Engine/Transmission (1994 - 1998) 98 12V grid heaters not working

[Texas Diesel]

The grid heaters themselves are good, the wiring is good, the grid heater relays work if I apply +12V to them. It seems there is no +12V coming from the PCM/ECM but the "wait to start" dash indcator works properly. Any ideas?

 

[dalton4]

Check for fuses. For my 96, as per service manual, there are two fuses between ignition switch and heater relays. One in PDC, fuse 3, and fuse 9 in the fuse block.

 

[C Schomer]

I haven't had that problem but there is a temp sensor on the top of the intake manifold cover. It looks like a water temp sensor and it controls the grid function - to what extent, I'm not sure. Maybe a problem with that - check the connector on it. Craig

 

[BV]

I haven't had that problem but there is a temp sensor on the top of the intake manifold cover. It looks like a water temp sensor and it controls the grid function - to what extent, I'm not sure. Maybe a problem with that - check the connector on it. Craig

I think your talking about the AIT (Air Intake Temperature sensor). Yes this tells the PCM to turn the grid on or off based on specific ohm values.



I would start by checking the wiring for dirty/loose connections then maybe pulling the AIT and cleaning it.

 

[Texas Diesel]

Ok, good, but if the wait to start light comes on when cold then the air temp sensor is good? right?

 

[BV]

Ok, good, but if the wait to start light comes on when cold then the air temp sensor is good? right?

I'm not sure but I think so. If that is true then the pcm is telling the light to come on but not triggering the heater relays - Again it could be dirty/loose wiring or worse - A bad PCM.