Got a used GP relay to replace an intermittent old one. Good news is the light pops right on (the old relay I sometimes had to smack with my hand to get it to reset). Bad news is it's little quick to extinguish the GP light. In other words, it's like 40 degrees here, and it pops the GP light on then off pretty damn quick, like instantly. The relay senses coolant temp right? Might the sensor be off? Remedy? It's a relay from florida. Maybe it's just acting weird like any Floridian that steps off the plane in Boston in March? I will try again today hoping for a longer GP burn and await your response. Thanks.

Oops. I said 617. But it's a 85 300Dt (CA car).

I've heard the light is driven via a transistor in the relay....it could be damaged or flakey...key thing it how does the car start...that light doesn't have to come on for it to function.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

There is a post about the light not coming on at all, but the relay still functioning - it's not that old of a thread.
Like BHD said, the light doesn't have to function for the relay to work. Listen for the relay to clink off. (this can take up to 90 seconds)

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D

The relay measures ambient temperature to control the time the light operates. There is an electrolytic capacitor in the relay that affects what this time is. If that capacitor is leaky, your light time will drop. The relay should keep running for 30-90 seconds, although the current draw decays exponentially.

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

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

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

Car coughs on startup. Never did before. But it does start. I guess I need to pull the multimeter out and see if it's staying on. I just got the GP relay, so I dunno, I may have to give it back. It's not a dash light issue. there's definitely somethin going on.

check to see how long you see voltage on the glow plugs when key is turned on...and see when it drops.


You may have glow plug(s) burned out.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Thanks for the input from everyone (forgot to note that in last email). My GPs are fine. I think the sensor is tweeked, but I will check voltage and it may be a non-issue if I get glow. It's just too fast for 40 degrees.

Just curious ... relay kicks on and sends voltage to plugs, fading to zero voltage over time to avoid burnout. Ambient air temp sensor decides how quickly full burn ends and fades to zero, or when to let the light go out to indicate ignition to the driver? In others words, does the sensor affect burn intensity and time like it does in a modern car, or is that static in a 85 benz and the only variable is the light telling you when to turn the key.

I assume this is dumb question, and it is the former.

I think the sensor just controls the time the light is on.

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

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

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

The glow plugs do not fade out the are on or off. They do heat up and cool down however, but this is just a function of the on or off of the relay.

I just had a problem with the glow plugs on my 87 td, they were not glowing i traced it to a corosion problem in the small plug on the relay. The inside of the ground(brown wire) had a white flacky film in it that was creating an intermitting contact. I cleaned it out and the glow has been good sense.

There is still a problem with my glow light it was working intermitently a while back but has not worked at all for a couple of weeks. I have checked the dash bulb, it works, and the glow plugs are all good. The black wire at the small conector on the relay feeds the dash light and it will light my test when the key is on, so i must have a problem in the wire to the dash.

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82 Rabbit diesel (first d), 84 Jetta turbo d (300k when sold), 83 240d (305k when sold), 84 F250 6.9d w/Banks turbo(parts truck), 86 F250 6 cyl.gasser(waiting for 6.9d), 84 300d ( 347k Sold 8/04), Y2k New Bettle TDI (185k miles), 95 740IL (wifes), 87 300TD (206k Sold 7/05), 05 Passat TDI Wagon

They actually do fade out. This has been discussed before:
Click Here
The plugs draw an initial current of about 120 amps well over the 80 amp fuse. The plugs have to fade to prevent melting of the fuse.

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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In the referenced posting it assumes that the battery voltage remains at 12V. It also ignores the resistance of all wiring involved.

It is highly unlikely that the current draw of GPs would actually be 120A, given the resistance of the wiring from battery to GP relay & to GPs.

If anyone has tested the current Vs Voltage applied to these GPs, over say 30 seconds, in a W!23 (for example) I would be very interested in the measured values.

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Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
Josephine '82 300D 390kkm White/Palamino int.
Elizabeth '81 280E, sporting a '79 300D engine.
Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.

Be a bit careful with the terminology here.

When the relay closes, the initial inrush of current may be close to 120 amps. The plugs are stone cold at this point. As soon as they begin to heat (well within 1 second), the current flow begins to drop. But, this can hardly be considered "fading" because they have not even achieved maximum temperature.

Within about three seconds (est.), the plugs reach maximum temperature and full resistance and the current draw is approx. 60 amps. to all five of them.

They remain at this maximum temperature and current draw for the next 37 seconds (approx.) with no degradation of temperature during this period (voltage drop in battery during this time frame is excluded). At the end of the period, the relay shuts off instantly and the plugs cool down very quickly. They drop below red hot within three seconds (est.).

All i ment was they do not get the power turned on or off to them slowly, like turning the volume up or down. Once the voltage is turn on to them they will heat up like the element in a toaster oven does, this may give the impression of fading but it is just heating up or cooling down. As they heat up the resistance increases and the current draw will go down.

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82 Rabbit diesel (first d), 84 Jetta turbo d (300k when sold), 83 240d (305k when sold), 84 F250 6.9d w/Banks turbo(parts truck), 86 F250 6 cyl.gasser(waiting for 6.9d), 84 300d ( 347k Sold 8/04), Y2k New Bettle TDI (185k miles), 95 740IL (wifes), 87 300TD (206k Sold 7/05), 05 Passat TDI Wagon

I have a 1991 350sd turbo diesel [no bent rods ---yet---] my car starts up quite normally with glow plug lamp functioning ok then goes out---sometimes it will come on again---after i'm in motion---stay on about 30 seconds then go out. doesn't do this every time---wonder whats wrong????

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mercedes 220s--sold
mercedes 300d--sold
mercedes300dturbo--sold
mercedes 350sd turbo [will outlast me]
jaguar xk140mc hardtop
mgb [several of them]
mga
mg-td with right hand drive[my 1st car]