OK, where's the fuse in my gp relay- grrrr
 

I have a 93 300D 2.5 turbo. The car won't start. I think it seems like the gp relay is bad. No light on the dash, no starting just a bunch of cranking. I see the new parts on fastlane and all these posts have an 80 amp strip fuse. Mine doesn't have one. The relay box looks about the same but no fuse at the end of it! What the heck? I don't think I am blind.

Later model cars have no fuse in the GP relay. There is a circuit breaker in there. I am sure that the picture you are seeing may not be the true representation of the GP relay for your car.

The official MB service manual offer a very good step by step method for checking the GP relay. Perhaps someone on this board has one that can help you with.

My car is an 86 300sdl and my relay has the fuse in it.

OK. I have done some more searching. It seems mine has an internal circuit breaker. So here are my next questions.

1) is there a way to reset?

2) is this like the fuse strip and just needs to be replaced or the whole relay needs to be replaced?

3) Is there something else that can cause this or rather what is the cause of this to break - just age? The car is 17 years old.

4) Can I put an 'older' style relay with a fuse strip in place of this one?

I do not think so. Your car has an afterglow feature that the older models do not have. Your GP relay gets a reading from a temp sensor in your head to determine the time the relay stays on, the older cars have the temp sensor in the relay itself.

You should be able to put a preglow relay from an earlier W124 provided it is a five-cylinder version. I don't know enough about Mercedes model-years to give good advice. The preglow relay from an OM617 will work but you will have to do some rewiring; it will not be plug-compatible.

The circuit breaker in your preglow relay is self-resetting; if it will not reset, you will have to replace the relay. Remember that there may be fuses in the main fuse compartment that cover power to the electronic part of the preglow relay. If one of those fuses is bad, the relay won't work. Likewise, the over-voltage protection relay (OVP) and its built-in 10 amp fuse may need to be good. I don't know how similar your car is to my '87, which is what my wiring diagrams are for.

Why not disconnect the relay and jump the glow plugs (for a few seconds) and give it a crank?

That will at least prove there is a problem with the relay/gp circuit?

Answer
 

It should have a strip fuse in the relay.
Enter this part number in FastLane, click the part picture to enlarge it.


MB# 007 545 99 32
Diesel Glow Plug Relay (Preglow Time Relay)
Chassis W124.128
Model 300D 2.5
Years 1990-1993



Have a great day.

I hear you on the 'what should be in there' argument. My local MB parts store (A better wrench in Chapel Hill NC) looks this up and they too said it shows one with a strip fuse. That's not what's in it. I'm going to do some voltage testing tonight to determine what is and isn't working. I'll also get a p/n off of the old one to cross and see what it is made for. I've had the car for 18 months with no problems in this department so whatever is in there was working, but sure as hell isn't now.

This is all very confusing. Everyone is telling me it should have a metal strip fuse. I just found this on E-bay (check listing 290325538558) which shows a good picture of the item and you can plainly see the MB part number on the side of if as 007 545 99 32. It also clearly does NOT have a metal strip. Somebody's picture or part must be wrong between these various sources. More grrrr....

A picture of yours might be helpful.

Yours Ain't got No Strip Fuse !
 

Nor does mine.
It is electronic
(and self resetting,
IF THE ELECTRICAL PROBLEM THAT CAUSED IT TO TRIP IS CORRECTED!)

'Vary rarely goes bad.

Much Nicer than having to fool with the Strip Fuses!

124.128 (s) from '90 to '93 all have this feature.

The pic that compressignite posted is indeed the same as my relay. The p/n is as listed above 007 545 99 32. So I have done some checking and don't understand the situation.

1) Under no key conditions, my batt has 12.46V and the red lug wire on the relay is the same.

2) I have good resistance levels on gp 1 & 2 (didn't check further). Their resistance is only 0.2 ohms or so.

3) Removed the gp pin connector and no voltage of course on the pins with no key.

4) Turned the key to preheat position. No light on the dash. However, I measure about 10.98V on the 5 pins that the gp leads go on. However, the motor is clearly not preheating. This seems to me that it may not be enough voltage???

5) I don't hear the relay clicking but clearly there is some connection changes that happen upon keying it.

6) Shut everything down and again the batt is back to normal 12.46V or so.

What's the problem here!!!

That relay is about $185. Should I change it out or is there more to check first.

I did get the car to fire this afternoon once it sat in the NC heat today. It started hard and rattled for about 5-10 secs and then ran like normal.

I had one fuse in the block that was gone. It was in slot 'D' and is a 16Amp fuse marked aux fan. I put a different one in for a bit and it didn't make any difference in the gp problem.

"I measure about 10.98V on the 5 pins that the gp leads go on."
 

Your GP relay is working.

If you've got power to the Pins like that...
'only two things left to check
1.Wiring from the pins to the Glow Plugs
2.Glow Plugs

I hate to say it but "Ohm-ing Out" GP s Ain't Always a 100% proposition.
(Twelve Volts TO THE Plug,WITH it out of the engine is the Definitive Test!)
You say your plugs are only a year old...
Did the Glow Plug Holes get reamed out to remove Carbon-ization?
Are the Glow Plugs Beru or (Second Choice) Bosch? [Anything else is Junk]

ran out and checked some more. When energized(key in preheat) I get nothing 0 V at the gp nut on #1 or #2 (didn't check further). The pins with connector off are getting 11 V.

Also, the resistance between gp1 and 2 or 2 and 4 is 1.6 Ohms. The same checking the other end between pin holes 1 and 2 (with connector off). Does this make sense? I'm not sure it does but am reaching my limits here. It is like something is amiss when everything is connected up, no voltage on any gp bolt that I checked. Is there a dead short somewhere that could cause this or is there something more sophisticated happening here?

Wiring
 

What's the Problem?
You've Diagnosed it Already!

"When energized(key in preheat) I get nothing 0 V at the gp nut on #1 or #2 (didn't check further). The pins with connector off are getting 11 V."

Resistance needs to be checked at each individual Glow Plug ...
Not "Between Them"

I'm not sure you're not having a "Semantics Problem" describing your checking
of the Glow Plugs???

If you have Proper Voltage at the Relay Pins and Improper at the GPs ...
It's The Wiring!
(Or Alien Abduction of the actual Copper wire inside of the insulation of the
wires from the relay Pins to the Glow Plugs.So you can't figure it out and
the Aliens get to enjoy driving you crazy.)
EDIT: The Aliens might be "Sucking the Electricity" out of your GP wires
whenever they see you testing...Hard to catch the little boogers!

Take each individual glow plug wire and test it for continuity with your DMM.

Then "Ohm Out" each wire (From one end to the other,with both ends
disconnected from the Car.) There's a formula online given the AWG or mm
of the wire and it's length for proper resistance figures.