I had this problem ever since I've had the car.

The GP light on the instrument panel would NOT come on. I checked and replaced GP's, checked wiring, checked lamp, and I even listened for the relay to 'clunk' at the end of its cycle. All GP's had the proper voltage at the correct times but I STILL couldn't get the light to come on. I switched my relays from my 81 300SD into this one and it worked.

After switching them back to the proper vehicle I just couldn't help my self and I had to crack open the relay. After doing some circuit board tracing I came across a transistor on the board that is connected to the lamp connector at the plug. It is a PNP type transistor part number that I have is 2N4126 that may be available from an electronics catalog. If anybody needs one and cannot find it let me know, I've got at least 10(ten) more transistors here. Anyways back to what I was saying. After replacing the transistor I re-installed the relay and IT WORKED.

I hope this may help anybody that has tried everything for thier relays and glow plugs but still no light.

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1981 300SD

Are you saying that your relay worked, except for the dash light?

Mine is intermittent. But the light is true to the operation. If the light is not on, the plugs are not on. Today when I left for work, the plugs would not light, but some cranking got it going. (-10C) When I got to work I shut it off and I tried the plugs a few time by turning the key. It worked. When I left at noon to go to another building, they didn't work again. When I got to the other building, I tested them again and they worked. When I left, they didn't again!

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Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

The same thing happens to my '87 190D; sometimes the light comes on and other time...nothing.

jake, this is really good info - I wonder how many relays out there could be repaired by simply replacing this transistor. I take it this transistor controls current flow to the dash indicator light?

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

Yes, my glow plug relay was working correctly as far as the glow plugs were concerned but the dash light itself was never coming on until I replaced that transistor. All a transistor is used for is an amplfier or a "switch" and in this instance it is a "switch" that operates off of the timer IC on the board and allows the lamp to come on and extinguish at the end of the timer for the relay. If your relay works intermittently but your not getting the light on all of the time it may just be the contacts in the relay itself need cleaning or you may have a GP about to go bad on you. If your relay works all of the time but you don't get a light on the dash, check your wiring to the lamp and the lamp itself and if that all checks fine it may be this problem here.

Search the site here for troubleshooting the relay and the lamp circuit and if everything checks out for you but you lamp will NOT come on I would bet on this transistor to be your problem.

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1981 300SD

Also, (and I don't know exactly where it is) I believe there is an electrolytic capacitor used to control the timing. If it fails I think the glow time becomes too short. Even in new relays there is some leakage and they need this capacitor to be charged (cycling the system does that) It would be great if we could identify several of the failure points of these relays because I think Bosch is only making the extended glow version now and not everybody's happy with that. Seems it improves cold running at the significant expense of glow plug life. I have a "spare" relay that didn't work quite as good - I'd like to check a few things inside and see if I could use it as a proper spare.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

Yesterday I replaced the 100 uF 16V capacitor with a 25V version and also a 470 ohm resistor that looked overheated. I also cleaned the connector pins and coated them with dielectric compound. The relay contacts looked new and I did nothing to them. I put the relay assembly back into the car and no problems so far.

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Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

W've been having cold start problems since December (when we bought our 83 300tdt). Today it's 64 degrees and it STILL won't start. My husband has been getting up around 2 every morning to warm it so that it will start in the morning. Last night we kept it in the garage, it didn't get below 45 last night; Cranks maybe fifteen times, drains the battery, has to charge, etc. The plugs are new, they're getting 12 volts from the relay. The light only stays on for 2 seconds or less.
possible ideas:
*We just replaced the starter and think it may be draining too much power?
*I'd been wondering if my glow plugs were staying on long enough and then I saw this post (quoted following)

Make sure your glow plugs are getting that 12VDC for about 45 sec, if they aren't then it sounds like it could be that capacitor in the relay that controls the timing. Also, are you getting 12VDC at the cable plug from the relay or at each glow plug? If it is at the cable plug, you may want to check the wires going to each plug and make sure they haven't broken. There is a strip fuse in the relay also that could possibly have a hairline crack in it and when it heats up enough it could open and prevent voltage/current going to the glow plugs. Take the fuse out of the relay and make sure it isn't broken somewhere that you can't see.

You may want to do a search here using the "SEARCH" function at the top of the page for this. There have been ALOT of problems just like yours in the past that have gotten figured out.

Hope that helps you.

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1981 300SD

Do not overlook the block temperature sensor for the relay. I do not know it's common failure profile but it does short time the relay if block is warm. That is contingent on it's proper functioning. There are two sensors on side of head and I believe the front one is for the glow relay. You should check with more knowledgeable member if you have no service information. Anyways a quick check is to check the resistance with an ohmeter with the engine cold and then after it warms up. If reading is the same change it.

We have been having problems starting our car after a recent oil change. Before the oil change, I could not turn the car off without lifting the hood and hitting the stop button. After the oil change, I drove it about 30 miles and then parked it outside. It sat outside over night in about 30-45 degree weather. The next morning it would crank, but would not start. It would drain the battery down and I would have to let it sit for a few minutes and try again. The glow plug light is not coming on, on the dashboard and I need to know how to check to see if the plugs are working. Can anyone give me some suggestions? Thanks.