Just pull the plug for the wires to the GPs from the relay (Bigger plug) Test resistance to ground from each socket in the plug. Should be 0.7 ohms, but many multimeters will show around 1.2 to 1.4. Look for anomalies amongst the 6 readings. Infinity = bad. Under 0.4 (if you have a really good meter that can detect small fractions of an ohm) means a plug going bad, which should also be replaced. If all 6 read within 0.2 of each other, they are likely all good.

Jim and Philip say the only way to test for good is with a ammeter and big wires. I've been successful always with just a cheap ohm meter. But they are right, the BEST way to test is with an Ammeter.


After checking GPs, with the plug still off the relay, turn the key to glow position (best done by a lovely scantily clad assistant) and check the 6 pins on the relay for +12v to ground. While you are in there, change the 80 amp fuse.

Floyd Thursby via Mercedes <mailto:mercedes@okiebenz.com>
March 7, 2018 at 5:51 PM
This post from one of the forums is exactly what is going on now that I cleaned up the EGR valve and got that sorted. Looks like a GPectomy is in order, or at least test them and the relay before I tear into them all. Max you said you had the stuff to deal with it?

First time starting after the manifold/EGR work took a bit of cranking, but then it has started fine since. So either at the beginning it was sorta dry from sitting, or the GPs weren't all putting out. it did belch lots of smoke too, FWIW. But it will start and run now so not a huge issue at the moment.

--FT

I am getting the following errors.

P0380---faulty glow plug relay

P1369---faulty glow plug

My questions is are both bad or is one causing the other? Each morning the glow plug light functions properly for the first start but after that it give the error each time. I hate to spend the money on a relay if the plugs are the problem but I also hate to change the plugs if its just the relay. Is there any way to know which it is or if it is both? Would really like to fix this before winter sets in. I have been down this road several times with my 99 E300-TD and, to my mind, the only solution is to do whatever is necessary to prevent future occurences. This is a very expensive, but necessary, maintenance item. Approximately each 100k km I have experienced these symptoms and have concluded that one or more glow plug is faulty. Given the labour to remove and replace the glow plugs, I change all six plugs, even though only one or two have failed. On the last visit, at @ 300k km, the relay was found to be "intermittent" and was also changed. This is about a $1000 proposition but, to my mind, cheaper that the 3-4 major tune ups that an equivalent gasoline engine would have required during the same 100km. That is why I would check the resistance of the individual glow plugs first! If they check out, then test the relay.

At first glance I can't see how the glow plug fault would fault the relay. The relay is either on for a period of time and passes voltage then off or it's always off or always on, both of the later create faults. Good luck the car computer can sense something wrong with the circuit, but can not pinpoint the bad item. That what multimeter is for. If the engine starts fine in the morning, that is indication at least some plugs work, meaning the relay is good. so if the relay tests fine it could be throwing the code only because of the plugs or vice versa? just want to get an idea if i will probably need to replace the relay no matter what or if there is a chance it is ok and the plugs are just confusing it. Those systems are fool proof. The relay is either open or closed, voltage does not get to the glow plug(s) or regardless of key position voltage is on and your battery drains.

You must test each glow plugs resistance, if they are low say 6-8 ohms they are still good. Then test the voltage at the relay, with the key in 0 and 2. If you get 12V with key at zero------whoooooooooooooops. If you get 12V after car runs for three minutes--------------whoooooooooooooops.

The relay is about 150 bucks.




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