Not sure what the problem is but my glow plug light won't come on and I am not glowing when it should be. So today I replaced the whole relay and.... Same thing. I checked and replaced the glowplugs less than 3 months ago so they should be fine. This is my only car so it is a little discomforting as we are expecting a foot of snow as we speak and it is already dumping and I need to get to work in the morning... Anyway, any ideas? I thought it might be a blown fuse somewhere but there appears to not be a fuse for the glowplugs in the fuse box under the hood...

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Andre Shoumatoff - Park City, Utah
Diesel Engine Conversion Community

First off, what car do you have?
On the 123's the GP fuse is in the relay box, a silver metal bar at the end toward the back of the car with a screw at either end. If you replaced the relay, then I'd guess the fuse is new too. I hear that they sometimes have hairline cracks in them that are hard to see unless you take them out. Do you have a hard time starting? Could the light in the dash just be burned out?

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Michael Roth

1982 300TDT Wagon
1982 240D Euro!, 4spd manual-Parts car now, dead engine
88 Jeep Grand Wagoneer-fixing up for offroading
1989 Ford F150 (rust bucket) For Sale!
1953 Dodge B-4F, 1 1/2 ton Stake Bed (new restoration project)

When in doubt, start with the basics:

Put a voltmeter on one of the glow plugs and see if you get 12V when you turn the key to the #2 position. If you don't, it's a relay problem. Either the relay has failed again, or the fuse is bad, or the ignition switch is not activating the relay.

If you have 12V at the plugs, then one or more than one of the plugs are bad.
Pull the plug out of the side of the relay and check each plug via the sockets inside the plug. Each should read less than 1 ohm. Doesn't make sense with all new plugs, but, you've got to cover the base.

Yes it is a 123, 1983. Perhaps one or more of the plugs has gone out. Does the relay have a mechanism that makes it not function if one or more of the plugs has burned out?

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Andre Shoumatoff - Park City, Utah
Diesel Engine Conversion Community

On your car what ever glowplug(s) are still working should glow regardless if one or more of the others are burned out, only the early cars were wired in series. This really sounds like a fuse problem. IIRC on the 123 the fuse is on the left front fender over the wheel, it is NOT in the regular fuse box. The IP fuse is a flat metal bar, about one inch long and held in place by two screws. There is no covering on the fuse itself but it is in a holder that is covered. Poke around a bit until you find it. Even if the fuse looks good remove it and make sure it is solid and clean. My GP fuse once cracked just under a screw and looked perfect untill I tried to remove it.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold

OK I will thanks. I figured it was a relay because the glowplugs wouldn't turn off until I hit the starter. Then the light didn't come on intermittently and now won't come on at all. I will check the fuses though... I was even going to drive to Salt Lake tonight to get some new glowplugs but good to know...

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Andre Shoumatoff - Park City, Utah
Diesel Engine Conversion Community

OK I checked and I cannot find an alternate fuse anywhere. I was under the impression the fuse was built into the relay - the metal strip that on my old one is slightly crooked (as in possibly burned out) but looks perect on the newer relay. I cannot figure it out, I am 99% certain I am not getting any power to the glowplugs because it starts like ****! So I need some help, I am hoping it is something easy I can fix tonight in town otherwise roadtrip and down to the shop where I have to pull a non running vehicle out into a foot of snow - so I would like to avoid this...

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Andre Shoumatoff - Park City, Utah
Diesel Engine Conversion Community

Do you have a voltmeter so that we can stop guessing what is wrong with the vehicle?

I do and all necessary other tools... down at the shop... The volt meter will tell you that the plugs are not getting voltage. Anyone tell me what color wires to check to diagnose if it actually is the relay or not?

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Andre Shoumatoff - Park City, Utah
Diesel Engine Conversion Community

If someone could give me more details on where to check for this fuse that would be great. The relay is notr brand new but was in a wrecked car so I assume it worked fine when the car was hit.

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Andre Shoumatoff - Park City, Utah
Diesel Engine Conversion Community

If the plugs don't get voltage, the relay is the most likely culprit. However, you definitely want to check to be sure that the key switch is sending 12V to the relay. I don't have the wiring diagram for the W123 but, someone else will hopefully chime in.

BTW, I lost the glow plug relay on the SD once. I made a jumper wire from five pieces of #14 stranded wire. Strip all the ends from five pieces, each piece about six inches length. Twist the ends together to form one fat cable on one side. Then take each of the remain individual wires and stuff them into the individual sockets in the plug that plugs into the side of the relay. This plug is the plug that sends power to the glow plugs.

Connect a jumper cable from the positive battery terminal to the fat end with all five twisted together.

Wait 30 seconds.

Start the engine.

Remove the jumper cable and jumper wire.

OK thank you guys... I will be at the shop tonight so hopefully I'll get it all sorted out. Luckily it has not been below 30 degrees so it has been able to start. Not very easily but it starts I'll post with what it is...

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Andre Shoumatoff - Park City, Utah
Diesel Engine Conversion Community