No extra wires just a multi wire plug on my '82.

If there are more wires... I nor my buddy saw them while changing out the old alternator.

The old alternator only had the one single plug.

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

Checked the old one and the multiwire plug has 3 wires.

Carefully checking the housing, there were no other wires connected to the old one.

Also did a current drain check which clearly shows the clock is running.
Motor off, current drain is .020 amp ave, runs .018 to .022amps.

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

Would "at rest" mean the engine is not running?

If so, I'm not quite accepting the fact that the reading at idle is less than the reading at shutdown unless there is a heavy consumer (glow plugs) that is engaged at idle but not engaged at shutdown.

If the glow plugs are engaged, this would exceed the capability of the alternator to maintain voltage at the battery and would signify your problem. The alternator works fine...........the vehicle is demanding more than it can produce.

At rest = not running.

I checked the glow plug voltage while running and it was zero. Im wondering if the relay is intermittently hanging.

I will recheck the readings.

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

Wow... motor not running voltage at ~12.5
Started and again was reading ~11.7
This time when I checked the glow plug I am reading about 4 volts.

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

No change while not running.

Still reading ~11.7 after starting. Glow plug still reading ~4 volts.

Popped the glow plug relay connectors and now 0 volts on the glow plug.
One quick rev and the reading at the battery is now well over 13 volts.

I can see where the alternator wouldnt be charging with the glow plug relay
hanging up and drawing 4 volts but with the motor off, why would I only get
a reading of .020 amps ?

The prime symptom is of course a battery flat enough that it wont turn the starter.

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

There's your problem..........intermittent glow plug relay.

With the engine off, you're only running the clock............20 milliamps is about right. Everything else, including the radio would be off at that current draw.

Ive got mine opened up and it looks very clean in there... gonna try burnishing the contacts and see what if anything that does.

Thanks for the help ~~!~~

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

If the alternator is overloaded the regulation suffers and the voltage declines.

It is entirely plausible that the GP relay is off when the car is shut down but stays on past the pre glow cycle.

Note that some relays have an after glow function, but that is supposed to last only a minute or so, then the relay should shut off and stay off.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Contact burnishing didnt help.

This was sure made much easier with my cheepie DVM. A great ebay deal. Thanks

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

I had a problem like this countless times on my 240D. Except for the one time the alternator was actually bad, it was a loose V-belt. Check the belt tension. I think on the 300D it is a dual belt system however...still worth checking though as it is an easy fix.

Ive got 3 gp relays. One off the '82. one from my '85 and one of an unknown origin from a buddy.

All 3 have different interiors, not just minor differences either, massive.
The unknown one wont even fire the gp dash light.
The 85 contacts were bad and even after cleaning did the same as my 82 gp relay did, which is to
only partially shut off. Had the cover off of both and the contacts do not release. One both I am
reading about 4 volt on the #3 gp the easiest to reach with a meter probe.

Does anyone have a schematic for the glow plug relay? Ive looked on line and thru postings here and have come up dry.

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

A schematic will make this a probably $20 repair instead of a $175 or so replacement.

ANYONE have a schematic ?

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

The FSM only shows "solid state" for the control in the relay. I don't think it'll help you in any way, but if you want the .pdf file, send me an e-mail.

FYI.

There's a 300D for sale on the local craigslist for $800 and it might have a malfunctioning glow plug relay.

Hmmm.....

__________________
daw_two
Germantown, TN

Links:
Sold last car --- 05/2012 1984 300D Light Ivory, Red interior
Cluster Needles Paint
New Old Stock (NOS) parts

Past:
3/2008 1986 300SDL "Coda"
04/2010 1965 190D(c) "Ben"
& many more