96 E300D glow plug indicator comes on 30 sec. after key is turned on
 

Quick back story... a rodent has died somewhere in the engine compartment. Thinking it was under the electrical box on the passengers side, I pulled the box out, no rodent. Put it back, turned key on, no glow plug indicator, a little warm outside, so it did start, a little rough, but smoothed out. Then the indicator came on. So if I turn the key on, wait 30 sec., I can hear the relay kick on. I did test the wiring to the glow plugs and it looks like there is at least 1 plug that is open. Would that cause the P0383 code, and it was just a coinsedence? Btw, think the rodent is in the crossmember, a few maggots dropped down in that area.. pretty gross and SMELLY! I had to tape off the vent area under the hood to keep the smell from getting inside the car.

My list of codes doesn't have a P0383!

A failed glow plug will cause the pre-glow light to act differently but the relay should still energize the other glow plugs. Are you sure the relay is clicking ON after 30 seconds? Most preglow relays click OFF after 30 seconds.

Jeremy

You're right, it's P0380..glow plug circuit A, and you're right again, it normally does turn on with key, and after a few seconds shuts off... not now.

After we bought the '96 E300 I downloaded a .pdf owners manual. It says this:

"If the preglow indicator lamp fails to light up, or lights up while driving, the preglow system is defective . . . "

When we got the car the CEL/MIL was lit. The preglow indicator behaved normally, IIRC. It woud beep and the multifunction display would read "check engine electronics" upon startup and again at shutdown. The OBD tool brought up code P0380. #1 glow plug was bad. I replaced all the glow plugs and reset the ODB system and everything has been normal since.

A wiring fault will also trigger P0380. You might test the "bad" glow plug at the head to eliminate the possibility of a wiring issue. Which plug is it? Maybe it's one you can reach without having to pull the intake.

What concerns me is that all was normal until I removed and replaced all the wiring and K components... The first thing I noticed after getting it all back together was that the glow plug indicator did not light up. I may need to get a wiring diagram for the glow plug assembly. In the meantime I will go back and see which glow plug tested bad and see if it is the plug or wire. When the light does come on after the 30 sec delay, and the relay kicks on, I don't know if it's actually delivering voltage to the plugs, so I'll check that too.

So # 4 plug is open, all the rest are fine. When the relay energizes, with the wires removed from the relay, I tested each prong on the relay and I'm getting 8 volts to each plug. Is that normal?

8 Volts sounds too low. Should be full battery voltage.

There is full voltage to the relay.. Does anyone know the process for the glow plugs once the key is turned on? Does the system do an analysis figure there's a problem with a glow plug, and then send lower voltage? Is there something somewhere else telling the relay not to send out full voltage or is it all in the relay?

I Checked mine last year and double checked a 95 relay, i also had on hand.
Both were putting out 12.8 v during the pre glow time after which it switched to 9 volts I think for the after glow duration. Having no light turn on with the key in position 1 and the glow lamp turn on latter indicates a short circuit with at least one glow plug.

It sounds l like pre glow is the issue.. I suppose replacing #4 would be an easy, cheap diagnosis. On the glow plug wiring assy., on right side, under lid, (which was replaced 5 or so years ago) there was quite a bit of what looked like light oil on the connectors.. is that normal from Mercedes?

A slight amount of oil on stuff under the intake manifold is not unusual. Make sure it's not getting into connectors and damaging the insulation or preventing good contact.

Glow plugs usually fail open but I suppose they could fail short-circuit (more likely the failure of the insulation allows wires to touch each other). Later pre-glow relays including 1995 contain a self-resetting circuit breaker rather than a fuse. I have never had an opportunity to observe this phenomenon.

I have a light added to my instrument cluster that illuminates whenever the #1 glow plug is energized. It stays on during afterglow and then goes out. This enables me to confirm afterglow and detect a stuck-on relay or a relay that fails to energize.

If the relay detects a failed-open glow plug, it still energizes the others but it does not turn on the glow plug light in the instrument cluster.

I have not conducted the test done by dieselbenz1 so I cannot comment on a reduction of voltage during afterglow; must measure that sometime.

Jeremy

Right, that's why the GP harness was replaced initially, due to failed wire integrity. When the new harness came, the connectors to the module had this oily like substance on them. When I took them out this last time, I sprayed electronic cleaner on them to clean them up a bit. I'll be replacing the #4 plug today and see what that does.

# 4 plug replaced.. all is as it should be! I would not have thought at all about replacing that plug with the light doing what it did, but just after removing all the modules and wiring I figured it was something that got disturbed... Thanks for all the help Jeremy, Greg and dieselbenz1!
Now off to replace tensioner shock, belt and side gaskets on fuel injection pump..

I kinda figured your problem was not related to messing with that electrical box. Glad it worked out.

Take some pics and post about your IP gasket job. Need to do that myself.

This thread got me wondering about CEL/preglow light behavior in relation to a bad glow plug. I could swear the preglow light behaved normally when I had a plug out and just had the CEL/MIL lit up. Gonna disconnect a glow plug and see what it does.

Jeremy, I'm curious about your "custom" glow circuit indicator light. Got a thread on that?

Greg, pulled the connector.. no light came on during pre-glow. Think I'll check voltage from relay now to see if it's back to battery voltage.

Belt and tensioner shock on, IP gaskets tomorrow. Thought about going from underneath, but already had the crossover pipe off, so will get manifold off and see what happens. I'll take some pics and hopefully it will be a success.