1 1/4 Ton Military Pickup Glow Plug Systems

branhamdiesel By branhamdiesel, 21st Jan 2011 | Follow this author | RSS Feed
Posted in Wikinut>Guides>Transport>Repair and Maintenance>Cars

I have lots of people call and ask me about trouble shooting glow plug systems on old military pickups and blazers. I hope these simple checks can help you out.

6.2 Glow Plug Overview

First of all, if your truck has a 12VDC conversion, you are on your own. This guide is specifically for help in troubleshooting the 24VDC set up. Why? Because the 12VDC system can be setup so many different ways, it is impossible for me to outline them all. If you are wondering if you should convert, don't do it. The 24VDC system gives you more power for cranking, and better battery draw for your glow plugs.

This guide is primarily for technicians or hobby technicians experienced in the use of a VOM or multimeter, test light, and vehicle electrical systems.

Lots of these military trucks are on the market, and if you like to tinker and love a good reliable truck, you can pick one up pretty cheap. I have to say, I love these trucks.

The system used 24VDC to supply power to your starter and the glow plug system. Before you do any repair you need to disconnect the batteries, however, you will need them hooked up to do this testing. The system uses at least two batteries in series, or 4 batteries in a series parallel circuit. The batteries power strip is located on the passenger side of the fire wall. The glow plug power tap is the furthest to the driver's side of the strip. The wire hooked to this terminal is not a wire, it is a fuseable link. The link is tied to a wire which then ties into the resistor. The resistor is mounted on the middle section of the fire wall behind a steel heat shield. The reistor is tied to the glow plug relay input via wire. The solenoid is connected to the glow plugs via the glow plug power harness. The solenoid is turned on and off by the glow plug controller.

This is very important. These systems do not use 24VDC glow plugs ( though they are available at any auto parts store), so do not buy 24VDC glow plugs if your glow plugs are burnt. Only install 12VDC glow plugs, your resistor drops your voltage from 24VDC to 12VDC. The plugs won't get enough juice if you don't install 12VDC plugs.

Glow Plug Relay

This pictures a glow plug relay that I recently replaced. The Chevy OEM replacement part is around $250. You can buy an aftermarket unit for around $30 - $40. The pictured glow plug relay is an aftermarket unit.

First thing you want to check is the power to the to relay's transfer input. This is going to be the big post fed from the resistor output. If you don't have any power there, refer to my section on the fuseable link.

If you do have power, then you need to hook a test light up to the small post on the glow plug relay with the purple wire and turn the key on. Once the key is on, a wait to start should illuminate on the dash, and the test light should illuminate. If it does, your controller is working correctly. If it does not, your controller either isn't powered or it is fried. Most likely it is fried. Test power input to the glow plug relay controller located under the dash. If the power input is good, replace the controller.

If your test resulted in the test light and dash light illuminating, you should have heard an audible clunk or click of the solenoid. If you did not, test the ground post ( the other small post on the solenoid). You can do this by hooking up your test light up to a power point, not a ground and the other small post of the solenoid. You don't need to turn the key on to see a result. If the test light does not illuminate, then you simply have a bad ground to the solenoid. Repair this ground problem and retest.

If your test resulted in the test light and dash light illuminated and you heard an audible cluck or click, then you now need to move your test light to the output post of the solenoid ( the other large post with the two wire, single eyelet glow plug harness.) With the test light hooked up to the output post, turn the key on. Your test light should illuminate. If it does not, you need to replace your solenoid, as I had to do in this repair.

If your test light illuminates then your power transfer across the solenoid is good. Refer to my section on the Glow plug Harness and Glow plugs.

Glow plug harness and Glow plugs

The Glow plug harness is the two wires with the single eyelet that is attached to the output of the glow plug relay.

First, you need to unhook the harness from the solenoid. Next you need to remove the harness connections at each of the glow plugs ( a total of eight connections. ) You now need a VOM or multimeter. Hook on lead to the harness eyelet that was attached to the solenoid outlet terminal. Now hook the other lead to each of the connections to the glow plugs on the harness. Each reading should not exceed .5 ohms. If it does, you have a too much resistance in your harness. Bad connection or wire corrosion. Replace the harness or repair failed part of the harness. Test all points of the glow plug harness. 2 or more bad connections can cause a no start situation.

If you harness test good, then you now need to test each glow plug. Hook one test lead to the block or good ground. Now you need to hook the other lead to each glow plug terminal that the harness was hooked to and record you results. The glow plugs should test between 8 and 2 ohms. If the do not, then they are bad and need to be replaced. 2 or more bad glow plugs will keep your engine from starting.

Fuseable Link

This is the fuseable link. Highlighted with the red circle in the picture. You need to test power before and after the fuseable link. Should be 24VDC on both sides of the link.

If you don't have power before the fuseable link check your batteries.

If you don't have power after your fuseable link, but do have power before your fuseable link, your fuseable link has failed and needs to be replaced.

If you have power after your fuseable link, but do not at the glow plug relay, refer to the resistor section.


You should easily find the resistor heat shield mounted in the center of the firewall. The resistor is mounted behind this shield.

Before checking your resistor you need to verify that you have voltage before and after your fuseable link.

It should have 24VDC on the passenger side of the resistor. When checking the output side of the resistor, you will need to load the circuit to determine if the resistor is bad. With no load on the circuit, you should read 24VDC on the output side of the resistor. With the circuit loaded the output side of the resistor should be around 12VDC. If no voltage is present on the output side of the resistor with or without a load, the resistor is bad. At this point, it may be cheaper to convert to a 12VDC system, unless you can find a cheap resistor.


1 14 Ton, 24Vdc, 62 Diesel, Diesel, Glow Plug, M880, M881, M882, M883, M884, M885, M886, M890, M891, M892, M893

Meet the author

author avatar branhamdiesel
I am diesel repair shop owner. I have spent nearly every moment since I was a teenager in a shop. I have worked for multiple dealerships includiing Cat, Freightliner and International. I have held positions at those dealerships of technician, fore...(more)

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author avatar Denise O
25th Feb 2011 (#)

My husband was a mechanic for over 20 years and now he sells cars. I am sorry but, even with him...
When he starts to explain to me what he is doing while working on a car, I go blank. I can do a lot of things but, this and math are my arch enemy.
I like the format, I like your writing. Thank you for sharing.:)

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author avatar Ed Cruz
20th Sep 2011 (#)

I am not a mechanic but I learned a lto and save money just reading articles from unselfish people like you. Thanks for explaining about glow plugs. Ed

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author avatar John b
20th Oct 2017 (#)

where is your shop located?

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