My Relay does not Relay - a Mystery Solved
 

Thanks to everyone that offered advice.

The mystery was why glow plug wires had 12 volts when not connected
to the glow plugs, but dropped to zero when connected to glow plugs.

After chasing and cleaning all electrical connecting points (ground cable to frame,
ground strap from frame to engine), relay strip fuse , etc...
and removing and testing all glow plugs (nice red hot glow on all).

I took apart the relay. Just to be sure.
It turns out that the relay will send 12 volts through the glow plug wire posts
with key on even if the big copper lugs are not closed (relaying).
I used a screwdriver to close the relay and like magic the volt meter jumped
up to 12 or more.

I cannot explain why the relay is designed to offer 12 low amp volts with key
on but it does, even though the relay is not closed.

I studied the circuit board and have decided a new relay (brand new) is
a better answer than trying to repair this one.
If I were to go after it I would start with replacing all the capacitors, the
diodes and resistors look clean, so does the transistor.

Anyway, what I learned is that 12 volts at the glow plug wires is no
indication that the relay is working.



The original post:
12 volts at glow plug wire when disconnected
http://www.peachparts.com/shopforum/diesel-discussion/268629-12-volts-glow-plug-wire-when-disconnected.html

Relay bypass.
 

So while I wait for the new relay to arrive I built this
relay bypass.

Originally I thought that the existing glow wire post would have enough oomph to trigger my external relay.

Although I am sure it worked while testing, it does not.
So I ran wires into the cab to use a momentary switch that will allow me to
trigger the relay to feed the glow plugs.

Notice that I get my 12 volts for external relay from the fused section of
original relay. Then 5 wires with bullet ends take the 12 volts and are plugged
in to the existing wire harness to feed the glow plugs.
This way I did not have to go to each glow plug and run new wires.

Almost did not seem worth it, but I want to use this car for commuting and
when the new part arrives I will eventually get around to installing it.

This cost around $25 to do.

Well, maybe the new relay that I put in did fix it! I like the mod that you have done. I was going to do the same thing if I could not get it working with the relay in place.

When I took mine apart, it looked like only one side of the relay was making contact when I pushed it down. Are both sides on yours touching when you manually push the contact down? The relay in my car is the same one that you have in your picture, make sure that you don't get one from a newer car, (post 1980 I think) it will not fit the wiring harness from the glow plugs or the ignition.

Cheers,

Reed

I noticed the same exact thing about the two contacts inside the relay.

Pondering this I wondered if it was to allow for usage and wear, like
after the first set of contacts wear out, the second set kick in.

After probing the relay with 12 volts everywhere I figured out that the
winding was bad on the main copper relay.

And I know what you mean about the model of relay.
My 1983 and my former 1981 both had a model that differs from the 80.
I kept the 1981 and harness as a spare, but no good for this car.

This relay by pass works nicely. I may leave it in as a backup.

One last time - relay needs a ground
 

So I purchase a brand new glow plug relay.
Install it and with tremendous confidence go to start car and it
does not start.

On the relay is the small connector that comes from cabin and does not go to glow plugs.

This small connector brings in 12 volts from key-on to tell relay to start.
I am getting the 12 volts on the wire when key is on.

What I am not getting is a ground from the brown wire that goes to the pin immediately below the positive lead.

So I apply 12volts to pin on top right, apply ground to pin on right side
second one down (below 12 volt pin). Big clunk sound as relay kicks in.

Then I get out old relay, apply 12 volts, apply ground - clunk.

Not ready to get in to why the brown lead no longer brings in ground
I simply added a wire to lower pin to ground.
Car starts from key, old relay was fine.

But what I realized was there are certainly a few specific tests we can do
when the relay is suspect.