Glow Plug Controller Question?

[Big Red Suburban]

How does the glow plug controller determine length of glow time? I thought it determined the glow time based solely on coolant temp. However, it was a little cool this morning (38F) so I let it cycle like normal (7-8 secs) then turned the key off and back on to cycle it again. But, it only flashed on for a 1/2 sec? This can't be normal?

I think my glow plug controller is going bad and is in need of replacement.

[diesel65]

PCM controls the glow plug duration, Your relay does not have a timer board like the early 6.5's and 6.2's.

[Big Red Suburban]

I understand the PCM controls the glow time, but based on what? I thought it was based upon coolant temp which is read by the pcm?

If not, how did the PCM know on the second turn of the key to the "ON" position that the glow plugs were just warmed up? It should have given the same glow time as the first (which was 7 secs), since the coolant sensor would be reading the same? But it just gives a quick 1/2 sec glow?

Thanks...

[kowsoc]

Sounds logical but I guess the 2nd cycle is recognized by the computer and it reduces time to not overheat the glow plugs. I use a second cycle for cold starting but I leave the key off for about 10 seconds between cycles and the second cycle will still only be a few seconds...but it helps mine start smoother.

If you unplug the coolant sensor, maximum glow time will be had so I guess a switch could be wired in to open the sensing wire when longer time is desired. Some people wire a switch to the glow plug relay to allow manual control of the plugs. Guys with the 18:1 compression engines need more glow time so they would know all the tricks. ;)

[HowieE]

The coolant sensor only limits when the glow plug controler CAN come on not how long it will remain on. I think the sensor opens when the block temperature is above 140 degrees thus once the truck is warmed up additional starts will not get a glow plug cycle since the engine temperature is hot enough to cause ignition without preheating.

[FarmerDale]

Then what do I have to replace to get mine to work properly? Mine truck glows for about 7 seconds each time I start, even if it has been shut down for only a minute. I now just wait long enough for the fuel solenoid to click on, then crank, knowing that the engine is warm enough to start.

New quick-heats from JK go in within a week. Do I look at the coolant sensor, or would that not be a concern when TCC lockup is normal (around 160* coolant temp)?

[HowieE]

If your glow plugs cycle every time you start, even when the truck is a full operating tempersture I would check the coolant sensor when the truck is hot. It should be open then but what you are saying I would expect it to be closed. If closed at high temperature replace it.

As far as I know the TCC lockup has nothing to do with engine temperature. I run my trans at 150 in the summer and as low as 105 in the dead of winter. Yes when I first start in winter there is about a 5 min. delay before the TCC will lock but I think that is an trans oil viscosity question.

[Big Red Suburban]

I went ahead and installed a manual glow switch (using info found here in the forums ;) ). If it acts up again I can atleast use my manual button to get the proper glow time. If it becomes a problem I will replace it.

I installed a diode to prevent the 12V from going back to the PCM when it doesn't expect it there, however, the glow plug light in the dash comes on when my manual button is pressed? I didn't expect that, but it is a plus smile.gif

Since I was in a wiring type of mood, I also installed a relay for the OPS and a manual button to be used for troubleshooting/priming purposes if necessary (the fuel pump I put in is much louder than the previous, however works ok).

[kowsoc]

The coolant sensor is not a "switch" with contacts but rather a variable resistance that varies with temperature. The PCM uses this info to vary the glow time, idle speed and injection timing according to coolant temperature. A -40 degree start will create maximum glow time (and a few short afterglow cycles), fast idle, and 5 degrees advanced injection timing. It is normal to get a glow cycle (although shorter time) when the engine is hot. It is not usually neccesary to wait for the plugs to cycle during a hot start. I usually ignore them and crank immediately on a warm start....but waiting for them will probably guarantee a quick, smoke free start. ;)

[moondoggie]

Good Day!