1998 E300 will not pass CA smog because of bad glow plug. What to do? - Page 4 - PeachParts Mercedes-Benz Forum

ROLLGUY · #46 01-31-2017, 06:32 PM

After two hours of making a soup can with two glow plugs, and hooking it up to run the first two plugs separately, I am having the same problem. Just for fun, I turned on the ignition and checked for power on all six terminals of the GP relay output, and found zero volts on number one. I went and got the relay out of my friends parts car, and it has the same problem. Could I have two bad relays? The guy that has the parts car has an identical one that has no problem with the glow system. I am going to check his relay to be sure it puts out power on the terminal for GP number one. I hope that is the case, and I will just order up a new GP relay from our sponsor. Fingers crossed......Rich

ROLLGUY · #47 01-31-2017, 07:14 PM

Dave just drove bye (he has an identical car) on his way home and I checked for power on terminal number one on his relay, and his does in fact have power there. I have a new one on order from Pelican! Hooray! Thanks for all of your input......Rich

ROLLGUY · #48 02-09-2017, 08:31 PM

I installed a new GP relay, and it too is bad. I took one of the old relays and dug out the silicone around the edge, and opened it up. What I found was the terminal for number one burned out (almost like a fuse). I can only assume that number one G P has a dead short. I will solder up the part in the relay, and then disconnect plug number one. It is a bummer that the new relay burnt out the same way, but I hope I can fix it.

Diseasel300 · #49 02-09-2017, 11:02 PM

That's a pretty heavy piece of copper to cook... Do the 606's not have a fuse in the glow plug relay anymore?

ROLLGUY · #50 02-10-2017, 12:11 AM

Quote:

Originally Posted by Diseasel300

That's a pretty heavy piece of copper to cook... Do the 606's not have a fuse in the glow plug relay anymore?

Evidently not. The new relay is the third one to have this happen to. Not knowing number one G P was shorted, I put one in from our parts car. I am sure it blew as soon as the ignition was turned on. Still not knowing, a new one was ordered from Pelican and it blew as well. I hope I can fix them, or at least the new one. I may have to hook up a dummy plug to number one after I fix the relay, as the CEL will still come on.

funola · #51 02-10-2017, 12:42 AM

Toò bad you didn't buy an ammeter to test 1 and 2 glow plugs. You would have blown the ammeter fuse (cheap) instead of 2 relays.

funola · #52 02-10-2017, 12:53 AM

It is extremely unlikely a glow plug is shorted. More likely a short in the harness. When you installed the parts car relay, you didn't measure for voltage in #1 output? Could have saved blowing up the 3rd relay.

ah-kay · #53 02-10-2017, 01:39 AM

I don't think the GP circuit goes thru any electronics, they are just controlled by a relay. The electronic controls the relay coil. I had GP shorted to ground before and it just blew the fuse. It happened a number of times and the GP relay never sustained any damages. It is hard for me to accept every GP relay has problem, you tried 3, right?

Anyway, this is what I would try.

Disconnect all GP from the GP relay, so NO current = balanced circuit and hope the computer will clear the code. If it does clear the code then smog test will pass. Rig up something to change the GP relay to manual for now to start the car.

jay_bob · #54 02-10-2017, 05:50 AM

I doubt that will fly.

I don't know the exact schematic of the unit but I've seen enough industrial controls to recognize what I see in there. The three copper ladders are shunts. They are tuned to an exact resistance by milling out the center of the copper until they are a very specific resistance (on the order of tenths or hundredths of an ohm). Then the voltage is sampled on each side of the shunt to determine the voltage drop across the shunt.

So most likely you won't be able to repair the burnt shunt by jumping it, but you may well be able to salvage one of the shunts from one of the other failed relays and use that to repair it.

Also this relay cannot be fooled by having zero current, it is not a current balance type like the old style relays, that had two counter flowing currents against a reed relay. This one is looking for absolute current flow in each leg.

This also explains why it throws 3 OBD codes, it has one shunt for #1 and #2, one shunt for #3 and #4, and one for #5 and #6.

funola · #55 02-10-2017, 09:39 AM

Looking at your pic on a computer monitor to see details. Are there 3 more of thes copper strips on the other side of the white plastic? Do all strips connect to the relay contact? Do they connect to anything else? If they connect only to the relay contact, and there is no main glow relay fuse, then those are fusible links for individual glow plugs.

97 SL320 · #56 02-10-2017, 05:17 PM

I counter that the shunts are milled due to the shape and rounded edges of the material. If this was flipped over, there should be a sharp edge from the part being stamped, this is called the break out side.

I've seen shunts in engine computers and none have any trimming makings.

The right hand shunt looks like it is seeing heat / high load due to the coloring near the top so check that circuit.

Pull numbers from the IC chips and perhaps the circuit can be figured out.

chronometers · #57 02-10-2017, 05:22 PM

Quote:

Originally Posted by funola

It is extremely unlikely a glow plug is shorted. More likely a short in the harness. When you installed the parts car relay, you didn't measure for voltage in #1 output? Could have saved blowing up the 3rd relay.

x2. The entire engine wiring harness of which the glow plug (gp) harness is part of is notorious for being bad. I was having glow plug issues with my '95 E300 and found the glow plug harness insulation gone throughout. Someone sold me an older gp harness from an older diesel (1987?). I guess you could buy the gp harness separately on older models. he 1987 was also a 6 cylinder....so harness is same and insulation more durable. Pick and pull one....splice it in and you may resolve your problem.

ROLLGUY · #58 02-10-2017, 08:53 PM

Quote:

Originally Posted by chronometers

x2. The entire engine wiring harness of which the glow plug (gp) harness is part of is notorious for being bad. I was having glow plug issues with my '95 E300 and found the glow plug harness insulation gone throughout. Someone sold me an older gp harness from an older diesel (1987?). I guess you could buy the gp harness separately on older models. he 1987 was also a 6 cylinder....so harness is same and insulation more durable. Pick and pull one....splice it in and you may resolve your problem.

I know for a fact that the harness is NOT the problem. I disconnected the wire from glow plug #1 (only one that can be removed without taking off the intake), and the wire has continuity, and not to ground. Plug #1 is the one I had a problem with last time I replaced them in this car. I could not replace #1, as it stripped out in the head. I suspect the element portion got twisted off of the main body when it would not turn in the head. I will have to pull the head next time more plugs go bad. Right now all of them except #1 seem to be working. I have got the new relay apart, and will try to replace the terminal/shunt with one from the old relay.