How to test the Glow Plug Relay? - PeachParts Mercedes-Benz Forum

Scott98 · #1 08-28-2006, 09:14 AM

I've got an '82 240D that I just bought (4 speed) yesterday. I suspect the Glow plug relay is bad. The dash light won't come on but I'm getting voltage at each glow plug. Also, when I pull the 4 pin connector on the relay and bridge the number 3 and 4 (or is it 2 and 4?) connectors, I can get the dash light to come on. The car is having lots of trouble starting. I don't think the plugs are heating up at all. Once it's warm, the car starts without a problem. I also just adjusted the valves.

Anyway, I suspect it's the main glow plug relay but before I drop the coin on a new one, is there a way to test it?

Thanks,

Scott

Brian Carlton · #2 08-28-2006, 10:17 AM

The glow plug relay is fine. Don't change it.

Pull the plug at the relay and connect an accurate ohmmeter to each of the four sockets (one to each glow plug). Connect the second lead of the ohmmeter to the battery ground.

Make sure you have an ohmmeter that can read 10 ohms........full scale.

Report back with results for each glow plug.

Scott98 · #3 08-28-2006, 07:14 PM

I have a cheap meter. The needle moved all the way over on three of the four plugs. The bottom right connector was the dead one. What does that mean?

Thanks,

Scott

Brian Carlton · #4 08-28-2006, 07:20 PM

Quote:

Originally Posted by Scott98

I have a cheap meter. The needle moved all the way over on three of the four plugs. The bottom right connector was the dead one. What does that mean?

Thanks,

Scott

The plug that is open has failed and needs to be changed.

Without a proper meter, the condition of the other three plugs is unknown.

Scott98 · #5 08-28-2006, 08:55 PM

Quote:

Originally Posted by Brian Carlton

The plug that is open has failed and needs to be changed.

Without a proper meter, the condition of the other three plugs is unknown.

The previous owner installed four new glow plugs within the past couple months. Could a faulty relay be to blame? Also, why isn't the glow plug light coming on? When I bridge the four pin connector it lights up.

Thanks,

Scott

#6 08-28-2006, 09:36 PM

Quote:

Originally Posted by Scott98

The previous owner installed four new glow plugs within the past couple months. Could a faulty relay be to blame? Also, why isn't the glow plug light coming on? When I bridge the four pin connector it lights up.

Thanks,

Scott

If you have one bad GP in a 240D the light will not light. Replace the GP and see if the light works.

Scott98 · #7 08-28-2006, 09:43 PM

Quote:

Originally Posted by Craig

If you have one bad GP in a 240D the light will not light. Replace the GP and see if the light works.

Would one bad glow plug keep the car from starting?

Scott

#8 08-28-2006, 09:49 PM

Quote:

Originally Posted by Scott98

Would one bad glow plug keep the car from starting?

Scott

Probably not in warm weather, it would probably start and run rough for several seconds. You may have other issues, but if you know one GP is bad, that's a good place to start.

It sounds like you have tested the GPs for resistance, have you verified the voltage too them when the key is in the glow position? If you have a volt meter, check the voltage to each GP, if they are getting around 11 volts, the relay is OK. If you don't get voltage to the GPs, we need to start looking at the relay.

Brian Carlton · #9 08-28-2006, 09:52 PM

Quote:

Originally Posted by Craig

If you have a volt meter, check the voltage to each GP, if they are getting around 11 volts, the relay is OK. If you don't get voltage to the GPs, we need to start looking at the relay.

Quote:

Originally Posted by Scott98

The dash light won't come on but I'm getting voltage at each glow plug.

....

beaulieub · #10 08-28-2006, 09:53 PM

Here's a good link on how to test the entire glow plug system. Also has great pictures.

http://dieselgiant.com/glowplugrepair.htm

#11 08-28-2006, 09:53 PM

Oops, never-mind.

bottom feeder · #12 08-28-2006, 09:54 PM

I just had a similar problem with gps that had only 12000 miles on them. Two were bad and the car struggled to start on cool (50F) mornings. Replaced them and it starts on half a crank.

#13 08-28-2006, 09:59 PM

Mine usually start OK with one bad GP, but I can tell when one is bad by the rough starting. Starting with 2 bad GPs can be a challenge in cooler weather.

As Brian pointed out, sometimes the resistance looks OK, but the plug isn't really getting up to temperature. When in doubt, I like to pull them out and "bench test" them with a battery and jumper cables..... carefully.

Scott98 · #14 08-29-2006, 08:40 PM

I just bench tested my plugs. Three of the four did not light up at all!!! I guess I found my problem. I also suppose they weren't new as I was told. I don't blame the previous owner. It was a college girl who didn't know the first thing about cars. I think someone ripped her off.

Scott