Issues with my 95 7.3 glow plug circuit - Diesel Forum

DAM1 · 10-28-2009, 01:16 AM

Trouble shooting indicated no voltage when glow plug relay activated on large terminal out to glow plugs. Relay was indeed activated. Replaced relay (aftermarket blue streak brand). New relay activating but apears still no volts out terminal to glow plugs and humming noise from relay. Supply voltage low? or is it possible the ground circuit in the controller defective? Need info on control circuit voltage. Is it possible to eliminate or bypass controler and manualy activate glow plugs for start only purposes. One other weird note, the control terminal and the gound side on the relay appear to have voltage when ignition is off. B.C. temperature 0 to - 10C already. Any help greatly appreciated.

Evel486 · 10-28-2009, 01:36 AM

Activation circuit is 12v key on. The PCM completes the ground. The supply voltage terminal is battery + constant.
You should have battery voltage on one large terminal all the time. One small terminal should have 12v key on only. The other small terminal is the ground to the PCM. The wait to start light is NOT an indication of glowplug activation.

**pjwoolw** · 10-28-2009, 01:44 AM

That is a glow plug relay and not a starter relay? The GPRs are isolated from ground. You probably know this already. There is no controller other than the PCM. Seems they all buzz a bit when new. Mine does. It should go away after awhile. If its making noise I suspect its working. So no 12 volts on the out post of the relay when key on engine cold? Know the part number of the relay you used? Just curious.

**Believer45** · 10-28-2009, 10:05 AM

Below is a cut and paste of a reply I made to a question about glow plug relay operation. It is specific to 94-97 trucks but the principles should be the same on newer units.

Quote:

Originally Posted by Believer45

To do a quick test of your glow plug system locate the glow plug relay under the engine cover (plastic hood looking thing on top of the fuel filter, draw a line from center of filter to right headlight and the relay is about 6” from the filter housing under that line). When the truck is having a hard time starting use a jumper cable and jump between the two large terminals for 90 seconds or so. This bypasses the relay - the glowplugs draw a bit of current so be ready for a spark or two. Disconnect the jumper and try to start the truck. If it starts fine then your problem is either the relay or what controls it.

To diagnose the relay further you will need a digital voltmeter. With key off you should have battery voltage to one large terminal and nothing to the other large or either of the small terminals.
With key on for the first two minutes or so you should have battery voltage to both large and one small terminal. The other small terminal should show ground. This shows the relay active.
With the key on and after the glow plug cycle has finished you should have battery voltage to one large and both small terminals. The computer activates the relay by grounding the second small terminal thus activating the relay.

It is possible to have battery voltage to both large terminals when the relay is active and still have a bad relay. If the contacts inside are pitted or corroded you may have enough voltage passing through it to light a test light but not enough to actually run the glow plugs. Generally you will lose voltage across the relay if this has happened. This is why you need a meter (preferably digital so you can test the glow plugs if you need to) rather than a test light to check the relay. With the glow plugs on (first minute or so after turning the key on in cooler weather) and the engine not running check the voltage at both of the large posts on the relay. There should be no more than .3 volts difference (less is better). You also should not have significant voltage loss from the battery + terminal to the always hot terminal on the right inner fender mounted starter relay, and again at the always hot glow plug relay terminal. Check this voltage loss with the key on, glow plug relay active. You will have to cycle the key a couple times to do all the testing unless you can get to all three terminals in @ 90 seconds or so.

If you determine the glow plug relay is operating as it should, check the information on the glow plug system.

Dave / Believer45

The above may give you some help testing. I have a copy of an excellent article on the glow plug system by Carpentractor, a "seasoned" member here who has been out of pocket for some time. I can send it to you in MS Word format if you wish. Shoot me an email at Believer45@gmail.com with GLOW PLUG RELAY INFORMATION in the subject line, let me know what email address to use and where you saw this and I will reply with two attachments (Carpentractors article and a copy of the above quote).

Hope it gets you going.

Dave / Believer45

DAM1 · 10-28-2009, 11:18 PM

-5 c this morning activated key on waited approx 30 +/- secs and engine fired right up. I did not confirm any voltages this morning, I am going to see what happens over next few days. Dissassembled the old glow plug relay and noted the contact plate was extremly arced (burnt ) current was unable to pass through the damaged terminal. Thank you for the responses

00y01e · 01-12-2010, 08:58 PM

Believer, Thanks for the cut and paste, it will definitely help in troubleshooting.
I have already determined that the relay is not activating on my 96 F250, but didn't know what the voltages on the small terminals should be.
What are my next options when I figure out what the voltages and grounds to them are ?

**Believer45** · 01-12-2010, 11:17 PM

May be a silly question but is the relay you are working with on top of the engine next to the fuel filter housing or on the inside of the right fender behind the battery?

The starter solenoid (relay) (on the fender)

looks similar to the glow plug relay (top of the engine).

Plastic engine top cover is gone, has been for 8 years or so. Pardon the dirt, this is a service truck and I seldom have time to clean her.

00y01e · 01-13-2010, 09:02 PM

Thanks for the reply, it is the one next to the fuel filter.
What I have found so far is that the key on 12v seems to be there but may be intermittent. This said, I have been trying to determine what is required to check for the ground on the other terminal when and if it is provided by the system. Of course all of this is dependent on the fact that sometimes it works and sometimes not.

I don't get to my email except in the evening as I live in the remote desert and only have satellite internet when my generator is running.