Do both grid heaters turn on at the same time?

I verified only one was getting power.

Checked both relays have ground.
Both relays have power on the battery side.
I can manually activate both relays.

Only thing left to test is the "on" signal from the computer.

Does the computer fire both relays at the same time, or does it do one or two depending on the temperature?

Is it PCM or ECM? Anyone know the pin numbers?


Thanks

 

I believe key on engine off, both are on,

when engine is running they alternate back and forth

 

This should help. http://dodgeram.org/tech/dsl/trouble...niford_htr.htm

 

Thanks, never really outright says both heater actuate at the same time 100% of the time, but the trouble shooting pretty much reads that way.

Best part of the article was to learn that the + trigger is from the fuse and the - is switched by they PCM. I should have guessed that, but I was testing it just the opposite.

Thanks again, I'm going to mess with it a little today.

 

be careful, some yrs use a positive signal and some use a neg signal...

I wired mine for manual control, light comes on in the dash like its susposed to, difference is I hold the switch down. I had my grid relays fail, they stuck on killing the batterys, so I replaced them with ford starter relays, so far its been fool proof, and it works for me.
The other deciding factor for me was mine always used to cycle on every start (yes even a hot engine) anytime it was colder than 10*C and at the time I was using my truck commercially and I felt it was a huge alternator load that was not required on every engine start up. (I went thru 3 alternators over the time I used my truck for courior work)

 

Just fixed it. Found out some weird stuff.

Mine is a positive signal. Fused at #9 on the interior panel.

Here's the weird stuff. The negative trigger wire is constant negative. The positive trigger is positive 12v when the heaters are "on". With key on, engine not running, after the heaters go off, the positve trigger wire becomes open (no + or -). With the engine running the positive switchs to a negative when the heaters are "off". Not an open circuit, they actually switch to a negative.

Not sure exactly what the issue is, but I found one of the + trigger wires to be frayed between the loom and the end. It had been taped by someone. Some how, this wire was not getting power. I got a weird continuity reading back to the fuse, but it was not faulted to ground. So I guess that was from the PCM. I was only getting 2 volts when the PCM was trying to fire the relay.

I ended up jumpering both relays together and just using the one good trigger wire. I backed off the nuts on post for the trigger wire and put a terminal under the nut, tightened down, put the one wire back on top. Shouldn't be any real load on the triggers, all it does is fire the relay; the one wire should carry both of the relays.

I guess this fix should hold out for another 166,000 miles. I'm guessing they installed two seperate sets of trigger wiring to have a redundant system. At least half of the heater would be working.

 

pretty sure its because they alternate when they cycle on/off.

 

The negative seemed to always stay as a true negative.