I have a 91 with the 7.3. The colder the temperature is the shorter duration the glow plugs heat. I put a test light to the glow plug output and ran it into the cab so I could time what is happening. on a 60 degree day the glow plugs will heat for 5 or 6 seconds, this morning when it was 7 degrees they would only heat for 2 or 3 seconds. I cycled the key half a dozen times and had the same results each time. After the 6 key cycles I tried to start it and it didn't start. I plugged in the block heater for about 10 minutes and then the glow plugs stayed on for 4 or 5 seconds. After cycling the key twice this time it started.

I tested the glow plugs with a test light and they are all good, however I don't know the resistance on them. I have about 9.5 volts to the glow plugs.

Anyone have a problem like this?

 

sounds like you have a few bad glow plugs.
hopefully they are motorcraft ZD-9 plugs and are not swollen in the head. the more bad plugs you have the faster the controller will cycle off.
8 good plugs under 30 degrees the relay should stay powered for 15 seconds before short cycling.

 

Spark-test the glow plugs instead of the light. Just use a piece of wire to B+, and if you get good sparks when you touch the glow plug, it's good.

I've occasionally seen really 'weak' sparks(compared to the rest) on one plug, and that plug was... partially bad. It still warmed up, but took much longer than the others(internal partial failure) and should be replaced.

Spark-testing will show you the difference very easily, vs a light which isn't pushing much current at all through the plug.

 

I replaced all 8 glow plugs, figured for $100 might as well. They stay on longer now, but the colder it is the quicker they shut off. Yesterday afternoon when I started it for the first time it was 50*. The wts light came on for 8 or 9 seconds, and the glow plugs stayed on for 15 seconds or longer. This morning when the temperature was in the high teens the wts light was only on for a couple seconds and the glow plugs went out after less than 10 seconds. This has me a little baffled. I'm going to replace the wiring harness to the glow plugs later this week and see if that makes a difference.

 

What brand of GPs did you get?

 

Motorcraft ZD-9 plugs. The same that was in it before.

 

check the connection at the glow plugs are tight. a loose connection will add resistance and make the controller think the glow plug is bad.

 

All of the insulators on the plugs are broken and some of the plugs look a little corroded. I ordered a 6.0 glow plug harness to modify, it should be here tomorrow. Hopefully that will take care of it.

 

check connections at relay and at the start solenoid...on the fender...this is the source of power to glows. look for a bad fuseible link

 

I need to ask you where you ordered your wiring harness that you are going to modify. The wiring harness to my glow plugs is very worn out. The mechanic working on my F350 wants to know where you ordered the wiring harness. Thanks.

 

I ordered a harness for a 04-07 with the 6.0 engine from ebay. I was not happy with how it fit, it was stretched too tight between some of the glow plugs. I ended up putting new ends on the original harness. I used electrical barrel connectors that fit tightly over the glow plug. I removed the plastic insulator from the plug and soldered them to the wiring harness, then put heat shrink over them.

 

old idi should stay on for 2 minutes. but if just one plug is not working the system freaks out and clicks on and off rapidly.

 

my plugs never run more than 3 seconds ... and the truck starts fine.... of course I use a manual system as the Controllers have always been problematic.

 

i think you are thinking of the 99 and newer trucks.
they glow for 2 minutes or more.
every IDI i ever owned never ran glow plugs for more than about 20 seconds.

 

nope that 10 second wait to start light is just to let the glow plugs warm up. they are supposed to stay on to help with warm up. why you hear your relay click off after its running. but pre 89 that system is relly dam picky becouse ford tried to be fancy. controller is known to fail easy and just 1 bad plug makes the entire thing stop working like it should. it was actually fixed in 89. i have a block heater so i never bothered to fix mine on the rare times i need to preheat. i live in a area its warm 11 months out of the year. sometimes all year.