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Old 11-14-2001, 04:24 PM
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ajb ajb is offline
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240d glow plug relay problem?

After checking out a 300d this past week, I am going to check out a 1979 240d this weekend. The car has 164,000 miles, auto trans, pw, ac, sunroof & leather interior. The car has been sitting for the past 11/2 years. The PO has died & the car is being sold by his friend. He claims the car has new glow plugs, but isn't getting power to them. The car will start if towed. (I'm assuming MB has a rear pump in the auto trans?)

My questions are:
1. Is there any way to check the glow plug relay and or fuse? Where is the relay located? What would the cost be of a new relay?

2. If it isn't the relay, where else would the problem be?

3. Is there any way to get power to the glow plugs by "going around" the relay? (a jumper from the battery to the glow plugs) As I would like to see if the motor will start without being towed.

4. Would towing the car to start it cause any harm to the transmission or any other parts of the car?

5. Is there anything else I should be concerned about?

Thanks in advance for your help!

Aivars Berzins

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Old 11-14-2001, 04:46 PM
SW SW is offline
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Join Date: Aug 2000
Location: Houston, TX. USA
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I don't know about the '79, but on my car the glow plug fuse and relay are housed in an approx. 1.5" x 4" black plastic box mounted on the drivers side wheel well sort of behind the lights. Pull the cover off and see if the fuse is not blown. I think the '79 also has the series type glow plug system unlike the later models which have a parallel set up. So, the problem could be with the wiring. I think the Haynes manual provides a good procedure for checking the system.
2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold
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Old 11-14-2001, 05:18 PM
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The '79 has a much different glow plug system. The relay is in front of the instrument cluster (firewall side). The fuse is in a small mounted fuseholder at the top of the firewall directly behind the engine. To find the fuse holder, just follow the wire from the rear glow plug to the firewall and there will be your fuse.

Visually check the fuse first, then use an ohmmeter to check for continuity from the wire from the fuseholder to ground. If you have continuity check for power to the other side of the fuse when you turn on the ignition. If you don't have continuity, start working through the glow plugs to find the problem. There are insulators on each glow plug which if not installed correctly will short past one or more plugs. These four plugs are all in series. Now that I've said that if the insulators are not on correctly, you can have continuity but have inoperative glow plugs. You will have to reason your way through this simple circuit and see that nothing is shorted around the plugs to ground.

Good luck,

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