Completely stumped. Will not cold start and put all new parts on it.

darkostoj

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Have a m998 with 9k miles on it. It's all original and in nice shape. About 6 months ago it stopped cold starting and have been trying to figure it out ever since.

So far:

Each lead to the glow plugs have been verified 24v when the wait light is on.
Wait light is on for approximately 10 seconds then turns off
Installed brand new glow plugs
Installed brand new temp switch on top of intake
Installed brand new cold temp advance switch on rear right of engine block

It will just crank and blow tons of white smoke out of the exhaust but not start. It runs great otherwise.

I am stumped! Help is needed!
 

Dock Rocker

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My guess is your box is toast. If it is toast your new glow plugs are now likely also toast.

What GP control box do you have and did you get the correct temp sensor for that box?

I would start by making sure you have current to the GP’s while the wait light is on. Also pull a few plugs and see what shape they are in. When the control box takes a dump many times it takes out the GP’s as well.

Likely you will need a new controller, temp sensor and glow plugs. Make sure you get the modern controller so you don’t have these issues again. Also toss on a ground harness to help with any potential ground problems causing future issues.


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darkostoj

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Not sure but have you tried a new smart start box?
Not sure but have you tried a new smart start box?
I have 2 M998's and the other one has no cold start issues. I swapped the box from the good truck to this one and it made no difference.

However, something strange did happen with the "known good" box. I didn't check the voltage on the glow plug leads, but the wait light would turn on and off after about a second.
 

darkostoj

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My guess is your box is toast. If it is toast your new glow plugs are now likely also toast.
What GP control box do you have and did you get the correct temp sensor for that box?
I would start by making sure you have current to the GP’s while the wait light is on. Also pull a few plugs and see what shape they are in. When the control box takes a dump many times it takes out the GP’s as well.

Likely you will need a new controller, temp sensor and glow plugs. Make sure you get the modern controller so you don’t have these issues again. Also toss on a ground harness to help with any potential ground problems causing future issues.
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So how could the box be toast if I am getting a constant 24v to the glow plugs? It is strange that when i install the known good box the wait light flashes.

When I ordered parts I bought specific M998 temp sensors, is there a specific one depending on which box you have?

Would brand new glow plugs go bad right away like that?
 

Dock Rocker

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So how could the box be toast if I am getting a constant 24v to the glow plugs? It is strange that when i install the known good box the wait light flashes.




When I ordered parts I bought specific M998 temp sensors, is there a specific one depending on which box you have?

Would brand new glow plugs go bad right away like that?
If you are getting a constant 24v to the GP’s they are toasty. Depending on what box you have they should only have 24v for a short period of time while the plugs are heating the holes. The oldest box gives you an afterglow for a little while after start.

Different start boxes work differently and they do have different temp sensors. They should be matched to the box you have in the truck.

If the GP’s were supplied with 24v for to long they will burn out almost immediately. Thats what goes wrong with most boxes. They die and then supply constant 24v to the plugs and then they die too.


Also did you unhook the batteries while you were swapping boxes? This is known to fry boxes of you do not unhook the batteries while you are monkeying around with the start boxes.

Re your flashing light with the known good box you likely have a mismatched sensor and box.


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darkostoj

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If you are getting a constant 24v to the GP’s they are toasty. Depending on what box you have they should only have 24v for a short period of time while the plugs are heating the holes. The oldest box gives you an afterglow for a little while after start.

Different start boxes work differently and they do have different temp sensors. They should be matched to the box you have in the truck.

If the GP’s were supplied with 24v for to long they will burn out almost immediately. Thats what goes wrong with most boxes. They die and then supply constant 24v to the plugs and then they die too.


Also did you unhook the batteries while you were swapping boxes? This is known to fry boxes of you do not unhook the batteries while you are monkeying around with the start boxes.

Re your flashing light with the known good box you likely have a mismatched sensor and box.


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So, I just went out and checked and the glow plug connector is showing 24v. However, it continues to show 24v even after sitting for 5 minutes. Based on what you said it sounds like the controller box is bad supplying a constant 24v to the glow plugs which results in burning up all the glow plugs.

I will say when I put the new glow plugs in it did start the first time I tried.... :lol:

Any recommendations on which box to get?
 

Dock Rocker

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Contact Michael Vaiden and Black Dog Customs in Georgia. He will get you fixed up and walk you through the process and get you the right green label box.

His prices are very Competitive and if you plan on keeping your hmmwv his phone number is a really good one to have in your Rolodex. He has help me personally on numerous occasions.


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dmetalmiki

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At least you now know the enging and fuel pump are good.
Thanks to all the advices, you now must be aware that the voltages to the glow plugs are not battery voltage.
most modern cars only shoot a third of battery volts to the glow plugs.
You should now research,
A) What is the Start off voltage.
b) the cranking voltage.
c) the 'run till warm' voltage.
T.M.s should reveal that. (If it applies).
That is why the truck fired right up first time..(As the glow plugs then continued on their 'merry way to destruction' and cook under the incorrect supply current.
As advised in previous answers, Go to the experts and get ALL the compatible components as a set for you particular truck, and then fit them with the batteries disconnected.
You will then have a good to go truck again.
(Also as suggested see if you can obtain a more updated better designed and reliable fault free set).
 

NormB

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So, I just went out and checked and the glow plug connector is showing 24v. However, it continues to show 24v even after sitting for 5 minutes. Based on what you said it sounds like the controller box is bad supplying a constant 24v to the glow plugs which results in burning up all the glow plugs.
I will say when I put the new glow plugs in it did start the first time I tried.... :lol:
Any recommendations on which box to get?
Look around on the evil auction site, and the various HMMWV groups on fakebook where it’s not uncommon to find the set (box plus sender) for under $400. Easier to find around $500, if they’re asking much more than that, keep looking.

I paid $490 shipped about 2-1/2 years ago - yes, it fixed the cold start issues - and about six months back I bought a sealed, NEW box for $360 plus shipping as a spare (you never know... I even have a spare, NEW hydraulic fan clutch I got for $300 last year) from I forget who, but they’re out there.

Keep us posted.
 

Wire Fox

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Box should step voltage down to 12volts
This is just incorrect. I've measured this while it's functioning correctly. The HMMWV is 24V at the plugs. At you possibly thinking about the CUCV? I haven't worked on those, but I believe I recall those being 12V, while much of the rest of the vehicle was at 24V.

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Edit: I want to say that in Sue's defense, if the reference material for that conclusion was drawn from that unofficial A1 Troubleshooting manual (that's generally very helpful!), it does pretty firmly state that the entire glow plug system is 12-14V. It's also the only obvious reference I saw that clearly stated what the voltage level "should" be (that answer seems to not be obvious in the TMs- they assume you know to always look for 24-28V). That A1 guide just so happens to be wrong in this area. My measured voltage is system voltage (24-28V), the wiring schematic that shows the logic flow of the start box implies no voltage divider or step-down, and the specs for the P/N of the HMMWV glow plugs are for a 24V system-which is what separates them from the 12V glow plugs used in the civilian variant of this engine.
 
Last edited:

Stonewall

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Be careful out there on the auction sites for a control box. I purchased a new one for a good price, thinking I was getting a great deal. The part number on the listing was correct for an EESS, but when I received the unit and installed, it would not work. I contacted a few on this site that I knew had a bunch of knowledge. They confirmed that the part number listed is what I wanted, but after seeing some pictures and hearing my issue, we decided that what they sold me was an old style GPC, which does not work with the current set up. After some argument, I was able to return for a full refund.
 

TOBASH

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I read here in another thread that if you connect your glowpugs backwards then current might not stop.
 
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