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Old 04-29-2003, 09:58 AM
Posts: n/a
Pre-glow timer relay shot?

When I turn the key to the pre-glow position, no dash lights come on, no outside lights will illuminate if I turn on the light switch, ---and a whirring/fast clicking sound comes from the pre-glow timer relay (the black box on the left side wall of the engine compartment near the front wheel). This sound will continue while I have the key at this position (although I have not let it continue longer than about 10 seconds, so far).

If I wait the normal amount of time I would normally wait for the plugs to warm up before starting, turn the key to the crank notch and try to crank --nothing. No solenoid clicks -- all dead.

The battery is fully charged.

The only thing alive on the dash is the tach needle --which jerks all over the place.

The big fuse metal strip thing (80amp, from reading threads) inside the preglow relay looks intact.

I don't have a volt meter.

Here's the question: Does this sound like the pre-glow relay is just shot and I should go ahead an replace it? I hate to have the old boy towed (never has been since 1979). Is it worth buying a meter or just replacing glowplugs?

I'd sure like to solve this myself...

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Old 04-29-2003, 10:00 AM
Posts: n/a
Car is a 1979 300SD W116, sorry, I forgot this part.
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Old 04-29-2003, 10:14 AM
Join Date: Jul 2001
Location: Old Lyme, Connecticut
Posts: 3,596

If you get nothing when you try to engage the starter you are not only having a glow plug issue. Starter juice does not check with the glow plugs or glow plug relay for permission on the way to the starter. You should be able to get the starter to run if the glow plugs are not connected.

The other symptoms also suggest a broader electrical issue. Check the ground connection to the battery at the battery and the frame, and the positive terminal for good condition. Any corrosion or crusty/powdery deposits of white, blue, aqua (or any color really) need to be completely removed. The cable clamp and battery post for each connection should be shiny clean metal when you put them together. If the battery is really in good condition it should readily power up the system. Your symptoms suggest lower than necessary voltage. You should also check the condition of the fuses in the fuse box. The little metal strips can build a layer of corrosion products that interfere with efficiently passing electrons from one surface to another.

And yes, I would get a multi-meter. I bought one from Sears a while ago (15 years ago?) and I believe it is a Fluke. Good machine, rugged and not expensive. You can use it all over, in the house, the car, boat, electrical gadgets, etc. Good investment. Good luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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Old 04-29-2003, 11:47 AM
Posts: n/a
Thanks, Jim. I'll double-check the battery, then check for corrosion, poor connections as you suggest.
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Old 04-29-2003, 06:51 PM
Registered User
Join Date: Jan 2000
Location: PA
Posts: 5,440

It sounds like a high resistance connection like Jim said. What happens is that 12 volts will get thru the bad connection when there is no or little current flowing. But as soon as the contacts close on the GP relay, a high current tries to flow thru the bad connection and all the voltage is droped across the connection. Thus the contacts which are magnetically closed pop open. 12 volts are again available and the contacts close again. This go on and on and is the buzzing you hear. Sometimes the starter solonoid will do the same thing.

Buy an electronic multimeter. They are a very handy tool for the car and home.


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