350SDL, original owner, 147K miles. Original glow plug relay/module finally indicated trouble, replaced all glow plugs and relay/module. New relay/module failed.

Opened both relay/modules up and found the relay arm on the the number 3 cylinder burned off on both modules. If I remove the plug to the glow plug harness and apply 12VDC to the number 3 cylinder glow plug pin the number 3 glow plug gets warm and there is no indication of a short because the wire does not get hot-hot. Repeat the process on the other 5 cylinders and no indication of trouble.


Mechanic is stumped.

We need to find the problem before we fry another relay.

All ideas welcome.

Thank you.

Check the wire for a ground, chances are the insulation has rubbed through somewhere not so obvious, and it is partially shorted. Dead short will blow the fuse, partial (high resistance) short will over-draw the current on that contact.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Peter,

The wire checks out fine....

We're still stumped.

Thank you.

I have what I think is a god ohm meter.

With the plug to the glow plugs removed from the relay, I am measuring all of the leads to the glow plugs buy putting one end of the meter in the individual pins to each glow plug and the other end of the meter to ground. They all read "0" ohms. I thought the correct reading is .6 ohms if all is well. What is happening here?

As soon as I can get my hands on an amp meter I plan to measure the current to each glow plug by going from the battery to the amp meter to each pin in the plug to each glow plug, is this a good diagnostic test?

Seems I have a short some where?

Thanks for any ideas.

Be sure you are using a meter scale that will read out below 1 ohm, such as a 10 ohm full scale (10.0) readout. If the scale is too big, such as 10k ohm, a 0.1 ohm resistance will cause your meter to read 0.

Jim.

My meter has a 1 ohm scale that I am using.

EZRider, that was one of those obvious suggestions that I've managed to overlook, so I thought I'd pass it along.

Brute force method - wire and 20/30A fuse. Power each plug separately. The one that blows the fuse is shorted???

Sorry I can't be of more help. Sounds like you have a good puzzle here.

Keep us informed.

In using an amp meter direct from the battery and measuring current draw at the glow plug wiring harness plug, each inividual glow plug starts out drawing about 20 amps and then drops to about 10 amps and stays there which indicates no short from the relay out to each glow plug.

What is curious to me is that there would be a total current draw of about 120 amps (6X20=120) through the glow plug relay when the key is first turned on . I'm surprised if this is normal?

All new glow plugs were installed using MB part no. A 001 159 17 01. Local MB dealer insists these are the correct ones. Is it possible we have the wrong glow plugs and they draw too much current for the relay to handle? Does anyone have the correct Bosch part number for these plugs?

We need help.

Any ideas.

Thank you.

OK, I'm guessing here, but it sounds like the glowplugs are acting normally. 20Ax12V is about 240 watts, tapering to 10Ax12v or 120 watts. That should make a nice, hot glow!

6x10A or 60A total current is not too much draw; I think I remember an 80A 'master' fuse link in the relay. Each relay contact is seeing 20A at start and 10A long-term, and shouldn't be enough to burn a contact.

If the glow plugs are normal, I'd have to suspect something in the lead wire from the relay to the plug, some kind of intermittent short. Those can be the devil himself to troubleshoot; look for rub spots on #3 wire?

Oh, one last thought. Was the relay replaced, or did the old relay (with #3 contact burned out) somehow get plugged back in, leading you to think it burned again? That's fishing, I know, but it is possible.

Somewhere there has to be an explanation. "The Truth is Out There."

Good luck, and good hunting.

Jim,

we're sure the relays were switched.

Thank you.