Engine/Transmission (1994 - 1998) Fuel heater

[Bigtoyfreak]

I'm wondering about the wiring for the fuel heater. I just swapped a 12V into my superduty and am trying to get the heater wired but I'm not really sure what the scoop is. Does this get constant 12V keyed power or is it controlled by the PCM? Any input would be great!

 

[Crowhurst]

The heater is wired via a relay. The relay receives battery power from fuse 7 at the Power Distribution Center (PDC) junction box. The switch wire for the relay coil comes from fuse 11 at the PDC, which receives power when the ignition switch is at start and run. The other coil wire goes to ground, of course, and the switched power goes to the heater element via its thermostat, and then to ground. There is no control by the PCM. My 98 service manual shows this at pages 8W-30-26, 8W-12-6, and 8W-12-2.

Because of the fires that have occurred in this area (including on my truck), and also the number of fuel and air leaks, I, and many others who don't go to the frozen North, prefer to remove the heater. I hope this helps.

Nick.

 

[CumminsPower98]

My heater and thermostat fried but didn't catch fire. One wire in the thermostat was just bouncing around in there for some time causing my lights to flicker. I couldn't figure out what was causing my voltage problems for some months but finally spotted this. I replaced with new parts and it fixed the problem. The new fuel heater is redesigned where the heating element leads are crimped onto the plug terminals rather then a spring loaded friction fit that corrode, heat or whatever and end up frying.

 

[Bigtoyfreak]

Thanks for the info guys. As it happens I live in the frozen north so the heater is required! Both the wires that come out of the plug are black... any idea which one is the ground and which is 12V+?

 

[Mundgyver]

Those two in the loom look to be different sizes to me. My thought would be the thicker one would be the ground. Best way is to unplug and take a voltage reading after you key up and make the relay or trace out and work with a resistance reading back to origin with out power applied.

 

[Bigtoyfreak]

Those two in the loom look to be different sizes to me. My thought would be the thicker one would be the ground. Best way is to unplug and take a voltage reading after you key up and make the relay or trace out and work with a resistance reading back to origin with out power applied.

Both wires are the same gauge and color.

 

[Crowhurst]

The heater is the middle part, and the thermostat is on top. I would want the heater element to be isolated by the thermostat when the thermostat is off, so I'd attach the switched power line from the relay to the wire going to the top of the assembly in your photo. The thermostat should switch on at 40 degrees F. The heater element is rated at 12 volts, 300 watts at zero degrees F. The service manual describes this at pages 14-72 and 14-98.

Nick