Out of the blue I'm not getting my 'wait to start' light illuminating and no cycling of the heaters pre or post start. Where do I begin this adventure?
Thanks

 

Swap out the relay with one of the others from something that's working. It's almost always the relay.

If that turns out to be the problem, spend the extra money and get high quality Bosch relays instead of the cheap Taiwanese wondercrap.

 

I was looking at the sticky link that had a couple of the FSM pages on this scanned. If the relay was bad shouldn't I still get the W2S light, or is it wired through the relay as well? (Wondering if I happen to have a bad bulb at the same time as the actual problem).
What else is on a relay that I can 'live without'? As far as new ones, I've bought relays to power my fan, fuel pump, and efi on my 70 Fury, but those are definitely not up to the power these guys draw, anyone got a good source for the right relay?

 

Remove the electrical connector from the thermistor. Turn the ignition key ON position, but do not start the engine. The wait to start should come on and the heater solenoids should click on. The WAIT to START light should stay on for 20 seconds. After 20 seconds the WAIT to START light should go off and the heater solenoids should click off. After another 20 seconds the WAIT to START light should start flashing. This is an indication that the thermistor circuit is open (because it is unplugged). If this does not occur you most likely have a defective module. Module is located on firewall below master cylinder.

 

module=computer, or is there a separate module for the heaters in addition to the engine controller? (FSM reads as only 1) At present I have no W2S light nor are the solenoids actually clicking on or off prestart, and post start they are not cycling.
If this is failing inside the controller does that also mean the KSB solenoid will be dead, and if so what does that mean?

 

Since you have no WTS light I suspect the PCM is either dead or is not getting power or ground. It doesn't really do much besides the grids on a '91.5 stick truck.

You should at least be getting a momentary WTS light as a bulb check when you turn on the ignition.

 

This lamp is illuminated by the electronic controller based on the input from the charge air temperature sensor. This light is illuminated when the ignition switch is turned to the ON position. It remains on for two seconds as a bulb test.

If the electronic controller receives a signal from the charge air temperature sensor indicating that the intake air temperature is at or below 59°F (15°C), and if the electronic controller has not received an engine crank signal, then the electronic controller will illuminate the Wait-To-Start lamp. If this light is illuminated, it indicates that the intake air heaters have been ENERGIZED and the preheat cycle has begun. This light remains illuminated until; the preheat cycle is finished, or the electronic controller receives an engine crank signal.

The Wait-To-Start lamp will flash ON and OFF if the charge air temperature sensor input to the electronic controller is above maximum value or below minimum value. If either of the previous conditions occur, a fault code will be stored in the electronic controllers memory.

 

Unplugged the sensor and no change in behavior. WTS light does not come on for any reason at anytime, not even the bulb check. Does the PCM do anything else on a 91.5 stick such that I can verify whether it's completely dead? Anyone got a wiring diagram for the pin outs on it?

 

hope this helps

 

These may be bigger so you can see the connector itself

 

Just for giggles, did you check to see if the bulb burnt out?

 

The bulb could be burned out, but that wouldn't change the fact that the grid heaters are not turning on. I could care less about the light, the click of the solenoids would tell me the heaters were working. No post start heating going on either, as evidenced by the voltmeter and the shaking, smoking truck.

 

Yep, that I can agree with, but before you suspect the PCM, I would check the bulb.

To make sure your Solenoids are good, use a VOM to see which of the small trigger wires are 12V and ground out the other one. If she clicks, then you can work your way back to the PCM.

After that, I will let the First genners take over, because I only slept at the Holiday Inn last night.

 

I can take the dash apart, but if I understand everything here, the only way the bulb would affect this is if the trigger wire for the relay is fed through the light bulb circuit and is after the bulb. Possible I suppose, but seems odd.
Anyone got the page of the FSM with that circuit?

 

You could have the bulb in your pocket and the grids would still work. The bulb has nothing to do with the function of them. You would hear the click and see the amp draw on your gauge. I see in your first post you do not see that happen. I would activate the grids with a switch if it was me. Much easier then messing with sensors and gremlins.