'76 W115 300D Glow Plug Question

[howardken52]

Friends,
Thanks in advance for any advice you might have! I've recently replaced the old series style glow plugs in my 76 300d with converted pencil style relay plugs and wiring.

I purchased a voltage meter today to see if they were working. To my dismay, there was no reading when the ignition in the "2" position. However, after some playing around (and dumb luck), I learned that the glow plugs do indead kick on in the "2" position AFTER a crank attempt. They each get a very nice, even 11.5 volts. They do this consistently, and shut off after about 30 seconds of the engine running. So, there doesn't appear to be any issue. Perhaps the relay just needs a jolt?

Has anyone else ever experienced this?

[barry12345]

Well there is an issue. You need the glow plug time before you crank. What is you glow plug indicator light indicating in the dash?


Also when cranking with the glow plugs on reduces available power to the starter. Almost sounds ignition switch related. Troubleshoot over replacing parts.


I personally have never heard of this particular fault in these cars. At the same time I do not know everything.

[Diseasel300]

Sounds like an ignition switch problem to me too, or incorrectly wired. The glow plug relay should energize the glow plugs when you turn to the preglow position and SHUT OFF when you energize the starter.

[howardken52]

Originally Posted by Diseasel300

Sounds like an ignition switch problem to me too, or incorrectly wired. The glow plug relay should energize the glow plugs when you turn to the preglow position and SHUT OFF when you energize the starter.

Scratch what I said earlier. The plugs do shut off when the engine is engaged. But they still don't warm up on the first try.

I am VERY impressed with these pencil plugs, though! They're fantastic!