I have an 85 300 td wagon and it is acting up electrically- And I haven't been able to figure it out...

The first symptoms I noticed were that it started to do weird things when I turned on the headlights- both turn signal indicator lights would light up dimly and the fuel and temp guage would jump and give higher readings ( the fuel sender stopped working and is disconnected. This was intermittent and didn't always happen.. The brake and flasher lights continued to work fine even when this was occurring.



Then one night the flashers spontaneously started flashing ( no key in ignition and I tracked that down to the switch which was melted from shorting. Replaced switch and went back to having previous symptoms.
Several people suggested I check and clean the ground wire ( Thanks for your suggestions ! ) I cleaned the connection at the engine/transmission connection and the one where the battery ground connects with the chassis. No Change ;-(

Then I started to notice a drain to the battery which has gotten increasingly worse. Now I need to put the car on a charger every time I come home and can't shut it off when I am out. I swapped another alternator and finally another starter. I put in a new battery. No Change.

I pulled all the fuses and tested each contact with a continuity meter. Fuse terminals # 6,8,14 & 1,3,7,9,11,13 all show continuity with the key off and the doors shut. IS THIS NORMAL?

I pulled out the instrument panel - No change Disconnected each wire off of the light switch- No change

I have added an additional fuel pump at the tank and injection line heaters to run WVO but these problems persist even when I remove this circuit from the equation. The back up lights also stopped working and I ran rep wiring to a switch on the dash to restore them. I haven't disconnected the wires from the tranny switch so those wires or the fuel sender circuit may be suspect.

Can anyone offer any insights or suggestions?


We are still offering to barter a free stay at our B7B on Molokai in Hawaii in exchange for somebody who wants to come and teach me how to properly tune the 617 and help sort out some of the places I am stuck but as yet we have had no takers. It must be pretty cold up north... :-)

Thanks for any help you might offer. I am getting desperate. Our other wagon is still out of commission with a runaway idle I can't figure out either.

PM Jay_Bob he's the electrical genius on this forum about the 87's and the 98's and he's learning the 08's...

anyway, try disconnecting the alternator plug and see if the drain stops.

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My drivers:
1987 190D 2.5Turbo
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1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Unplugged the alternator plug No Change :-(

Now I have noticed that the glowplug relay is making funny sounds. not sure if that is because the voltage is too low or it is bad. I replaced it with another used one I had and it buzzes too...

This sounds like a defective ignition switch back feeding through the glow plug relay (and possibly to some degree other circuits).

You also need to remove the cluster.
Look back in the exposed area for a massive collection of BROWN wires.
I suggest you (one ground bolt at a time) remove, clean, reinstall, all of the grounds.


.

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Got your PM.

I can second what whunter said, a bad ground or a bad key switch are both plausible. I am leaning towards bad ground myself. Weird problems like this almost always turn out to be bad grounds.

Best thing to do is download the wiring diagram from the online factory manual at startekinfo.com and start tracing.

First thing I would do is turn to the "ground distribution" charts in the wiring diagram and verify every ground point has good connections and no corrosion. Use the "component locator" to identify where the grounds are. They will all be tagged with a W, i.e. W1, W2, etc.

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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

First of all- thanks for all the suggestions and advice ! It is hard living in the dark dungeon of discouragement alone and out of ideas....

I swapped a key switch out of another car and no change... :-( Even with a fully charged battery that is reading 12.5v and a jump of the charger up to 13.5 I could barely get the starter to turn over. I disconnected the glow plug relay at the feed wire on the relay and it seems to turn over a lot better. ( I have also cleaned all the battery to frame and the frame to engine and starter grounds and the small connector block by the battery...) What is between the glow plug relay and the Key switch?

I had a bear of a time downloading the wiring diagram from the link you gave me JayBob ( Thanks that site is an awesome resource ! ) but I am horrible at reading them and knowing what to do with them. Any suggestions for a next step?