Well my flat bed will not start cold...it cranks and kicks...kicks...kicks...dies. So replaced all glow plugs today. However, only one checked out bad. The relay is only months old. replaced it spring time. replaced all with motorcraft glow plugs. test lighted posts on relay...GRP-109 napa relay, had power with key on between both big terminals. and the small wire outback. the other small post didnt have power...Do both smaller posts need power? Someone put in a glow plug switch...no idea what it does. This poor truck has been wired and hacked bad...

 

Hey Kris. Use a volt meter and check the voltage on both big posts. If there is more than 0.3 volt (I think) drop on the load side of the relay, it is bad.

 

One small wire is PCM controlled ground.
Jump both big terminals for 30-45 secs. and try to start.

 

alrighty....so if I jump and nothing happens, bad pcm? so maybe someone used the switch as a ground..?

 

If you jump it and it fires up then the gpr could be bad or like you said they could have the ground on that switch.

 

I just noticed in your first post both big terminals have power So the gpr is turning on. It just may be bad

 

Just in case if jumping the relay as Mike suggests dosen't help, how many miles are on the injectors? My brothers Superduty got so it didn't want to start unless it was plugged in at 50 degrees. The glow plugs and relay were good. The odometer was at around 265k, and it started dumping fuel in the oil. I believe it still had the original injectors. Replaced the injectors and both problems went away.

 

Ok. I was tired...mixed both truck issues into one...my crew cab has the switch for do something with glow plugs. My plow truck is the one that I did glow plugs to yesterday. Only one was bad. So it shouldn't be a glowplug issue. I did a gpr early spring. Truck has 149k. Gonna test gpr later on..if both big posts have power means relay works..but I'll check for voltage drop..

 

The relay could be closing, but burnt contacts inside could be limiting the current. If you jump the two big post with a screw driver or something for a minute, then try to start it and it fires up, then it's probably a bad relay.

 

+1; and this is why, as said, you MEASURE the voltage with a digital voltmeter, one that goes to tenths of a volt at least, on BOTH big terminals while it's energized. Burned internal contacts will almost always show as a major (more than 0.3V as mentioned above) difference in voltage. This is why a test light is useless in diagnosing GP/GPR issues.

 

12.57 on left post....11.05 right post with key on.

 

You need to check BOTH posts with key ON and compare.

 

For once....I'm actually stumped. The crew cab has 12.54 key on at the hot side. 0.21 at other post...it's a brand new relay. The switch someone installed does nothing. Flip it on. Nothing. Only starts with silicone spray. once warm it will start all day. Thinking bad idm...

As far as the other truck I did glow plugs on. I'll have to check it out in the am. When it's cold. Never had any issue until this week.

 

Also, crew cabs tach stays at zero

 

IDM has NOTHING to do with glow plugs, nor with the tach. Does the WTS light come on in the CC truck?