Early, early grid heater modification

[cmann250]

I have "early model" grid heaters in my '89, manufactured in 9-88. They work the same, but they are triggered differently and operate at different temperatures than anyone else's toasters. It's a FSM documented condition. The gist is my truck starts better at 14* than 32* and with this mild winter, its a pain.

Here's the wiring diagram



(Third edit might be the charm) I'm thinking I'm going to have to switch S21 and S22. I'm going to have to find out what kind of voltsge those wires put out and duplicate that with a switched wire. I'd essentially have a parallel circuit with a switch in one and normal operation in the other. I'm thinking a 3 posisition switch would be the ticket. I don't have it 100% figured out. Can anyone else make sense of this?

[BobS]

The "thermistor" is in effect a thermal sensing switch, single pole single throw normally open. it's also known on here as your grid heater switch. When the temperature falls below a certain temperature the switch goes into the closed position and it makes contact. By design, the air heater module is a relay. When the thermistor closes and the module gets energized by the grounding action of the thermistor. This in turn energizes the yellow/black lead which puts 12 volts to the grid heater relays. Make sense or as clear as mud?

To manually bypass the system to run in manual mode you simply need to get a single pole single throw MOMENTARY NON-MAINTAINED [starter push button] switch connected across the terminals of the thermistor.

[cmann250]

I think I understand. So if it gets cold, the thermistor closes and completes a circuit? If I were to splice the wires of the thermistor and switch them, I could "fool" the intake heater control unit into thinking it's cold. True? If I left the switch open, all would function as normal. If I closed the switch, I would short circuit the system and turn on the grid heaters. If all that's true, that's exactly what I'm trying to do.

Thank you!

[BobS]

To verify if the thermistor is normally open or closed: Take a multimeter or a volt ohm meter and check the thermal switch to see if it's open or closed at room temperature. To activate the circuit at room temperature you need to do the opposite of what the switch is doing at room temperature.

[cmann250]

I'm thinking about this again.

I tried splicing the thermistor wires and I got manual control of the heaters. A side effect was with the switch open, the original circuit was left open. That resulted in the grid heaters turning on regardless of the temperature and then the WTS light would blink, indicating an open circuit.

I'm back to my original idea. I am going to wire up a "redundant" S21 and S22 that are manually controlled. In doing this, I leave the automated part of the system untouched therefore it should work as before. My manual control circuit will be an undetectable redundant piggyback, more or less. This will allow me to use the grid heaters at 30* and they should still work as the factory intended below 15*.

I feel that I am doing my truck a favor since it sounds like the engine is trying to jump out from under the hood when it's running on 3 cylinders at 30*. I find that with this early design of grid heater control, plugging the truck in for anything less than over night is a waste of time. With it being plugged in for 2 or 3 hours in the morning, the coolant is warmed up and the heat comes on quick, but the truck starts hard. This manual system will let me plug it in if I remember to, yet still start up easily with the grid heaters.

I appreciate the help of those like BobS, but I cannot make sense of electricity from someone else's explanation. I just have to figure it out myself, usually the hard way.

[Tacoclaw]

Looks to me like you could simply splice into S20 between the thermistor and the control box, then run your wire to a switch in the cab. Connect the outlet of the switch to ground, then break or connect the circuit with it depending on how it works. That'll leave your thermistor to operate normally as well.

[cmann250]

I think that's what I tried. Many other variables could have made it not work, but I'm going to try feeding the solenoids first.

[DodgeFreak]

you could tee into the thermistor wires to by pass the thermistor and it should function as normal. and then when you push the switch it will close the circuit around the thermistor, then when you release the switch go back through the thermistor