Testing Glow Plug Relay off the truck - Diesel Forum

BroncoGlenn · 03-03-2010, 02:43 PM

I'm curious if it is possible to effectively test the GPR when it is off the truck. I failed to notice the whole part about how the GPR doesn't actually trigger unless the orange wire post opposite the red wire trigger post is grounded. So, having already thrown the new GPR on and wondered why it wasn't working, I dug around here until I found that handy bit of information. I just thought it was like a regular old relay, power the trigger and a supply and output posts bridge. Which explained why my brand new one didn't appear working until I grounded out the orange wire post and was greeted with a loud click and 12 volts on the meter. This also tells me I didn't properly test my old relay and perhaps it is still good.

So, that having been said: If I have a relay not installed and I use a 12 volt power supply (a 12V battery charger) and connect the positive line to the 12v trigger and supply posts of the relay and the negative clamp to the (orange wire) post opposite of the trigger of the relay, I should should get a 0 ohm bridge between the two large posts with 12 volts, correct? My old relay just sits there and buzzes faintly with infinite resistance across the posts and no juice.

Granted I could put the old one back in and test it properly, but I don't want to spend the time, and I managed to swap them once without dropping anything into the valley of the lost and don't want to press my luck.

Also, how cold does it need to be for the glow plugs to come into play? As I recall it is based off of oil tempature (HP res or pan?). Ambient temp here is 60 degrees and the truck has been off for a couple hours now and is still a little warm from running around this morning. Of course the glow plug light on the dash comes on, but apparently this light doesn't really mean the glow plugs are being powered.

**klhansen** · 03-03-2010, 03:35 PM

You can get a GPR to pull in (assuming the coil isn't burnt out) by applying 12V across the two smaller terminals (polarity doesn't matter). But that doesn't mean the contacts aren't burnt and cause a high resistance connection (or none) across the larger terminals (again not polarity sensitive). I've also had one that was sticking closed. Took 3 or 4 tries before it would click. That one got trashed.

There's a picture in my photo gallery of a GPR torn apart showing burnt connections. It was intermittent in operation.

The GPR activation is controlled by the oil temp measured at the HPOP reservoir. It needs to be below 130F for the PCM to pull in the GPR. The WTS light is independent of GPR operation.

So you may need to let your engine cool off more to get a valid test.

**Excavator** · 03-03-2010, 03:42 PM

He wants to check the old one that he removed

I would not worry about the old one if it has been on there for a few years.

If it now starts good leave it be.

**klhansen** · 03-03-2010, 03:44 PM

Quote:

Originally Posted by Excavator

I would not worry about the old one if it has been on there for a few years.

If it now starts good leave it be.

I agree totally, Jim. Keeping old GPR's laying around is a waste of time.

**Flywheel** · 03-03-2010, 05:39 PM

I noticed mine was "going" bad when I went to check my fuel pressure while I cycled the key after a filter change. The relay was smoking....

It started to bulge the plastic and was melting.

That one had some problems and prolly looked like the ones in Kevin's pictures....

BlueMuleTrucking · 03-03-2010, 08:44 PM

How do I check which GPR is bad on the truck? I really don't have a second set of hands, so a second replacement at about 30 dollars, I don't have too big of an issue replacing both, Work is slow till the 15th or so, so I want to pinch pennies the next week or two.

**Excavator** · 03-03-2010, 08:48 PM

Hard/No Start? Check here first.

**klhansen** · 03-03-2010, 08:53 PM

Quote:

Originally Posted by BlueMuleTrucking

How do I check which GPR is bad on the truck? I really don't have a second set of hands, so a second replacement at about 30 dollars, I don't have too big of an issue replacing both, Work is slow till the 15th or so, so I want to pinch pennies the next week or two.

You only have ONE GPR. It's the relay mounted closer to the firewall. The one in the front is the Intake Air Heater relay, which rarely is called on to close.

BlueMuleTrucking · 03-03-2010, 09:02 PM

Quote:

Originally Posted by klhansen

You only have ONE GPR. It's the relay mounted closer to the firewall. The one in the front is the Intake Air Heater relay, which rarely is called on to close.

What are the two similar looking units on the front next to the Oil Filter?

**klhansen** · 03-03-2010, 09:13 PM

Quote:

Originally Posted by BlueMuleTrucking

What are the two similar looking units on the front next to the Oil Filter?

If you meant fuel filter, the front one is the IAH relay, and the rear one is the GPR. I guess I threw you off talking about "closer to the firewall." Sorry. Both relays are actually toward the front of the engine in the valley.

Here's a pic. The circled one is the GPR.

photo credit to F350_6

BlueMuleTrucking · 03-03-2010, 09:20 PM

Yes I meant fuel filter. Yes you did throw me off. When I go to change it, I'm printing out the picture so I don't reverse anything. Thanks for the help.

BroncoGlenn · 03-04-2010, 03:36 AM

Quote:

Originally Posted by Excavator

He wants to check the old one that he removed

I would not worry about the old one if it has been on there for a few years.

If it now starts good leave it be.

Reason being for checking the old one is to ensure it really was faulty and I don't have anther problem elsewhere, ie, bad plugs, that I won't know about until next winter, and hopefully not be stuck somewhere as a result.

Being in southern Nevada this time of year isn't cold enough to cause cold weather starting issues.

**Flywheel** · 03-04-2010, 10:12 AM

To test out of the truck - put 12v to the small terminals and put an ohm meter on the large ones. Should be pretty low resistance.

John

**klhansen** · 03-04-2010, 12:56 PM

Quote:

Originally Posted by BroncoGlenn

Reason being for checking the old one is to ensure it really was faulty and I don't have anther problem elsewhere, ie, bad plugs, that I won't know about until next winter, and hopefully not be stuck somewhere as a result.

You can check the glow plugs without removing them. Check the Hard/No Start link in my signature for the procedure.

BlueMuleTrucking · 03-04-2010, 05:59 PM

Got the GPR switched out. Took about 20 minutes total with, looking for tools. I dropped one of the washers for the connections, and the boot on the left terminal pretty much fell apart. Is it worth getting a washer and fixing the boot with tape or shrink wrap?