem with it installed go over the electronics in the original relay. Not much sense trying to chase external problems yet.

There should be a time constant ciruit in there amoungst other things. Your could have the original now very old glow plug relay with aged components. What shape are the high current contacts in for example?

I particularily never found European electronic/electrical things to be that good in general. On the otherhand their glow relays seem to have stood up well on average over the years. They still can and do fail though.

Murdock · #19 01-06-2014, 02:18 PM

I am overdue for a parts yard run so I might go this weekend. Since I can make the relay work 100% manually I am guessing there is a component inside out of tolerance that doesn't like any dip in voltage.

My next test will be to "y" off of the pins and do a voltage test at the connections while it is plugged in, then I can see the actual voltage going to the relay from the ignition switch. I wonder if it is slightly below 12v which the relay no longer actuates to. Maybe I will pull it out this week and get it on the bench and see if anything sticks out. I already spent too much time yesterday, might as well see if I can find something.

tangofox007 · #20 01-06-2014, 02:24 PM

If you do not attempt a start, how long does the relay remain closed?

Murdock · #21 01-06-2014, 02:31 PM

That's the thing, it very seldom actuates. Maybe once in 50 starts? When it does it acts as normal.

funola · #22 01-06-2014, 03:25 PM

If you are going to Y off anything I suggest Y-ing off the relay output and the wire coming from the ign switch to the glow relay using 2 incandescent bulbs so you can pop the hood and see instantly what's going on without having to pull out a volt meter. Since I have done that (y-ing 2 bulbs) the problem has not occured. Figures!

willw · #23 01-06-2014, 03:52 PM

I was having the exact same problem, Poor starting, light going off quickly, etc, and then total failure of the GP system.

Thank God I listened to the masters on this web site and had a spare GP relay from the junk yard in the trunk.

When I opened the old relay I didn't see much wrong, just a little corrosion on a couple of pins, which I re-soldered. I went further and pulled the actual relay apart by taking off the spring of the relay and looking at the contacts.

There are 2 contacts on the relay armature, both had huge pits in the centers. One was worse than the other. Before I took the relay apart I had
tried to measure the contact resistance of both contacts. Since they are connected together I expected that they should be the same resistance
to the frame of the relay, but they were not. The one with the worst pitting was more resistive that its sister contact.

So here's what I think happens.

If you use a Ohm meter to test the relay it works just fine :-)
But putting operating current, dozens of amps, across the contacts cause the relay contacts to go highly resistive, heat up, not provide the proper power to the GP's, and cause the system to think that the excessive heat build up means the system is warmed up before it really is.

That's my story and I'm sticking to it!

Will

Murdock · #24 03-12-2014, 03:17 AM

It has been a bit, I have been distracted by other life elements. I feel a little dumb by wasting time on this after I found the solution. I was finally able to get to a car at a local pick-n-pull before the vultures got it. Swapped in the $11 relay I got and trouble over.

What was deterring me from all of this was 1) I didn't want to drop $100 on a relay if I didn't need it and 2) I could make the old one operate manually with test leads.

Thanks all for the suggestions and knowledge. This blue ox is coming to life more and more as I knock items off the list. Next is a transmission adjustment, but I'll take that topic over to the proper thread.

Maxbumpo · #25 03-12-2014, 12:11 PM

I'd be suspicious of your ignition switch at this point.

If you haven't changed ALL your fuses recently, I recommend you do that. A surprising number of electrical gremlins have been cast out by new fuses...

Murdock · #26 03-12-2014, 12:24 PM

That's a new one for me. I checked the fuses, most of them look pretty clean. I also cleaned the ground behind the instrument cluster as well as on the front rIght fender. AT this point its working as it should. My only concern now is being a part out of a yard I have no way to know its life expectancy.

Maxbumpo · #27 03-12-2014, 12:28 PM

Get some of the proper copper fuses, change them all. Doesn't make much sense (my BS is electrical engineering) but it DOES work. I learned long ago not to argue with success...

If your "new" relay works now, it may work for years. The capacitors age more quickly when they are not in use. If/when it does die, you may be able to repair by replacing the capacitors.

cooljjay · #28 03-12-2014, 02:41 PM

This is why you need to remove and replace the fuses...

Murdock · #29 03-12-2014, 02:46 PM

I was thinking I would take the relay I pulled out and test/replace some of the the components.

Cooljay - that makes sense, probably due to vibrations over time it wears those fuses out. That might explain my intermittent rear left window motor. I wonder if anyone has gone through the trouble of swapping in a more modern fuse block?