Glow plug relay testing.
 

I thought I saved the info on my computer on how to test the glow plugs and relays of all the Ford Diesel models, but can't find it now.

Anyone know the link to the info?

I have a 1991 F250 and now that it's getting colder the wait to start light comes on and goes right out with no clicking and a smoke plume with a hard start.
I need to fix it before it gets too cold as I have the block heater to get me going in the morning, but when I try to start it after a full 10 hours at work, I will be stuck.

To test the glow plugs:

Disconnect the gow plugs from the wiring harness.

Take a 12V test light and connect the alligator clip to the 12V side of the battery, then touch the prope to the glow plug prong. If the light comes on, the glow plug is good. If not, its probably dead.

Your model year of truck has a solid state controller, so if more than 2 glow plugs are dead, the controller will not come on long enough to properly heat up the remaining glow plugs. The idea being to prevent burning out the rest of the glow plugs.

Chances are you have a couple of dead glow plugs in there and thats what I would check first.

I'll get out and test them this Sunday, and I have a full set that tested like new from the wrecking yard, but what about testing the relay?

I used to have a full set of factory manuals for all the F and E series trucks from 87 up to the 92 model, but they got tossed a few moves ago.

Only way I know is to connect everything up and see if it works properly. I don't know of any way to bench test a glow plug controller.

Well, the controller is cycling, it's just cycling way to quickly and not letting the glow plugs warm up enough.

I did get it to start without the block heater, I just had to cycle it 10 or so times real quick and it just barely caught and ran rougher than it ever did before on a cold winter start up, and it was not cold enough outside to freeze water yet.

Last winter it would start right up after the wait to start light went off.

As far as Glow plugs, you can just look at it and see if it looks OK (not having a swollen or missing tip) and test the resistance comparing it to a new one.

I have a set of extras.

I need to know how the whole system works.

Because it cycles, and the glow plugs test OK, I doubt there is a grounding, power, or a main wiring issue.

What I don't know is what is getting me stuck.

I did see that at the end of the wiring harness by the passenger side front glow plug there is an electrical connector hooked up to a small black block (about one inch square) that just hangs there.

What is it?

Is it a temp sensor that tells the relay how cold it is out, and could that sender, or it's wiring be the issue?

I don't know.

I do not want to, or can afford to spend money to replace it and find I did not fix a thing.

If you look in Dave Sponaugle's tech gallery, you might be able to find the connector you are decribing along with an explanation. Otherwise you could try and post a pic of the plug so we can get a better idea of what that connector is.

However, I believe there is a 2 tang temperature switch that threads into the front of the passenger side head that provides temp signal for the high idle/cold advance. Its kida hard to see and you have to look down the back of the goose neck for the thermostat housing to see it. I'm not completely familiar with the 87 and up controller wiring, but since its not threaded directly into the water jacket like the older ones, its possible that this is used as the temp signal instead.

This might help:

http://www.intellidog.com/dieselmann/glow3.jpg
This covers 1987 up to 1994 IDIs
Looks like there is an outside temperature switch (closed when cold, open when hot), so that loose connector could be your problem.

Any chance you could post a picture in your gallery?

The 91 glow plug system control is all in the relay/controller box.

There are no other sensors tied into the glow plug system.
The only temp sensor in that drawing is for the fast idle/timing advance switch behind the thermostat housing, nothing to do with the glow plug system.

The only place I can think of that could be causing problems is the ground connection for the black wire on the controller or the engine to chassis harness connection near the dip stick.

The ground wire must have a good ground.
If there is any corrosion in the engine to chassis connector, the glow plug circuit is not recieving full power when the glow plugs are on and they will heat slower.

Your glow plug supply wires should be either yellow or tan, check all the connections between the relay and the battery end of the supply wires to make sure they are getting full power.

If you have a volt meter, how much does it drop when the glow plug are heating?
Mine shows over 2 volts drop with 1000 CCA batteries.

If everything above checks out OK, you can get a relay for the old style system before 86 and swap the relay on your controller.
If the controller is cycling, the relay has good power, and the glow plugs are all good and showing between .5 and 1 ohm resistance, about all that is left is the relay.

I am running a 93 engine harness on mine, and it does have a connector in the area you describe with 4 wires in it if I remember right.
Flat plug about 3/4" square.
I assumed it was for the FILP sensor on an auto tranny truck, it hooks to nothing on my manual tranny truck.

if it only cycles a few seconds, no matter what the temp then you probably have a bad glow plug. i know this because i had this happen, i ordered new gps and installed them last week, doesnt cycle for only a few seconds like it did before.

Nope.
Our digi-cam is crap


The system pulls the volt gage and cycles the same as if it's working correct.

It's just that it works like it's 70 degrees out no matter how cold it get's.


If the temp control for the glow plug circuit is internal to the relay, then it has to be the realy.

It is not the F.I.P.L. connector, or anything to do with the fuel filter as it's all connected up, but on the front passenger side of the intake manifold right by the fuel filter and lines, and it does plug into something.

It's square, black and looks to be about one inch on all sides.

It is not mounted to anything, but just hangs off the end of the harness that runs along the fuel lines, just sitting there on top of the intake.

Now I just have to talk my job into letting me plug the block heater in an hour before I get off work until I get it fixed.

They all tested good.

I turn the ignition on, the light come on then goes right out, then a couple seconds later, I hear the relay clicking and cycling like normal for warm temp start up's, but it's almost freezing out.

I can start it, but the key must be cycled 10 or more times quickly and then it barely catches and smokes like crazy.

thats odd, mine did almost the exact same thing. only it was my gps that were bad. have you cleaned the connections on the gp controller/relay?

It just acts like it does not know it's cold out.

I went through the glow plugs last summer when I got the truck and they test the same now as then.

I should have snatched an extra relay or two when I got the extra Glow plugs from the wrecking yard.

I've been told that some of these solid state GP systems are notorious for their contacts getting corroded, just enough for the controller to sense the wrong resistance. That will cause the short WTS light time and cycling of the GP's. It's the very same symptoms as having one or more bad GP's.

Check your connections on the controller/relay and make sure they are clean.

What kind of glow plugs are you running?

When you tested the glow plugs, how did you test them?

A test light only tests if the element is open as if it were burnt out.
A resistance check with a decent ohm meter can show weak plugs that have high resistance, or plugs that are shorted internally when they burnt out.

The resistance does not feed through the relay, that is what the small AWG yellow and green wires on the controler/relay are picking up and sending to the controller.

https://www.ford-trucks.com/user_gal...id=189823&.jpg

If I am reading you right, the wait to start light is coming on for 10 seconds, and the truck is not starting well when the light goes out.

Something is telling me that the glow plugs you are running are not heating up in 10 seconds.

So either the glow plugs you are running, the power supply to the glow plug relay is supplying low amperage, or the relay mounted on the controller has contacts that are not carrying full current to the glow plugs is where the problem lays.

The only thing that times how long the glow plugs stay on is the resistance change in the glow plug circuit.
Has nothing to do with temperature in any way.

When you turn the key ON, as soon as the wait to start light goes out, the engine should start.
If you set there with the key ON, when the relay starts clicking again it means the glow plugs have cooled off enough that the controller heats them up again.
But since they have cooled off, the engine will not start as easy as it would if you started it as soon as the light went out.

As an afterthought, is the bimetal strip on the relay damaged in any way?
That strip is also where some of the resistace change is measured on a normal system.
You may want to check the small yellow and green wire terminals for loose or corroded connections.

I am helping a friend with 1990 F-250 he bought a month or so ago.

A test of at the glow plugs shows 6V (+/-).

Is this normal ?

The Truck PO had "Frankenstiened" the electricial system -the two Yellow cables (12V) go to a switch inside the cab, then back out to a terminal at the top of the GP controller.

Did Ford make 6V glow plugs for this year. Or will a few dead GPs cause a 6v drop?

Thanks