been out hunting this week. i noticed at least twice, when the air temp was in the mid 50s and my truck had been sitting for at least 10hrs(and unplugged), my grid heaters didn't kick on(wait to start, idle, or, low speed driving). someone suggested that there might be a fuse link in the wire from the battery to the relays, but both had voltage thru the wire when i checked it with a DVOM. what should i check next? or am i jumping the gun, and its not cold enough for the ecm to start the grids? ISX does the 12v PCM run the grids? didn't you wire a switch for those?

In your case the IAT sensor sense the temp reports it back to the ECM and either uses one or both grid heaters to warm the intake. I would check for error codes to see if the P0380 or P0382 pops up that would tell you if you relays are good. There is a fusable link near the batter running down to the grid heaters. So check and see if the at the relays (studs). Then using a live data tool like a ScanGauge II or simular check and see if the IAT is dropping below 65*F or is showing a invalid temp say like 90*F on a 50*F morning. This points to a failed sensor within limits but will not trip a error code. Tell you the truth you don't need grid heaters till about 40*F...

The operation of them is kinda weird. Mine works by actually controlling the negative (green) wire. The orange and yellow have a constant 12v positive any time the key is on and it energizes the relay coil by switching the negatives on and off. I actually just figured this out a week ago. I thought by tieing the 2 positives for the relays together, it would make both grids turn on all the time, not the case. They still work exactly the same. So the switch actually works like it should, though I really wanted it to turn both grids on every time it cycled, so I need to tie the negatives together. With the engine off, the ECM defaults to grounding the relays to negative. I can put a positive jumper wire onto the relays and they will turn on. But if the key is on, it opens the negative source so I have to put the jumper wire on the negative rather than the positive (since it will be constant positive if the key is on).So, you can do like Mike said and check for codes, or you can just leave the truck off and run a jumper wire from a positive source to one of contacting bolt things that the yellow or orange wire caps go on to. You will hear them click, if you are deaf you can use a volt meter on the battery and obviously you can watch the voltage plummet. Test both of them. Most likely something will not work, otherwise you have ECM issues. Check continuity between the positive terminal of the battery and the 2 terminals of the relays that connect directly to the battery terminal. If they are infinite, the fuses are shot.

Those grid heaters are throwing me for a loop. I am working on an isolated setup and went out and checked everything and here is what I figured out. With the engine off, BOTH wires (green and orange/yellow) are negative. I can put it on resistance and there is no resistance between the green and orange/yellow when the relays are not on, key on or off. When the grids turn on, it makes the GREEN one a positive and that is what energizes the coil. Any time the grids are off, everything is a negative. Sorry for messing up. :banghead:

Check continuity between the positive terminal of the battery and the 2 terminals of the relays that connect directly to the battery terminal. If they are infinite, the fuses are shot.

did that while i was at the camp site(b4 i got home and posted this thread)

So, you can do like Mike said and check for codes

there were no codes when i checked after i got home...

otherwise you have ECM issues.

don't you put that curse on me, Ricky Bobby

Solenoids/relays failed???

So the fuses are good, that just leaves the relays. Just get a jumper wire and stick one end on a positive of the battery or fuse box lead and the other on one of the relay coil terminals, leave the key off. It should make the relay click. If it clicks, check for 12V on the grid heater side of the relay to see if it is actually switching the power. Should be if it clicks but never know.

To test the ECM, just ground one lead to something and put it on 12V dc and leave everything hooked up and just touch the other lead to the coil wires, you can just touch the little 3/8" nut since it isn't covered by the wire cap. Turn the key on and the green should change to positive 12V. If not, do it again but try the other wire, might be reversed on yours or something. If nothing changes to positive...your cursed.

I will make a video on this in a couple hours for you.

--- Update to the previous post...

Here you go

http-~~-//www.youtube.com/watch?v=oM_jlieIgqY

You can also just ground one lead and stick the other on the wire by the grid heater, then you know for sure it is either on or off if it says 12v or not when the relay energizes.