Hi guys. Well after installing new glow plugs, the glow plug light does it's normal cycle but when I put it in drive and take off, the glow plug light turns on and doesn't turn off. I believe the relay is DONE! This worries me because I've read similar issues on here where people's light turned on and it eventually drained their battery.

So the idea I have is this (while I wait for a new relay to arrive...) Can I unplug the relay after I turn the car on. So the glow plugs did their thing and now the glow plugs are not going to turn on when I drive because they're unplugged. Will this work?

I have to use the car a lot this weekend. I don't mind unplugging and plugging the relay. I just have to have an option to drive the car.

digi

__________________
1984 300td euro turbo

Anyone? Sorry for my impatience!

__________________
1984 300td euro turbo

Yes that will work, start the car normally, then unplug the 6 conductor plug at the relay. Don't leave the top of the relay uncovered if the pins are actually hot with 12V, anything metal dropping onto them could short and you'll lose that fuse.

The thing to do is get the car running and the glow plug light on as in the present fault situation. Then get out open the hood and pull the plug off the pins on the relay and with a multi meter or test light see if there is 12V between those pins and ground, if there is the relay is faulty, if there is not 12V at the pins the circuitry to the indicator lamp itself is faulty.

Awesome! Thank you!

__________________
1984 300td euro turbo

So I grabbed my multimeter and tested the relay. It's ok... I also tested the glow plugs for power when the light came on when driving, no power. So I don't have to worry about my battery going dead... So obviously, one of my glow plugs is dead. Could it be something else? My car turns on without hesitation in 30 degree weather. Can this be achieved with a bad glow plug?

So my question is... Would the light be on for any other reason? I ruled out the relay...

Also... Can I test the glow plugs without having to remove them?

digi

__________________
1984 300td euro turbo

Yes. If you have a 20 amp scale on your DVM it's very easy. Unplug the connector from the glow plug relay. Connect one lead of your DVM set to 20 amps) to battery + terminal post. Connect the other lead of your DVM to one of the 5 terminals of the glow plug harness connector you unplugged. It should surge to around 18 amps and then drop slowly to around 12 amps.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The light is an artificial indication controlled by a solid state device that is part of the timer.

You guys are swell! Thank you!

__________________
1984 300td euro turbo

Not obviously. Not sure how you got there...
Pretty good indication your plugs are good. Does it run rough then smooth out?
The relay is the most likely source. The relay applies 12V to the light. There's no real easy way to check this except at the dash light which would only tell you there's 12V there which you already know because it's lit. The only way to be sure is to rig up the relay's 4-pin connector so that you could disconnect pin #3 without turning off the car or pulling the plug to the relay. This would entail disassembling the connector and temporarily installing a jumper, or building a short cable assembly with connectors on each end that would allow you to do the same thing. It is entirely possible that the electronics could turn the light on without energizing the glow plugs. I don't know if this is a common failure mode or not.
Yes, the current check is a good way, but you can also check resistance to ground from the connector at the relay. Resistance should be less than one ohm, so you need a good meter to check that.

__________________
mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

The relay only powers the glow plugs. If the relay also powered the indicator lamp, the lamp would remain illuminated for the duration of the glow cycle.

A short or a cracked trace in the instrument panel causing the light bulb to light up with no connection to the actual GPs.

You say the light does it's normal thing prior to starting - which includes going out, then you put it in drive and it comes on.

Does it go back off when you put it back into park? Or stay on until you shut down?

Do your other lights work normally? (low fuel, brake wear, brake warning, and alternator come on prior to starting, then go out and stay out).

Mine was on while driving a while back, it was because I had taken the connector off the temp sensor for the GP system and forgotten to put it back on. Might make sure your sensor connector hasn't fallen off. You'll probably find two on the same side of the block as the GPs.

__________________

The light is never connected to the glow plugs.

Completely N/A for the subject model.

Okay, how about:

A short or a cracked trace in the instrument panel causing the light bulb to light up with no connection to the actual status of the GPs.


Happy now?

What a useful post. I'm glad you provide the info to support your thoughts. Maybe an explanation of what is wrong with my statement other than just touting that I'm wrong and you know everything about every MB model.

I do know that at least some of the 123s have two temp sensors in the block. One for the temp gauge and one for the glow plugs. They must have gone to one sensor at some point.

__________________