Any reason we could not use the intake heater relay as a GPR? Looking at it, it is almost twice the size of the GPR. I have the heater removed anyway so it's just sitting there useless. It looks like it would carry the current judging from the guage of the wire that used to lead to the heater element.

 

I believe the intake heater relay is a self latching relay and it would stay on and burn up the glow plugs. I will test one that I have to confirm this.

 

Kwikkordead, I have been told you cannot switch them but as long as the intake heater relay is rated as high as or higher than the glow plug relay I don't see why not, unless it is the amount of load the relay itself puts on the controlling circuit. I can't imagine that being the case but just trying to think it through that is all I can come up with.

I know you cannot use a starter relay rated for 200 amps in place of a constant use relay rated at 200 amps (even though they look the same and are wired the same) for the cab power in a school bus we bought for our church many years ago. The starter relay is not designed for hours on end of power running through them. It seems to me the glow plug relay would actually see shorter duration use than the intake heater relay, but then I do not know the parameters for their respective controllers.

**ADDED IN EDIT**

I had not thought of that part of it. Maybe that is why I was told they cannot be interchanged.

Dave / Believer45

 

Never heard of a self latching relay, but I can understand it's theory. I was under the impression that the intake heater was controlled by the PCM which would turn the heater on and off at will according to certain ambient and engine temperature conditions. If that were the case a self latching relay would be a bad thing.
But I could be way off track here. I don't know the design strategy in this case and so am just guessing here. The main reason I bring this whole thing up is simply the thought that a bigger relay would be more able to handle the load of the glow plugs and be less apt to burn out as so many of them do. I hope I am right here.

 

if it were self latching relay how then would it unlatch it self !
if contacts are rated at or above current draw of glow plug the relay imo would work just fine

 

well thinking about it for a little while i guess if it were a current relay instead of a voltage relay then it wouldnt work

on a current relay the current flow throught the coil holds the relay on
once the current flow stops the relay drops out
i guess you could call that a self lacthing relay

 

A self latching relay needs a pulse of power on one coil to flip to the "on" position....then it stays there without any further power being applied. A pulse of power on a second coil will unlatch it and turn it to the "off" position.

A normal relay is "on" as long as power is applied to the coil, then turns "off" when power is removed.

The glow plug relay is a normal relay. I have no idea what kind of relay the fuel heater relay is, but it makes sense to be self-latching since it needs to be on the whole time the truck is on. But the above description should help you understand what the differences are.

Duncan

 

that make sense to me but wouldnt thier be 3 wires then

a gnd and one to activate each coil

 

The old 6.9 glow plugs had a latching relay to provide after glow once the engine started to reduce the white smoke.
The latching relay was timed by a timer circuit that recieved voltage from the alternator once it started to put out voltage.
The actual relay still only had two wires on it though. The latching was controlled by the glow plug controller that was elsewhere in the circuit.
With the right controller circuit you can make a starter solenoid act as a latching relay.

The fuel heater does not need to be on the entire time the truck is running, only till the return fuel lines start to provide enough heat that the fuel does not gell and plug the filter while cold.
On the IDI engines three things are tied to the coolant temp switch that closes till the coolant temp reaches 120 degrees. That is fast idle, timing advance and fuel filter heater. Once that temp is reached all three turn off and stay off.