Just got back from a cold elk hunt. Truck was old reliable during the first part. Started right up first time every time. Then I noticed the truck wasn't doing the post heat cycle after start up for the air intake grid heaters. Sure enough, next morning it would'nt start. Had to plug the block heater into a generator for an hour or so to get things going. Put an amp probe on each heater lead during the "wait to start" period as well as a voltage meter. No current draw or voltage to either lead. I'm thinking the IAT sensor might be bad resulting in the ECM not telling the relays to kick in or the relays them selves could be the culprit. I was hoping one of the many informed and smart readers of this forum might be able to shed some light on which part of this circuit tends to be the weak link. Shop manual shows replacement procedure for the IAT sensor but nothing to do with diagnosis. HELP...I want old reliable back!!! Thanks in advance.

 

most of the time it is the relay for the grid heater. mine was on the inner fender, but i hear tell there is another one somewhere. mine stayed on so i just un-hooked them.

 

Thanks....actually, both relays are on the left inner fender under the battery and are sold as a unit. The only other relay is for the fuel heater with lives in the power distribution center. Regards

 

yeah its one piece right. my problem was the relay sticking on and never turning off. killing my batteries. i replaced that but it didnt fix the problem.

 

Is your wait to start light coming on? If so, I would bet the IAT is fine.

The diagnosis portion is covered in the powertrain diagnosis manual, but you can check the following, I only have a 01 manual, but it should be the same.

Yours is switched 12V for the relay, believe 01 was the switch year for this, you have 4 wires controlling the solenoid, 2 Black/Tan wires going to ground and a Yellow/Black wire to heater #1 fired by ECM pin 47 and a Orange/Black wire for heater #2, fired by pin 28 on the ECM. Check for 12V there, if so, replace the solenoid ( During the heater on cycle. )

BTW, Best price I found for the Solenoid pack was from Rock Auto, Standard parts, but mine came with the factory part number on it, was 50ish back then Vs. 75 or so good guy price from the dealership.

 

Great info!!!! Thanks. Yes, the wait to start light does come on.

 

probably here's what you need