Glow Plug relay diagnosis -- and how the system works -- '94 7.3 Turbo IDI

[QuercusRubra]

Hey guys, I have a 7.3 Turbo IDI, and I'm just looking for some clarification on the glow plug system and whether I need a new relay.

My problem is sometimes the glow plugs aren't running when they should, even though the WTS light is on and power is going to the relay. I can tell because if they do come on, I can see the volt meter in the instrument cluster dive down and the headlights dim -- and sometimes that doesn't happen.

I have a switch in the signal wire to the relay, so I can turn it off and not run the glow plugs if I don't want them -- either I'm monkeying with something, or the vehicle is hot. If the glow plugs aren't working, I can usually cycle my switch and fiddle with things and eventually they come on. Also, if the glow plugs don't appear to run, the engine won't start, if they do appear to run, it starts right up.

So, my question is do I need a new relay?

and I'm wondering how this system really works. I've heard mention of a glow plug relay (looks like a starter relay?) and also a glow plug controller... I guess I don't know what that is... are they the same thing? And then, how does the WTS light work? When my plugs don't light, the WTS light does come on for the expected duration and then goes off. If the signal power switch to off, then the WTS light does not light. So just wondering how that thing works and if there's possibly something else I need before I go buy a new relay.

Thanks in advance, and please let me know if any further clarity or information would help answer my questions.

 

[laserjock]

Check out this thread. Lots of info about your system. The relay is on top of the controller I believe.

http://www.oilburners.net/forums/sh...something-similar&highlight=thermal+gp+switch

Since its intermittent, I would check the connections on everything including your bypass switch.

 

[helidiesel]

You should have at least 10 volts on the ribbon resistor side (thin curvy metal strip) of the relay and 9 volts at each gp lead end (test with connection installed on the gp).

 

[QuercusRubra]

When the glow plugs work, they work, and when they don't, there is no sign of large current draw... and I do hear the relay click when energized. So I suspect the relay internal contacts are just worn out and it's not making the connection -- new relay needed. I just wanted to see what others thought. I don't really understand the controller: the WTS light still lights up for a reasonable amount of time, regardless, apparently, of whether the glow plugs have glowed. That doesn't make sense to me. Maybe it does... the timing circuit is reading change in resistance on the plugs somehow...? so that's independent of whether they get energized. Is the WTS light triggered by that 'other' wire on the low power side of the relay? There are two that come in where my signal comes in.

 

[OLDBULL8]

Your best friend when troubleshooting electrical problems is a DVOM. Digital Volt Ohm Meter.

http://www.oilburners.net/forums/showthread.php?36169-How-To-Manual-Glow-Plug-Switch

 

[laserjock]

It could be your engine harness. Check the big connector for corrosion. The controller and relay could all be working and you aren't getting power to the hot side of the relay (big yellow wires).


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[QuercusRubra]

UPDATE -- Glow Plug Relay

Ok, I'm confident the relay is bad... a little testing and I see that power is going to the signal terminal, and the hot side main terminal, and the relay clicks, but power never goes to the glow plug side main terminals. I replaced the relay and now it all works.

A few curiosities though:

1. I got the "right" part from NAPA and it looks the same, but the metal tabs where it bolts to the controller below it are 90 degrees ccw so when I set it on the studs, the connectors aren't in position... Any one have experience with this? do I have the wrong relay? Any problems with this? I rotated it so it hooks up right and just put in one bolt and it seems ok.

2. There is now a buzzing sound from the area of the relay when the glow plugs run. Pretty sure it didn't do this before... maybe that's a symptom of the glow plugs not working before? or is something wrong? I haven't figured out exactly what it is. It modulates somewhat through the glow cycle

3. Also, it seems the glow plugs don't run for as long now... and the engine does start. Haven't had a chance to test totally cold yet. Maybe that's a sign they weren't working right before either?

 

[OLDBULL8]

When you bought the replacement relay, you had to specify that it's for a glow plug system, or you might of got just an starting relay which won't last long, they have a smaller current carrying contact. That's why the one you got won't mount the same as the original. Without two screws to hold it down, you will get the humming noise. It will work, but not for long, then you'll have the same problem you had before. Cost is about the same for either one. Take it back if you can, and get the right one.

 

[helidiesel]

^^^^ I like the above advice.

 

[mblaney]

Throw that relay away and buy the part number in my signature. Last GP relay you will ever need!