I assume my 1975 300D is one of those odd "in between" years for glow systems. I have the old series type plugs with a key start system (i.e. not the pull knob glow/start system), but I do not have the 80 amp fuse in the engine compartment. With a little help from some forum members, I traced the glow plug power source back through the firewall to a caged relay (a little square box with a couple of springs in it, each with what looks like a thermal switch piece of metal under it). This device does not have any obvious faults, but I am not getting power to the glow plugs when I switch the key to the glow position.

The caged relay has four wires exiting to the engine bay, and two coming in from the dash area. Can I assume that the two coming in should carry 12 volts across them when the key is in the glow position? If so, I think I may have a faulty igition switch, as I cannot get voltage to measure at the relay at all.

Any advice would be much appreciated; as it stands, I have to jump the battery to the plugs to start the car (which works fine every time).

Thanks!

Chris
cscmc1@eiu.edu

Withot the wiring diagram it is difficult to diagnose your problem. so I have to go by what I think should be happeneing. When you turn the key to the glow position, 12V should appear at one of the 2 wires coming from the key switch. The other terminal may be a ground. If one of the wires is brown that should be the ground. You can use a light bulb tester. Connect one end to 12V and the other end to either of the 2 leads. The brightest connection will be ground. Then connect the tester to ground and the non ground terminal. When key is turned, the bulb should light. If the light doesn't light, the key switch is probably bad, but it could be a loose wire someplacer a bad ground connection

Do this test and tell me the results.

P E H

Thanks for the suggestions. I realize now that I may have been shooting myself in the foot when I checked for voltage at the relay. I measured for voltage across the two wires coming from the ignition switch, and got nothing, but maybe the wires are BOTH hot 12v? Perhaps I need to get back in there and see whether either provide 12v when the hot lead is attached to the wire and the other lead is grounded to the chassis. I can't believe this didn't occure to me when I was testing in the first place. Go figure.

I am finally getting over a cold, and will get out as soon as possible to check again, then post a follow-up. Thanks for the help!

Chris

Chris - I found the large "main" contacts in my relay built up with severe corrosion. I cleaned it up with emery paper and CRC contact cleaner and will see if it fixed it this weekend. My is the later style though.

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Jim

Thanks, engatwork! This may be a good time to ask what your relay looks like. Mine is difficult to describe... it's in an unpainted metal "cage", about 1.5" square I suppose, and has two wires coming in from the dash to two posts unde rthe cage, which in turn feed two long metal strips that appear to be thermal type swicthes. These have contacts at one end that look like the "points" in distributor-type gasoline engines, and yes, the strips themselves looked corroded.

On the "other side" (the one w/o the cage), there are four lines "out" to the engine bay. This side is the side that is exposed when the relay is mounted to the firewall: the cage side is on the cabin side of the first firewall (access is actually gained by removing the grille beneath the windshield), and the other side (the one with 4 wires out to engine bay) sticks through a hole in the firewall.

Whew... hope that makes some sense... Sorry if it just confuses you! Anyway, does this setup sound similar to yours? It is difficult to find any illustrations and specific troubleshooting techniques for my system as it seems to be an "interim" system used for just a few years (???).

Thanks again,

Chris

The later style like mine is totally different. It is a black plastic covered circuit board a little larger than a pack of cigarettes with plug ins and a couple screw on connectors. There are four phillips head screws that I have to remove to open mine up.
Take a close look at the "points" - you may want to clean them just like you used to do points. Can you open yours up without destroying it? If you can - open, clean and inspect it. Clean it good with electrical contact cleaner.

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Jim