glow plug problem

[farmerf1]

I have a 1994 Ford F-350 7.3 IDI. A few months ago the glow plugs started giving a problem. The glow plugs checked ok except one I had to replace. I replaced the glow plug relay but it didn't help the problem. When the switch is turned on the relay clicks on and off with out heating the plugs. I also replaced the engine coolant sensor. Is there other controls that tell the relay when to operate and for how long? I could use some help on this. It is hard to find a wiring diagram for the IDI engine. Thanks.

[1957F100]

How long does your Wait to Start light stay on?

Also how did you check the glow plugs?. Last year i started to have a hard starting problem when the truck was cold. Somebody told me to use a test light to check them. I did that and found that two were dead so i replaced just those two. Still had the hard starting. So after talking to a friend of mine about the problem he asked how i checked the plugs. He told me with regards to testing the plugs with a test light it will just show which ones are lighting and which ones arent. Even though the test light shows the plugs are lighting they may not be lighting enough because they are old. Apparently the correct way to test and see how hot the plugs are getting is with a multimeter. But i went ahead and replaced all 8 plugs with some beru ones from ford and my truck was starting right away in the mornings.

The reason i asked about how long your WTS light stays on is because i noticed mine was only coming on for about 4 seconds with the old plugs. after i changed the plugs it would come on for about 10 seconds.

[farmerf1]

the light just comes on for about a second. I check the glow plugs by hooking a battery charger on them. A good one will pull about 25 amps. The relay just clicks off and on when the switch is turned on.

[farmert]

If your wait to start light is only staying on for one second I would say that you have some bad glow plugs. Having just one plug burned out will cause a shorter cycle time on the system on your truck. Tom

[1957F100]

Originally Posted by farmert

If your wait to start light is only staying on for one second I would say that you have some bad glow plugs. Having just one plug burned out will cause a shorter cycle time on the system on your truck. Tom

I second that.

Do you know how old the glow plugs are?

[Dave Sponaugle]

With all the glow plugs working the circuit draws about 192 amps to heat the glow plugs.

The best test is a digital multimeter, test on the ohm scale.
You should have between .5 and 1 ohm resistance between the threads and the wiring connector.

The resistance goes up on old plugs, which means less current draw and less heat.

It is also possible for glow plugs to short to ground when they burn out, which would show a dead short of 0 ohms.
If your plugs shorted to ground, the test light will still light, but the plugs do not heat.

There is no external control device, the glow plug relay/controller senses the resistance change as the glow plugs heat to time the cycle.

The reason using a test light is so common is they are cheap and fast.
A very cheap meter may show a dead short when testing .5 ohms which is almost a dead short.

The meter I use is about 50 bucks, a test light is 5 bucks.
Also you will have to pull the glow plugs to test with a meter.

[farmerf1]

Originally Posted by 1957F100

I second that.

Do you know how old the glow plugs are?

The glow plugs are 3 or more years old however 7 of them pulled 25 amps, which is what a new one pulled, and one was open so I replaced it.

[northern hawk]

Here's the glow plug wiring system on a 87 6.9 , similar to the 7.3 IDI to 94.

http://www.intellidog.com/dieselmann/glow3.jpg

[Dave Sponaugle]

What condition are the connections on the relay like?
Do you have the black ground wire hooked to a good ground with a clean connection?

Look at the connector near the dip stick where the engine harness plugs into the chassis harness.
The two large tan wires at one end of that connector are the poswer supply from the battery to the relay.
Are the connectors in that connector in good condition?

Those same tan wires should hook up to the battery terminal of the starter relay on the fender.
Is that connection in good shape?
What condition is the wire from the starter relay to the battery in?

Any connection between the battery and the glow plugs that has any corrosion in it will alter the voltage supplied to the glow plugs.
The solid state controller will not function propperly without propper voltage, and since the controller uses resistance feed back to time the cycle, any corrosion in the connections will alter the cycle time.

When you say you replaced the relay, did you replace the entire unit with the controller under it, or did you just replace the relay?

[farmerf1]

Thanks for you guys help. I think I found my problem. The owner of the truck before me had the two brn glow plug wires hooked to the wrong side of the big resistor up on the solenoid.

[farmerf1]

1994 f-350 7.3 IDI I am having another problem . By replacing the glow plug relay, new glow plugs and engine temp sensor, the truck now starts real quick when cold, now however after you run it and and shut if off for 5-10 minutes or longer it starts hard, like having to run the starter a couple of times for 5-10 seconds. I replaced the o-rings on the injector bonnets because we thought it might be leaking air back into system and we thought the starter wasn't turning fast enought so we hooked to a charger booster which turned it over faster, but neither one helped. If you just shut it off it will start right back up, it's when it sits a little while that it starts hard. Any suggestions? Thanks