Engine/Transmission (1998.5 - 2002) Grid Heaters keep cycling

[Richterno1]

It was about 75 degrees out the other day and my grid heaters were cycling the whole day. what do i need to do?

 

[WestTN]

Check your IAT. Intake Air Temperature sensor. Sounds likely it has failed.

 

[Cummins724]

Are you sure that it was the grid? Is it possible that your A/C compressor was cycling or may be low on refridgerant?

 

[rweis]

I would check the IAT first. Measure the voltage and compare it with the temp to see what it is sending the ECM. If it is not reasonably correct, then clean it.



The IAT mearly provides a resistance to the ECM to tell it what the temp is by a resistance table = temperature table.



There is a thread about puting a resistor in parallel with the IAT to change the resistance it reports to the ECM. IE, it is 30* OAT but you do not want the grid heaters cycling, you use a variable resistor to change the resistance so the ECM will think it is 60* OAT when it really is 30* OAT.



There was a thread about removing the grid heater and associated wiring, but the ECM thinks that the grid heater is malfunctioning and throws a code indicating that fact if it can not see the grid heater relays etc.



IF that is your objective (ie you live in a place you do not need the grid heaters) then there is a thread on adding the parallel resistor to the IAT so you can dial in the temperature you want the ECM to see.



Probably changing the IAT is the best bet and just let it do its thing.



Bob Weis

 

[mhenon]

It was about 75 degrees out the other day and my grid heaters were cycling the whole day. what do i need to do?

Until you figure out the problem you could disconnect the relays going to the grids. Just reach down next to the driver side battery and you'll see two relays. Pull one of the wires from each (shouldn't matter which one) relay and your grids will be off line.

Mike

 

[PC12Driver]

The ECM disables the post-start grid cycling above 17mph-ish or after 3 minutes, regardless of the temperature. If they are still cycling above that speed or after the timer is up, then it isn't an IAT sensor problem. I'd start looking at the ECM at that point.



What indications are you getting?

 

[Richterno1]

ac was not turned on, i know it is low on refrigerant but would that make it click even if it was off?

 

[PC12Driver]

The only settings where the compressor doesn't run are floor-only and panel-only (and off, of course). It won't energize in these modes, regardless of the refrigerant level.



The compressor will cycle in all the other modes (both a/cs, both defrosts, and bi-level).



When the grids cycle you'll get a pretty good swing on the voltmeter. I don't think it moves much or at all with the compressor clutch cycling. That should help narrow down the source.

 

[Cummins724]

I was just throwing that idea on the table because a friend of mine had the same thing happen thinking it was the grid, but turned out that he was low on refridgerant & had a bad A/C cycle switch.