(Have briefly checked the archives and find little discussion on the early type glow system and its diagnosis; please pardon if this has already been discussed):
Last night I tried starting my 75 300d after it had set all weekend, and the glow plug indicator on the dash lit only briefly, then extinguished and never came on again after susequent attempts to start the car.
It does not feel like the plugs or series connectors are heating when the ignition is turned on; this car is too early to have the 80amp relay, right? I did not see one anywhere under the hood. Is there another relay or fuse I can check?
I tried to check voltage at each of the plugs with the ignition switched on, and did not get readings. However, it was awfully dark out, so I'd like to try again (in better light) to confirm the lack of power there.
Can anyone explain how the thermal switch works? The only thing I tried differently on this starting attempt was to run the block heater for about an hour and a half before trying to start the car. That may explain why the glow plug dash indicator lit so briefly initially, but not why it wouldn't light this morning. Maybe the thermal switch stuck open somehow? Would this prevent voltage from being present even at the "in" line back at the firewall glow plug?
Thanks in advance for any help!