The engine pre-heat grid circuit is not working properly on my son's 1992 D350 diesel. The circuit will work at temps around 40 degrees, but takes longer to activate as the outside temp gets colder until at around 20 degrees it ceases to function. After starting the engine w/o the cold start working the circuit will start cycling on and off after the engine begins warming up. This has been a progressive problem. Last winter the circuit would work down to below zero, but it took several minutes for the circuit to engage at those cold temps. Does anyone have an idea on how to isolate this problem? He's already taken it into a Chrysler dealership and they couldn't find anything wrong.

 

sounds like the relays need to be changed out. I would also check on the condition of the battery, have it load tested if it is getting old. the heater grid pulls a pant load of amps at 12 volts.

I cannot give you the exact location of the relays that control the heater grid, but on my truck it sounds like they are somewhere on the passengers side of the truck.

 

We replaced one of the relays already and tested the others by swapping with the new one [they are all the same], which didn't help. The battery seems strong since it starts the truck during cold temps w/o the cold start. The malfunction seems to be influenced by ambient temperature. There are two modules located on the top of the grid. Are these temperature sensors? Also, would a worn ignition switch cause a problem like this?

 

I see my son Jackson is logged on, so I will let him carry the ball from here. Thanks for your reply.

 

since the relays didnt work I would look at the IAT sensor. (Intake Air Temp) that can be found just after the air filter assembly, but before the turbo.

if the computer senses a temp of 50F or below it will kick the heater grid on.