My truck is a 96 F250

Like a lot of people I'm going through the cold start problems. I tested the glow plug relay the other day and it wasn't sending power the glow plugs at all. It had power to the hot side but no power to any other of the terminals. I replaced it and it started fine but it wasn't cold so this doesn't count. I drove it for about 30 minutes and put a test lamp on the relay and it showed that it was sending power to the glow plugs. My understanding is that when the engine coolent is below 165 degrees, the glow plug control module energized the glow plug relay when the key is in the wait to start position which sends power to the glow plugs enabling the truck to start when it's cold outside. I think that the glow plug relay continues to send power to the plugs for about two minutes. So here's my question. If I put a new GPR in and turn the key to the wait to start position and the GPR isn't energized and sending power to the glow plugs, does this mean that the glow plug control module is bad? If it is, where is it located and is there a way to test it? Any ideas why I would see power to the glow plugs 30 minutes after starting? I pretty sure this has toasted my GP's if it has really sent power to them for that long.

 

check the smaller wire going to the relay. that is the control wire that turns the relay on/off. You should see the small wire energize, likewise power out of the relay for a minute or so on a cool morning. I don't think it would come back on after engine is warm. If you suspect the relay is still on, look at the small wire again. If you have power out of the relay, but no power to the small wire, I would suspect the relay has welded the contacts on. Check with an ohm meter to both big terminals with the key off. Should read open, or very high ohms. If you read low ohms or a direct short, disconnect the wires and check the relay itself. Be careful, the hot wire will remain hot unless you disconnect the battery!!!. If you find the small wire is turning on the relay after the engine is warm, then probably a bad control unit, although that is unfamiliar territory to me.

 

Are you sure your measuring voltage on the glow plug lug?

Here's what I know, hope it helps. I have a 1999 and the relay has four lugs, 2 small and 2 large. One at 3, 6, 9 and 12 oclock. The one at 9 oclock is battery and is always hot. The one at 3 is the glow plug lead is only hot when glow plug heating is called for. Any time the key is on there is 12 volts on the 6 oclock lug. I don't know all the terminology but, when the controler is calling for the plugs to heat, there will be a ground on the 12 oclock lug, when they're hot it goes open and the relay should open too. lariatbob may be right about the relay being welded closed but I think a short to ground on the 12 oclock lug could cause this too.

 

Here is how it works. I just replaced mine tonight due to a corroded terminal that ate through the brown plastic. If you are at the front of the truck the 2 terminals North and "South" (the smaller 2 terminals) are the coil of the relay. One has constant power and the other is waiting for the Ignition signal. The larger post to the "West" is the constant 12v+ (Or Common terminal on the relay). When the ignition is turned on the coil fires and passes 12v+ through the relay to the "east" terminal. Hence where the glow plug hot lead is for both banks. To test the relay make sure you have 12V+ to the "west" larger terminal and when the inginiton is turned on and glow "wait to start" is on the voltage (12v+) is at the "east" terminal. Meter from the lead to ground on the crank battery. Hope this helps bud.