Help! My HMMWV Won’t Start! HMMWV Glow Plug and Start System Maintenance
Why won’t my Humvee start?
There are LOTS of reasons why a HMMWV might not start normally. Recently the Project Humvee Battlewagon was getting progressively harder to start. The “Wait” light on the dash would just blink on for a fraction of a second and white smoke billowed from the exhaust when the truck was cranked. Some quick troubleshooting revealed that all eight of the HMMWV HMMWV glow plugs where fried. Changing HMMWV glow plugs is pretty simple… when they come out easily. However, I didn’t know WHY they failed. So, I went a bit further and upgraded the major starting system parts:
- Start Control Box
- TSU – Temperature Sending Unit
- GPs – Glow Plugs
I also continuity tested the wiring harness to be sure all glow plugs were getting juice. Luckily, the wiring harness was fine and did not require replacement.
What is the Start Control Box?
The Start Control Box controls the various electrical systems required to start the HMMWV. Most noticeably, the “Wait” light on the HMMWV dash. They also control the cycling of the glow plugs.
It seems that over the history of the HMMWV the purpose of the Start Control Box has evolved. I don’t claim to be an expert in the history or function. So, I’ll keep this brief. If you are an expert in the Start Control Box please educate us in the comments. 🙂
What you need to know is that your Start Control Box needs to work for the HMMWV to start normally.
Where to buy a Start Control Box
Some of the popular online milsurp parts stores have them, but I’ve consistently seen them for the best prices on eBay (link).
Start Control Box versions:
- Older versions are called PCB – Protective Control Box – They use a Glow Plug Controller instead of a TSU and are generally not favored. Some versions, like the LAU PCBs have been known to start HMMWVs on their own and burn trucks to to the ground.
- Then there was the EESS – Engine Electrical Start System – These use TSU.
- Newer versions are called S3 – Smart Start System – Also uses TSU. Not sure what the difference is between EESS & S3. Leave a comment if you know. KDS Smart Start System (S3) P/N 6500391, CR-2701. NSN 6110015427901. Replaces P/N 12479192, NSN 6110014912158.
You MUST disconnect the batteries before connecting or disconnecting a Start Control Box or it WILL be damaged. You should also check the Glow Plugs before installing a new Start Control Box, as defective GPs can fry the box.
How to test HMMWV Start Control Boxes
I read somewhere that the old PCBs could be tested with common electrical test equipment… mainly a multimeter. However, it is recommended to pitch the old PCBs in favor of an EESS or S3 box. I have put my google-foo to the test and have not yet found a way to test EESS or S3 boxes.
What are Glow Plugs?
What you need to know as HMMWV owner is that there are eight glow plugs, one for each engine cylinder. The engine should start even with faulty glow plugs if the ambient temperature is about 70F or higher. The further below 70F, the less likely that the engine will start without some source of heat applied to the engine, ideally from the glow plugs.
How to test HMMWV glow plugs
All you need is a multimeter set to measure resistance in Ohms. I set mine to 200 Ohms max and checked each glow plug for internal resistance. Ideally all HMMWV glow plugs will test at between 1.5 – 5 Ohms. Don’t forget to first check the resistance of your meter, then subtract that from the number on your meter when you test the glow plugs. In my case, all eight glow plugs showed as open circuit… ie. resistance was off of the scale (set to 2oo Ohms) and the “1” on the multimeter never changed.
You can test each HMMWV glow plug while still installed in the engine by removing the electrical lead. Place one of your multimeter test probes on the glow plug connector tip. Place the other test probe on the glow plug threads or nut.
- If the number on your meter doesn’t change when the test leads are touched to the glow plug (stays on “1”), then the glow plug is dead.
- If the number on your meter changes when the test leads are touched to the glow plug, then the glow plug is not totally fried. However, you need to verify whether it is still within specs (1.5 – 5 Ohms). If not, then replace.
Where to get HMMWV glow plugs
I ordered a set of eight 24v HMMWV glow plugs (link) and received them a couple days later with free shipping via Amazon Prime.
I had read that the best glow plugs are “non-swelling” from Kascar. However, when I called to inquire I was told that those were a limited test production run from a German company that was trying to convince the DoD to stock them. However, since the non-swelling HMMWV glow plugs were more expensive, the DoD opted to stick with the standard Wellman 24V glow plugs. So, the German company abandoned the idea. Kascar has since sold out and does not expect to ever get any more. Leaving the standard Wellman HMMWV glow plugs linked above as the sensible option. I have heard of another brand making some sort of “improved” glow plug, but haven’t found any hands-on data. If you know of other options, please leave a comment.
How to remove swollen glow plugs
I was lucky that none of my glow plugs were corroded, rusted, or otherwise fused in place in the engine head. All would turn with a 10mm deep socket. However, two of the glow plugs would not come out. You have to be REALLY careful when removing glow plugs so that none of the glow plug heating elements breaks off and fall into your engine. If that happens, then you have a much bigger problem, as the glow plug fragment needs to be removed (one way or another) before the HMMWV engine is started.
Not wanting to have to fish a broken glow plug element out of the cylinder head I ordered the most basic “glow plug removal tool” I could find. The OTC 6005A Glow Plug Remover tool from Amazon is just a hinged split nut that is camped on the threads of the glow plug to allow it to be backed out slowly. Depending on how far the glow plug sticks into the engine there are also spacers that can be placed between the cylinder head and the split nut. I didn’t even need to use the thinnest spacer under the split nut.
HMMWV Temperature Sending Unit
The TSU replaced the Glow Plug Controller (GPC) that was used with early PCBs. It resides in the coolant crossover manifold near the front of the engine. I don’t know much about these. I’ve read that they report the temperature of the coolant, which may influence whether the Start Control Box cycles the glow plugs. You have to be sure that you install a TSU that is compatible with your Start Control Box.
How to test HMMWV TSU
I am not aware of how to test a Humvee TSU. If you know, please leave a comment. It seems to me that if it is sending a temp signal, then identifying the pins should allow you to take readings when cold and when hot to see if the unit is working. However, that is untested speculation.
HMMWV A1 Troubleshooting Document
I found this “AM General Corporation MILITARY HMMWV M998A1 Series Vehicles Component Testing and Troubleshooting” PDF file online. It has a variety of interesting tasks that can help identify the root cause of various issues, including starting the HMMWV.
HMMWV Start Control Boxes overview
I found the following online many moons ago (sorry, don’t recall where) and thought I’d share, as the author appears far more knowledgeable on this than I am.
> I see a need here to explain the starting system in the older Humvee
> 998 series and here is some scoop of the control boxes that are used in the
> older HUMVEE series. This should help some folks here. I have discussed
> this recurring problem with AMG, Wellman and KDS system engineers at length
> over the years and all agreed that this early control box is a piece of
> junk. The problem lies with the gov’t specs and it took Uncle Sam over 17
> years to realize that this is getting to be an expensive ongoing problem
> that had to be fixed.
> Glow plugs are pretty much glow plugs whoever manufactures them.
> Wellman of Shelbyville, Ind. manufactured most or all of CUCV and Hummer
> series glowplugs for the gov’t. Their engineer there explained many things
> to me which was very useful in finding problems. You see, there are NOT
> really “12 volt” or “24 volt” glow plugs. A plug is rated at say 12 volts
> for 20 seconds to reach a temperature of about 1700 degrees. This plug can
> be left on for all day and will remain at 1700 degrees with NO harm to it or
> will “swell” out. This is what they’re supposed to do. 1700 degrees is a
> good temperature to get the engine to start. Now when you put 24 volts to
> the same plug, it will reach temperature in about 4 seconds which is how
> they do the “quick start” system. HOWEVER, if you leave 24 volts on it, IT
> WILL MELT or “swell” as most people say and will burn out after deformation.
> Now we get into the sorry controller that the gov’t specs called for.
> In the old system if your engine didn’t start, you would naturally turn
> the switch off and repeat the same process. Trouble here is that you would
> create a condition known as “STACKING’. You would be lighting the plugs
> again and if you repeated this several times, you would melt the plugs. The
> original controller box would NOT recognize this condition and would ruin
> the glowplugs! Then things went from bad to worse! The next generation
> controller had a propensity to catch the wiring harness ON FIRE or try to
> start the system by itself when no one was around. This box had the extra
> glowplug wiring harness that had to be installed. When Uncle Sam discovered
> this, all existing units were scrapped through the DRMO. Here is one
> instance where all of them should have been destroyed. (Like they normally
> do but noooo in this instance!) I have seen a few of these turn up on EBAY
> and either they don’t know or don’t care when selling them. Funny thing
> that when I would email the selling party about their control box they would
> ignore me or tell me to mind my own business. That’s all right though, I
> sent all prospective bidders the website so THEY would know. A few years
> back AMG lost about 6 brand new military only Humvees in the factory parking
> lot due to this condition. One spontaniously caught on fire and
> damaged/destroyed others around it. They never really said much about the
> incident but they knew.
> ALL/MOST later and older units used a “glow plug controller” which
> was/is a temperature sensor in the upper water manifold on the right side of
> the engine. Its’ problem(s) is that it has some electronics encapsulated
> inside it and because of the extreme temperature changes, would have a
> premature failure rate. It was a common problem and the most obvious one
> was when turning the switch to “ON”, the “WAIT” light would NOT come on
> along with the glow plugs as well. You can take a new one and plug the
> harness into it (without removing the old one just to check) and see if the
> problem is corrected. Easy fix too.
> Kascar has a replacement controller (for about $600) with box and a set
> of very expensive plugs that are not supposed to swell in this application.
> Bear in mind though that ALL glowplugs will swell if enough voltage is
> applied to them and left on. Kascar had a re-engineered box and it supplied
> a different voltage to the plugs or would not stack. This is one way to go
> but their box is already obsolete! KDS systems (electronic manufacturer
> that makes pretty much all of these boxes) has a new, improved box part
> number CR 2699 that eliminates the need for a sensor in the water manifold!
> The sensor is inside the box itself and it like Kascar’s has a sensing unit
> inside that prevents the condition known as stacking. If you can find one
> of these CR2699’s, it will prevent many of the earlier problems and failure
> rates for a lower price. Also it is a direct retrofit and you just ignore
> the plug to the old controller and insert a plug into the water manifold
> 1/2″ pipe thread hole. I hope this answers some questions and helps folks
> out there. If you have a question and I cannot answer it, I know who to
> ask! Thanks for reading this, sincerely, Julian Burke
Where to get Humvee parts:
- Probably my “go to” for real military parts is HMMWV parts on ebay
- You can get HMMWV books on Amazon
- Actually, you can get a LOT of upgrade parts on Amazon. We have made it easy by adding a HMMWV section the Gear Report Amazon store and add parts to it as we find them.
- Southern Metals Recycling… if you happen to be close to Wilmington, NC. They scrap HMMWVs for the government and can sell some parts.
More HMMWV articles can be found under the Humvee category menu above or by clicking HERE. You might also like the HMMWV section of our Amazon store where we have listed lots of replacement and upgrade parts for Humvees. We have also found a lot of HMMWV parts on eBay (link).
Please leave a comment if you have suggestions on other HMMWV upgrades should consider, or you have done upgrades that we can share on Gear Report.
The Project Humvee Battlewagon
Gear Report acquired a real HMMWV (High Mobility Multi Wheeled Vehicle) from the US Army for use as our official field and shooting range vehicle. We have already posted about some of our upgrade projects (Project Humvee Battlewagon article links) and have a lot of really cool projects on the way. Consider subscribing so you don’t miss any of the cool reviews as we post them.
Here is our Project Intro (link) and our list of HMMWV upgrade projects.