Archived Grid heater wiring

[Magruder]

Ive searched the forum with no luck. I have a 99 3500 cummins that has had a wire melt down at the grid heater. It blew the 40 amp fuse that powers the relay. I unhooked the 2 big melted cables from the grid heater to diagnose the problem but dont have any power down to the 2 relays on the inner fender. I have battery power to the big cable of both relays, but nothing at the other 4 small wires with 90 degree boots on them. I need to know where these 4 wires get power. Im assuming that 2 of the wires are hot with the key on and that the other 2 are powered on by the relay in the fuse center which is controlled by a ground from the pcm according to air and oil temp? Does anyone happen to have a wiring diagram for these? Any help would be appriciated! Thanks, Aaron

 

[B.G. Smith]

The wires coming from the PCM are 18 ga. , both are black with tan stripe, these go to the - side of the relay coils. The other wires come from the ECM and are also 18 ga. and go to the + side of the relays, the one going to #1 relay is yellow with black stripe, the one going to #2 relay is orange with a black stripe. You will only have a completed circuit through the coils with the ign switch on and the inlet air temperature below 66 degrees. (the wait to start light will be on). bg

 

[KWentling]

I have the service manual for a '98. 5 and I would think it would be the same. The hot to fire the relays comes from the ECM and is determined by air temperature. The ground circuit is unbroken. There is no fuse. Is the 40 amp fuse that blew feed the fuel heater?



Kim

 

[Magruder]

Thanks much for the info! Yes the 40 amp that is blown feeds the fuel heater. Im guessing this is a separate circuit all together? Will any of the 4 18 gauge wires to the relays have 12 volt through them? Did the relays sticking on cause the heater wires to melt? If so looks like ill be replacing those and possibly the heater. Or atleast the gaskets to them because it actually got hot enough to melt the plastic oil dipstick tube a little. Thanks guys for your help!

 

[tareynolds]

the pcm controls the grounds on these trucks, they ground only, no 12 volt supply to any cicuit from the pcm or ecm. they uses switch drivers that convert readings to a 5 volt signal to tell the pcm or ecm to give the relay ground. this keeps the controllers from melt down. you have a problem from the battery feed circuit to the heaters, check closely along the fueul lines and at the left side battery for any chafeing, or corrosion at these points.

 

[tareynolds]

do you have any problems with your starter at any time? most of the time that I see something like that, it has come back through the starter. there are cures for these problems, even some very good aftermarket buys that cure these things, hope I can help you with this, if you have blown fuses, you have got to figure out why. I will help you all I can, I have the knowlege and the books if you need help.

 

[KWentling]

I believe the PCM does supply the ground on all of the 12-valves (it does on my 98), but is very clearly shown in the 98. 5 factory service manual that the ECM controls the + power and the ground is unbroken. If that is also the case on a 99 you should have 12 volts on a yellow/black wire on one relay and orange/black on the other if the temp is below 59 and above I believe 15 degrees F for 10 seconds after the switch is first turned and the wait to start light is on. Below 15 and above 0, 15 seconds, and below 0, 30 seconds. You should be able to tell if the grid itself is ok by disconnecting the wires to it and checking with a meter for resistance between the hot terminal and the ground. It should be very near zero. If its open its bad. And yes the fuel heater is a whole different circuit. If the grids are bad it is likely the relays have stuck on.



Kim

 

[Magruder]

Thanks for all your help! Ill check the heater itself today with a ohm meter like you suggested. Ill also leave the heater unhooked and hook up the relays to see if I can get them to come on. Last time I checked them I was in my shop so the ambiant air temp was probably warm enough no to switch them on. Ill see if one relay is sticking on. Thanks alot! Aaron Oo.