I think my glow plug relay is a goner... - PeachParts Mercedes-Benz Forum

Chris Blanchard · #1 12-12-2002, 02:35 PM

I went out to start my 84 300SD this morning, and the glow plug light would not illuminate. Having been through that before, I suspected that I had one bad glow plug, but the other 4 would be sufficient. I tried starting the car and it would not catch. It was about 40F in the garage, so it wasn't _that_ cold. No go.

I measured the battery output with no load: ~12.5 volts. I monitored the voltage while the glow plugs were _supposed_ to be glowing and saw no voltage drop so I suspect that they are not being energized.

I measured the resistance of all 5 plugs - all were within spec. I examined the 80A fuse and it was fine. The plugs are Bosch, and are about 9 months old.

While cranking (at a healthy speed, I might add) I did notice smoke from the exhaust that smelled very rich, so I believe I have fuel.

So, I am thinking my glow plug relay is toast. I am ready to run to the dealer and grab one right now ($130) and give it a go.

Anybody have any better ideas?

Chris Blanchard
1984 300SD 165k
Cincinnati, OH

#2 12-12-2002, 02:59 PM

The first time I had such a problem, I didn't think to disconnect the 5 prong plug at the relay box while I went probing the tips of the glow plugs themselves, so I got uniformly good readings, but a few minutes later I gave myself a big dope-slap and went back and rechecked with the connector disconnected -- then I found one plug with infinite resistance and one very bad, along with three good ones. I had realized that there has to be a point where those five circuits come together, so if the connector is left in place, electricity will take the path of least resistance and give good readings as long as there is even one plug which is ok.

Chris Blanchard · #3 12-12-2002, 06:00 PM

I measured the resistance of each glow plug by connecting the negative terminal of my voltmeter to battery ground, and then touching the positive terminal of the voltmeter to each of the female ends of the 5-prong plug on the glow plug relay.

My readings were all below one ohm, and I believe spec. for these is about 0.6 ohms.

I will perform one last test - measuring the voltage provided to each of the glow plugs with the glow plug harness disconnected. I should get about 12.5 volts there. If they ain't getting voltage, no current will flow......

At least the dealer had one in stock and was offering a good price on one - about $120 (I get 10% off for being an MBCA member) which is a better price than I found at a local europart place ($130) or on the internet from my usual supplier ($120). I was pleasantly surprised.

Chris

psfred · #4 12-12-2002, 08:06 PM

Chris:

Replace the 80 A strap fuse on the relay -- it's toward the fender at the "top" of the relay, probably broken, will bend if you poke it. Held on by two cross head screws, you only need to loosen them.

Mine did the same thing last Sat nite -- fortunately the engine was warm enough to crank right up without the plugs.

Peter

#5 12-12-2002, 08:24 PM

Was wondering...

Lets say the fuse is ok and the relay is shot. In an emergency could you run a wire from your battery to the load side of your relay and fire the plugs? On a series system I know from experience it will work to jump the bad plug with a wire to get it to start.

psfred · #6 12-12-2002, 10:40 PM

It will work just fine. Use clips, though, don't hold the wire in your hand -- if you don't have really good contact, it will get HOT!

Peter

CHAPP · #7 12-13-2002, 05:58 AM

My glow plug light comes on 75% of the time. Two years ago my mechanic ran a jumper that made the relay turn on by a small button under my dash. I don't need it very often... so have never replaced it. Car starts perfect...even after sitting for a week and temp. 10 F. I think he hooked the wires to the circuit that warms the engine after it has started...fooling the relay into thinking the engine is running. This way, when it is real cold outside I can run the glow plugs after the light turns off...to make sure it starts smooth...and keep the switch pushed until engine is smooth. I do this is because the CAT loaders I operated had manual glow plugs, with a timing chart for the temps & mintues to heat the glow plugs... pasted on the dash CAT told you you to heat the engine after it started until "did not run rough". I operated equipment for over 20 years...with very breakdowns.

Chris Blanchard · #8 12-13-2002, 09:05 AM

As I stated earlier the 80A fuse was fine.

I purchased the glow plug relay yesterday and replaced it this morning before work - took about 20 mins (the screw that holds it to the fender on the right side is tough to reach with all the stiff wires from the headlights in the way).

I tried to start it and SUCCESS!!!

I noticed that the glow plug relay I pulled out was stamped '05/84' which indicates that this is the original one. The fact that it lasted ~18 years is pretty incredible.

The piece of diagnostic work that I did that I think is probably the most reliable (and easiest) was the fact that the voltage across the battery did not droop at all when the glow plug relay was _supposed_ to be energizing the plugs. That method is only useful if you are really confident that your plugs are in fine shape (mine were less than a year old, and this car has never failed to start even in the coldest weather).

Chris

dave240 · #9 12-13-2002, 09:18 AM

Chris,

FYI, I just went through these same gyrations last week with my 240
I found that my relay was good, but 2 of 4 glow plugs were bad
even though the tested good using omh meter.
I determined that they were actually bad by phyically removing
them and energizing with 12.5 volts individually. The good ones
glow to a bright orange. the bad ones do nothing.

Now, I don,t know if you have other means of transport,
but if you have time to order, you can find great deals on the
internet. FASTLANE is among them.
But I bought four new Bosch Plugs, and a new and improved version
relay, for less than $100. Some places offer free shipping.as well
My car starts like a big-dog at 15 degrees,
before the new plugs/relay it would not start even at 50 deg.
good luck.
(note. the newer improved Bosch relay, allows plugs to remain gloowing
for up to 3 min, after startup)

#10 12-13-2002, 10:59 AM

My 84 300D's glow plug relay went bad (glow plugs' resistance good, but no voltage to them) and I was able to fix it by replacing the largest of the two electrolytic capacitors on the circuit board. Unless my memory has gotten worse, it was a 100uF, 16v cap. Hope this info is of use.

Jerry

Chris Blanchard · #11 12-13-2002, 02:36 PM

I considered opening it up and having a look, but I "assumed" that the circuit was potted and that no components would be serviceable/replaceable. Besides, even if I did replace a couple of caps, who knows what other components are nearing the end of their servicable life - knowing me, I'd be worrying about the things I didn't replace.

I did notice that with the new relay, the light seems to stay on a bit longer and that the engine idles a bit better right after a cold start. My guess on that is that the caps were slowly degrading and the RC time-constant that they helped to create was becoming shorter. Just a guess...

I'm not sure if the relay I got keeps the plugs glowing longer (even after the dash light goes out). I've seen that type of relay advertised but I'm pretty sure mine doesn't do that (it's the same p/n as the one it replaced).

On to my next projects this weekend, replacing the rear rotors and pads, and adjusting the valves and doing the rest of a 15k service.

Although I've owned this car for a bit over two years, I'm now just getting to the point of _only_ regular maintenance and the _rare_ repair. I need a project vehicle (my wife would kill me).

Chris Blanchard
1984 300SD 164.8k miles

lrg · #12 12-13-2002, 06:39 PM

Chris,
I think virtually all these systems keep the plugs glowing a bit after the dash light goes out. For an easy test try turning the key to glow at night (or in a closed garage) with the drivers door open. You'll see the interior lights dim when the plugs glow and stay dim after the dash light goes out. After a total of 45-60 seconds or so you'll hear a "click" as the relay cuts out and the interior lights will brighten up a bit.

coleman kelly · #13 09-08-2003, 11:28 PM

I have one trick to add to this thread. I also noticed my dash light for the glow plugs not coming on and engine wouldn't start.
Testing at the 5 pring plug I found not current going to the plugs, to I popped the cap on the glow plug controller board( is this what you are calling the relay?)
Anyway, I foudn that the engine started quicky if I held down the contactor amnually for about 20 seconds, it'll emit a tiny blue spark when contact is made.
I also figured the board needed replacemtn because of a few spots where it had browned(like overheated)
Good emergency start. Looking forward to getting the updated board.

P.E.Haiges · #14 09-09-2003, 12:12 AM

Chris,

You have to remove the connector that goes from the relay to the GP to measure the GP resistange otherwise you read parallel resistanace thru the other GP. Then its easiest to measure the individual GP resistance using a banana plug in the connector holes.

I found out that when my GP relay didn't work it was because there wae a bad connection between the other connector on the GP relay. R&R it about 10 times fixed it. There was probably corrosion on the contacts and the R&R wore the corrosion away.
The relay has been working OK since.

P E H

Brian Carlton · #15 09-09-2003, 12:15 AM

I had the same problem last winter and was also unsure if it was the plugs or the relay. Could not start it at 12 degrees. A weak battery added to my woes. But, I made a little jumper with five leads on one side and all the wires twisted and soldered toether on the other end. Stick each of the five wires into the sockets in the plug (after disconnecting it from the relay) and run a heavy jumper cable directly from the battery to the single soldered lead. Let is sit about 30 seconds while still connected and attempt to start the vehicle. In my case, two plugs were bad and this solution did not work. The relay was fine. But, I keep the jumper in the trunk just in case I need it in the future

Brian Carlton
1984 300SD