glowplug relay wiring
 

I have no power to the glow plug relay in my 85 300d turbo. The wire connecting to pin 15 on the relay does not light my test lamp when I turn the key on. From the wiring diagram in the Haynes manual it looks like it goes stright to the ignition switch. Am I reading that right or is there a fuse somewhere in between.

Somewhere there's link to the W123 Chassis FSM online
 

Using the "Haines Book of Lies" for Wiring schematics is like getting your Great Aunt Drunk (then blindfold her) and ask her to draw the Diagrams.

http://www.mez.co.uk/haynes.html

What's Pin #15 on that chassis?
 

The Ignition Sequence Trigger Wire for the PreGlow ReLay?
OR
The Hot at All Times POWER feed?

On the W124 Chassis the Hot A.A.T. POWER feed runs from a Positive Buss
in the Left hand side Kick Panel of the Driver's Footwell.

(I understand how the Engineers routed the Cabling the way they did,BUT NOT
why they didn't use a Dedicated Cable Feed for the PreGlow Relay Directly from
the Battery.The Cable that feeds the Sub Panel Positive Buss is not as large as
the Cable from the Buss to the PreGlow ReLay.)

123 car
 

Ignition sequence. 123 chassis

Err...
 

O.K. I'll not suggest Anyone Torture themselves by testing CONTINUITY between
the Ign. Sequence Terminal (on the Backside of the Ignition Switch) and the
opposite end of the VERY Same Wire at the PreGLow relay.

Unless,the Ign. Sequence Cable is dependent upon the PreGlow Relay's GROUND WIRE being:Clean Bright and Tight !
[IOW No Ground...for the Ign Sequence to operate against.]

If You've got a Funky Ground for the Pre GLow ReLay,It'll do Funky things!
It does have it's own SEPARATE grounding Point.

To my knowledge, there is no fuse on that side of the glow plug system. Have you tried hot wiring pin 15 to see if the relay works then?

To be clear, it's circuit 15 (hot when ignition is in start or run) and the pin is #1 in the GP relay assembly connector. There is basically a direction connection from the battery (circuit 30) to the ignition switch so no fuse, and through the switch in Run/Start to make circuit 15. If you pull the small plug in the glow plug relay assembly you should see 12V at pin 1 when in the key is in the Run position, and pin 2 should be open. (Use pin 4 of the connector for ground to make these checks if you want to really verify the circuit. If you get nothing, then see what happens when you switch the ground wire of the meter to the chassis or engine block.) If you want to check it continuity wise, disconnect the positive post on the battery, and check continuity from the + connection (not the battery post, that's out of the circuit now) to pin 1 of the connector with the key in Run. Turn the key to start and you should see a connection from the + connector to pin 2 of the GP relay assembly connector. If you check continuity from pin 3 to ground that will tell you the dash lamp is probably okay. Pin four of the connector is ground.

If you have a bad connection somewhere, it may not show up with continuity checks, but only when voltage is applied and the circuits are connected normally. That's because with no current in the circuit there are no voltage drops. So if you are looking at pin 2 of the GP relay assembly connector in the run position and are seeing no voltage, either you have a bad connection between the battery and the GP relay assembly, or no connection to the relay assembly. If you had a bad ground through the connector and everything else was fine you would still see 12V.