HMMWV Glow Plug Gospel

[Wile E. Coyote]

I just had the short cycle light on a cold engine. Garage is 40 degrees
engine started well though and I was able to use the truck. I left it running the whole time. It is 9 degree Fahrenheit outside.
Ugh. So I’ll be crawling through all this info as well. Should it have started so earsily if the PCB box or relay box failed?

Unfortunately there are many variables here. Next time it short-cycles, don't go to START, but return it to OFF. If you have a set of jumper cables handy and someone to serve as Soldier B, clip one end of one cable to a clean chunk of metal on the engine block, and have Soldier B hold the other end to the clean and shiny top of one of the 7/16" head control box mounting bolts near the main electrical connector under the hood just in front of the driver. Turn the switch to RUN. If the WAIT light stays on this time, you very likely have a grounding issue (very, very common.)

I've had boxes that didn't turn on the WAIT light or short-cycled the WAIT light despite the fact the system was still cycling the glow plugs correctly. If you suspect that might be the case (because the machine started after all) - next time you try from cold and the WAIT light short-cycles, look and see what your instrument panel voltmeter is doing. You can usually detect the glow cycle by the change in alternator note coupled with the action of the voltmeter (and the headlights will dim a bit if you have the old sealed-beams), and after the initial glow period most boxes will continue to cycle the plugs a few seconds on, a few off, and on again for the Afterglow period.

Have a look under the dashboard behind the steering wheel at what type of control box you have fitted - specifically the label. You need to note the label color and manufacturer at the very least, and a p/n if you can see one without having to remove the box. That will tell us what generation of PCB/ EESS/ S3 box and system you have. If you still have one of the original Prestolite silver-label things that the USMC stuck with for years while everyone else tried other things - those are 30-plus years old now. I have a stack of them with failed mechanical relays.

The other boxes that work with or without a temp sensor are all solid-state except for one model with one relay, from what I remember, but the solid state ones don't like uneven glow plug loads (which is why they say to replace all the glow plugs as the same time you install one of the newer type boxes - so the spectre of that particular problem never crops up.) For those boxes, when you have a few burned-out plugs, some types of them will short-cycle the glow light. At that point sometimes you get away with just changing the glow plugs out (as a set) and trying again - and sometimes - you don't (like pretty much every KDS Green Label box I've ever come across which blows the SCRs like expensive fuses.)

As a matter of course I would measure all the glow plugs with a meter if you have one. Pull all the orange hoods off, pick a low Ohms range on the meter, put one probe on the tip of the glow plug and touch the other to a clean ground. Bad ones will read 0.00 (or whatever you meter usually reads to indicate infinity) which indicates the heater coil is open. Willing to bet you have 3 or 4 bad ones.

 

[McSpeed]

Just checked. All glow plugs are from 1.7 to 1.9


i have the yellow striped intake sensor. Pulled the plug. Looked like some corrosion on a few pins. Cleaned and fluid filmed them and snapped back in place. Now I have a wait light that does the normal timing.

It is also 55 degrees in the shop. I just pulled it outside. It is 25 out now and I’ll let it resume that temp and see what it does. Might have just been that connection????

 

[Wile E. Coyote]

Glow plugs are good then.

It may have been the connection as that's the temp sensor meant to inhibit glow plug operation if the water temperature is within the cutoff range. You probably have the KDS Yellow label system then, which is one of the least problematic (in my own experience anyway - mileage may vary.) I've had a couple of annoying vehicles that are hard to start when you're in that sort of grey area where the temp sensor figures you don't need glow action and inhibits the box from performing a glow cycle - yet you really could've *used* a glow cycle. And 55-ish is when it tends to happen. What I did on set once was grabbed a water bottle from the fridge and dumped it on the temp sensor to fool it into thinking it needed to perform a glow cycle. Then the WAIT light came on as per typical cold start and away we went Onlookers figured it was some form of black magic, I think.

I've had wires in the harness go open at the temp sensor and again at the main connector where it goes from the engine compartment into the box, but...maybe...twice in 20 yrs. The temp sensor end gets the most abuse because over the years every time you change the temp sensor (or GPC in earlier iterations), the plug orientation never winds up the same way twice, so the connector gets twisted around until it lines up. I think the temp sensor actually only uses two of those contacts anyway but I'm not 100% sure.

 

[McSpeed]

Here is my controller.

 

[Wile E. Coyote]

That's the latest version I know of, so you should be good. I have those in two of the ones at work with no problems encountered. So far

 

[benalco1]

I have been following all of these threads because I’m having similar issues. I have the Green label KDS..... the wait light has been short cycling for the past week (glow plugs are not being energized)... it has done this in the past, but after a day or two it returned to normal operation. It doesn’t appear to have the upgraded grounding harness which I intend to install. I did try the jumper cable method mentioned above with no success in making the wait light (smart box) operate correctly.

What’s are “blown SCR’s” that you refer to as “expensive fuses?” Could maybe the box need to be grounded (more so than the jumper cable method) to have it function properly?... or does the box need to be replaced? (All glowplugs tested 2.0-2.1ohms)

 

[Wile E. Coyote]

The old box design used Clum and Trombetta mechanical canister-style relays to send power to the glow plugs. KDS Green and KDS Yellow used SCRs, which are sort of transistor switches (3 mounted on a heatsink in the one design and 4 in the other, from what I remember.) KDS Yellow uses a yellow-banded temp sensor mounted in the water jacket at the front of the engine where KDS Green just ignores whatever's there and does all its decision-making within the box itself.

I don't have a single KDS Green left in service because they all failed. Some you could see a nice burn mark in the middle of one or more of the SCRs when you opened the box up - some showed no evidence at all. They'd all short-cycle. I think that since the TBs all said to change out all the glow plugs before installing KDS Green/ Yellow that the boxes worked primarily on a current-sensing principle, and having some bad plugs would cause unanticipated current values which would cook the boxes. But the boxes cooked anyway, and only one of mine had a couple of bad glow plugs. Go figure.

It *could* be that if the engine, box, plug bodies and battery aren't all at the same ground potential ('floating ground') that it might cause some unintended circuit path wthin the box itself, which blows stuff. Because KDS won't release any circuit details it's really hard to know exactly what's going awry.

Either way, if your plugs are all good and your grounds are all good - and you did the harness-wiggle test in the earlier post - you'll be changing out your box. And there's nothing cheap out there right now.

 

[benalco1]

The old box design used Clum and Trombetta mechanical canister-style relays to send power to the glow plugs. KDS Green and KDS Yellow used SCRs, which are sort of transistor switches (3 mounted on a heatsink in the one design and 4 in the other, from what I remember.) KDS Yellow uses a yellow-banded temp sensor mounted in the water jacket at the front of the engine where KDS Green just ignores whatever's there and does all its decision-making within the box itself.

I don't have a single KDS Green left in service because they all failed. Some you could see a nice burn mark in the middle of one or more of the SCRs when you opened the box up - some showed no evidence at all. They'd all short-cycle. I think that since the TBs all said to change out all the glow plugs before installing KDS Green/ Yellow that the boxes worked primarily on a current-sensing principle, and having some bad plugs would cause unanticipated current values which would cook the boxes. But the boxes cooked anyway, and only one of mine had a couple of bad glow plugs. Go figure.

It *could* be that if the engine, box, plug bodies and battery aren't all at the same ground potential ('floating ground') that it might cause some unintended circuit path wthin the box itself, which blows stuff. Because KDS won't release any circuit details it's really hard to know exactly what's going awry.

Either way, if your plugs are all good and your grounds are all good - and you did the harness-wiggle test in the earlier post - you'll be changing out your box. And there's nothing cheap out there right now.

Thank you for the quick response.... that’s what I was dreading. I’ll try the grounding harness and let you know - I’ll probably be buying a box soon. Based on what I’ve seen, it appears as if I’ll want to get the newer KDS cr-2700 (S3) or the Natron version. I noticed the KDS cr-2700 on eBay for $499.99 this morning.....

 

[Milcommoguy]

Not SCR's. Old school Power MOSFET. Each about 60 Amp rating. Common 30 year old design and components. (same as HMMWV) Inductive kickback (spike/s) from control relay and starter KILL boxes. Two ways to fail... Blows open, good for saving glow plugs, but now can't get truck to start. BAD... shorted. After sixty seconds, time for a new set of plugs. Grounding straps good Idea but not the end all.

IMO too much going on to start such a simple engine and it's tested EMP proof? Somebody made a killing selling these to the Government.

See the Glow Plug Movie
CAMO

 

[sagreen83]

I'm having some starting issues in the cold. Glow plug light comes on, cycles after 10 seconds and I see an amp drop on the meter. Problem is that it wont start cold unless I give it a little throttle. Then when it does fire up I get a massive amount of white smoke.

Does this sound like glow plug problems?

Is there a written procedure for replacing the glow plugs? I bought new Kascars, but it doesnt look like a 10 minute swap, at least on the passenger side

Scott...

 

[90Jimmy]

Having to depress the throttle during cold start in not uncommon, it’s actaully how you start the vehicle during cold weather conditions. I believe the TM states half throttle above 32F and fully depressed under 32F. If the plugs weren’t working it wouldn’t fire up. The condition you describe is exactly how my 998 functions during cold weather start, with the older style control box. TM does cover remove and replace precedures for glow plugs...
While you are changing them don’t forget to enjoy the process....it’s a hobby right.

 

[Lionel]

In case no one has mentioned it so far, it is highly recommended to disconnect the batteries before doing all the unplugging and plugging back in. I know there's been more than a few controllers smoked by doing it.

 

[sagreen83]

Having to depress the throttle during cold start in not uncommon, it’s actaully how you start the vehicle during cold weather conditions. I believe the TM states half throttle above 32F and fully depressed under 32F. If the plugs weren’t working it wouldn’t fire up. The condition you describe is exactly how my 998 functions during cold weather start, with the older style control box. TM does cover remove and replace precedures for glow plugs...
While you are changing them don’t forget to enjoy the process....it’s a hobby right.

Glad to hear that!

One thing I've been trying to do is use the least amount of throttle during cold start as possible. 2 things happen.

1) I get a lot of white smoke. I literally turn on a big fan in the garage to blow the smoke out while its settling down.
2) Im trying to keep the diesel engine clatter to a minimum. When its cold and I use a little throttle there is a LOT of clatter. Takes about 5 minutes before it settles down to a normal sound.

Are both of these normal?

Thanks!
Scott...

 

[Dock Rocker]

Glad to hear that!

One thing I've been trying to do is use the least amount of throttle during cold start as possible. 2 things happen.

1) I get a lot of white smoke. I literally turn on a big fan in the garage to blow the smoke out while its settling down.
2) Im trying to keep the diesel engine clatter to a minimum. When its cold and I use a little throttle there is a LOT of clatter. Takes about 5 minutes before it settles down to a normal sound.

Are both of these normal?

Thanks!
Scott...

Yes. Both are very normal. Diesel engines need heat to operate so until a certain cylinder temp is reached you are not burning all of the fuel hence the white smoke.

Because it’s not burning all the fuel it’s not running very efficiently hence the clatter.


Sent from my iPhone using Tapatalk

 

[Retiredwarhorses]

Glad to hear that!

One thing I've been trying to do is use the least amount of throttle during cold start as possible. 2 things happen.

1) I get a lot of white smoke. I literally turn on a big fan in the garage to blow the smoke out while its settling down.
2) Im trying to keep the diesel engine clatter to a minimum. When its cold and I use a little throttle there is a LOT of clatter. Takes about 5 minutes before it settles down to a normal sound.

Are both of these normal?

Thanks!
Scott...

the clatter is normal and goes away after the cold advance expires and retards the timing.

 

[NormB]

In case no one has mentioned it so far, it is highly recommended to disconnect the batteries before doing all the unplugging and plugging back in. I know there's been more than a few controllers smoked by doing it.

Even though I have a battery disconnect switch, if I do something electrical more than, say, changing a headlight, I actually disconnect a battery TERMINAL. Not paranoid, switches fail.

 

[NormB]

Glad to hear that!
1) I get a lot of white smoke. I literally turn on a big fan in the garage to blow the smoke out while its settling down.
Scott...

Last spring I bought a 45 degree silicone pipe connector with a 3" ID to match the exhaust pipe, THREE lengths of the press-together gas water heater chimney pipe at home depot and one of those connectors that allow you to make any bend in the pipe up to 45 degrees.

Connections go like this: silicone connector, 3" chimney, adjustable connector, two lengths (I think they're 6 feet, IIRC) of chimney and out the door so's the unburned diesel vapor doesn't permeate the garage and house. Literally takes a minute to connect.