'77 Relay problem
 

My 77 has the upgraded pencil glow plug and original relay. Recently the glow plug light wouldn't come on unless the starter is engaged. The relay isn't providing juice to the plugs when the key is in "glow" position but if you take the relay out then the car won't start. Any ideas on what is going on here? Also can't hear the relay trying to click in glowing position and I have tried unhooking the temp sensor so the relay would think it is very cold out, no dice.

I would first make SURE the GPs are good. Try a used relay from eBay or the bone yard. You could use a push button switch and forget the relay but you'd need a solenoid for the new 8o amp GPs.

I would not rule out the possibility of a fault in the ignition switch. We know from repeated posts on this board that the switch can fail, causing the starter not to engage. It seems equally possible to me that the switch can fail in such a manner as to fail to energize the relay. I'd try bypassing the ignition switch and providing 12 volts directly to the relay. If it works, the likely source of your problem is the ignition switch. The fact that the light glows in the 'start' position seems to show that the problem is not in the relay itself.

Have you checked the copper fuse, on firewall directly behind the engine (small black box)? Mine failed years ago, replaced it and no problem since, I have the pencil type in my 77 300D.
Ron

Thanks for the replies. It turned out to be fuse #14 was blown. Repalced and life is good. Can't figure out why the relay worked only when cranking but that is for another day