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post #1 of 12 Old 10-18-2019, 08:16 PM Thread Starter
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Help. No Power To Glow Plugs

Hi all, and thanks in advance. I’ve been a forum reader for a while now and just signed up as a member.

So here goes, I fell into a good priced 6x4 diesel, around a late 90’s to early 2000, and have been fixing it up.
Recently I was testing the glow plugs buy holding one against the block and turning the key on to see if it was heating up. It did, but then I think I shorted something.

I figured it was the glow plug relay on the right rail under the bed, however a new one did not solve the issue. So I have constant power to the relay harness and I can hear it click when the key is turned on, the glow plug light also comes on the dash for a few seconds, however there is no voltage to the glow plugs.

I confirmed the wire from the relay harness to the plugs is good. The ground wires in the harness test good to ground. There is a yellow set of wires in the harness that provide 12 volts with the key on, they appear to go to the injector pump, and the last wire in the harness connects to the temperature sensor on the thermostat housing; the wire from this sensor also shows 12 volts in the relay harness with the key on.

Still no power to the glow plugs! 😬. Am I missing something??
Also, if anyone has a diesel gator would you be able to check your voltage readings in the harness with the key off and with the key on? It might be a big help.

Thanks again!
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post #2 of 12 Old 10-18-2019, 11:38 PM
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Quote:
Originally Posted by Gator6x4Diesel View Post
Hi all, and thanks in advance. I’ve been a forum reader for a while now and just signed up as a member.

So here goes, I fell into a good priced 6x4 diesel, around a late 90’s to early 2000, and have been fixing it up.
Recently I was testing the glow plugs buy holding one against the block and turning the key on to see if it was heating up. It did, but then I think I shorted something.

I figured it was the glow plug relay on the right rail under the bed, however a new one did not solve the issue. So I have constant power to the relay harness and I can hear it click when the key is turned on, the glow plug light also comes on the dash for a few seconds, however there is no voltage to the glow plugs.

I confirmed the wire from the relay harness to the plugs is good. The ground wires in the harness test good to ground. There is a yellow set of wires in the harness that provide 12 volts with the key on, they appear to go to the injector pump, and the last wire in the harness connects to the temperature sensor on the thermostat housing; the wire from this sensor also shows 12 volts in the relay harness with the key on.

Still no power to the glow plugs! 😬. Am I missing something??
Also, if anyone has a diesel gator would you be able to check your voltage readings in the harness with the key off and with the key on? It might be a big help.

Thanks again!
My 2 cents, id hook up a little glow plug switch to the relay and mount it on the dashboard.
Especially nice if you have harsh winters.

For the long answer youll have to wait as its too early for me to read long texts
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post #3 of 12 Old 10-20-2019, 10:14 AM
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Your description of the power to the relay suggests only 1 wire has power. A relay needs two power (battery voltage) wires to work. One wire will larger in diameter. This is what supplies the high current to the GP and will have 12 volts at all times (Key on). This is the wire that gets switched by the relay. The other 12 volt wire will be much smaller in diameter and carry very little current. This is the command wire that determines when the relay switches. This wire will have power in sync with what the dash light indicates.

Pull the relay out and verify the two power wires exist. As noted above one will have power all the time and the other will cycle with the dash GP light. If these are OK verify the relay is switching by supplying power to the low side and verify continuity across the high side of the relay.

If the above is OK, I'd verify the female terminals that attach to the male tabs on the relay provide a tight connection and there is no corrosion. I had this happen to me on a tractor GP circuit. Relay was tight installing and removing from relay mounting block, but when the individual female terminals were removed from the mounting block there were two terminals that provided very little interference fit. Too little to carry the relatively high current loads of the GP's. Your situation sounds somewhat similar to what I experienced.

Also verify the fuses are OK for the GP circuit.

Good luck.

2012 JD855D OPS Black Poly Roof, Deluxe Cargo Box, Front and Rear Protection, Power Lift, Front Hood Rack, Rear Screen (homemade), 800 Watt Inverter, 2500lb Winch installed in cargo bed, Front and Rear LED auxiliary lighting (4" square 27 watt), Front and Rear CV Guards, Koplin Side Mirrors
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post #4 of 12 Old 10-20-2019, 06:10 PM Thread Starter
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Thanks for the thoughts!

So I have the constant power, it is on always even with the key off. There is a wire from the coolant temperature sensor that runs into the relay (module) harness; this wire provides 11.9x volts when the key is on.
Another yellow wire also comes into the relay harness as well and then continues on to the injector pump; this wire also provides 12 volts when the key is on.

One and or both of those wires somehow must be the switch power for the relay to switch the heavy wire 12 volt constant to the heavy wire for the glow plugs, however even with the power from these lines there is no power to the glow plug line.

Two notes. The yellow line also appears to have continuity to ground when tested with the key off... a short or designed? The temperature sensor is a one wire sensor and must be sending some kind of resistance to tell the glow plug module when to activate the glow plugs. Perhaps the sensor is bad and sending a warm signal?

Thanks again
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post #5 of 12 Old 10-22-2019, 03:54 PM Thread Starter
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Anybody by chance have a diesel 6x4 that can check some electrical readings?
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post #6 of 12 Old 10-22-2019, 06:37 PM
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The shop manuals have great wiring diagrams and details on how the glow plug circuit works. If you plan on doing much work, I suggest you purchase one. If possible, consider one of the vendors who support this site.

Sorry but I have a 855D, so likely not the same.

2012 JD855D OPS Black Poly Roof, Deluxe Cargo Box, Front and Rear Protection, Power Lift, Front Hood Rack, Rear Screen (homemade), 800 Watt Inverter, 2500lb Winch installed in cargo bed, Front and Rear LED auxiliary lighting (4" square 27 watt), Front and Rear CV Guards, Koplin Side Mirrors
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post #7 of 12 Old 10-22-2019, 10:14 PM Thread Starter
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Hey 200, thanks I am certainly working on one, pretty expensive.
If you have the same glow plug module Part AM138537 it would still be helpful. I think that’s what my issue is.
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post #8 of 12 Old 10-24-2019, 11:51 PM Thread Starter
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UPDATE:
I found a tech manual online. So far all tests as it should in the connector harness for the glow plug module (relay).

I took the cove off the old module and found a bad solder connection. Re-soldered it and it now powers the glow plugs correctly for the timed duration, but does not turn the glow plug light on in the dash...

The new module from the dealer powers the dash light, but doesn’t send power to the glow plugs. I think this new relay is bad. The dealer says they will have another one in tomorrow, I’ll update then.
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post #9 of 12 Old 10-25-2019, 07:46 AM
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Glad you found part of the problem.

Is it possible to post a picture of the bad solder joint?

You should be able to bench test the relay to verify its condition.

Can you post a link to the manual?

2012 JD855D OPS Black Poly Roof, Deluxe Cargo Box, Front and Rear Protection, Power Lift, Front Hood Rack, Rear Screen (homemade), 800 Watt Inverter, 2500lb Winch installed in cargo bed, Front and Rear LED auxiliary lighting (4" square 27 watt), Front and Rear CV Guards, Koplin Side Mirrors
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post #10 of 12 Old 10-25-2019, 01:26 PM Thread Starter
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UPDATE:

Thanks 200. Picked up the second “new” glow plug module from the dealer and it works as it should! Glow plugs and dash light now both function.

I just put the cover back on the old relay, but good thought on the pictures. I’ll try to open it back up later for pics.

Here is the link for a technical manual I found for an M-gator. Diesel engine and same wiring as the one I’ve got.

http://radionerds.com/images/4/4e/Te...TOR%281%29.pdf
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post #11 of 12 Old 10-28-2019, 02:45 PM
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Garage
Read your post. My 2011 855D dash light glow plug light went off in June and I just kept cranking it (although I would wait a few seconds before cranking after turning key)

Lo and behold the temp finally dropped around here (in Memphis) and the light started working again. The only explanation is that there must be a sensor that tells the glow plugs they need to come on.

BTW I have a set of doors that might fit yours.
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post #12 of 12 Old 10-28-2019, 03:45 PM
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Quote:
Originally Posted by dmwmbacpa View Post
The only explanation is that there must be a sensor that tells the glow plugs they need to come on.
There is a temperature sensor that dictates how long the GP's are activated on the 855's.

2012 JD855D OPS Black Poly Roof, Deluxe Cargo Box, Front and Rear Protection, Power Lift, Front Hood Rack, Rear Screen (homemade), 800 Watt Inverter, 2500lb Winch installed in cargo bed, Front and Rear LED auxiliary lighting (4" square 27 watt), Front and Rear CV Guards, Koplin Side Mirrors
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