Troubleshooting Smart Start, the Glow Plug system, what a nightmare!

[DHAN12435]

This sucks..... Ok so here is the scenario. Hope its not to much information but more is better right..

I have three M998 vehicles. They all ran when purchased. One would crank after running the starter ran say 5-10 seconds and once cranked it smoked a little until it warmed up (lets call it Humvee 1). The other two basically worked as they should and would crank right up.

I pulled the smart start yellow label boxes and swapped them between two units to see if the issue with the one that was harder to start would crank easier. I assumed the glow plug control was not working properly as I never saw it cycle or draw and amperage. Well basically I now I have two Humvees that wont start even after putting everything back as it was. I did not swap the Glow plug Controllers in the manifold, my mistake...

What I see now is Humvee 1 outputs 24v all the time to the glow plugs when in the run position. The other unit which I used as the spare parts (call it Humvee 2) now only outputs 4v to the glow plugs. Both Humvees now have burnt up glow plugs.

Ok I figured lets get some new and improved Green Smart Start boxes and new Dual coil Glow plugs.

Parts came in. I replaced the glow plugs in Humvee 2, Installed the new smart start box with a matched Glow plug Controller ( temp Sensor). Still reading 4v on the glow plug wiring. I never connected them to the glow plugs because I do not want to replace them again. I have good circuit breakers, ground seem ok.. What else could be causing this? I have searched through documents trying to find some information about the electrical system but no luck..

My next step is to probe my last working Humvee and record what voltages should be on each wire going in and out of the Control box to compare.. I will post my findings if it amounts to anything that could help someone else in the future..

 

[Action]

First, it sounds like you put a bad box in a good humvee and messed ghose plug up.
Unhook batteries. Remove engine harness plug by washer bottle. Look in manual to see which of the 9 wires is for glow plugs. Stick a multimeter in that socket and the other lead in all the other ports looking for continuity, including a ground. This may help find a short if there is one.

Jason

 

[Wire Fox]

...did you disconnect the negative cable from your batteries prior to disconnecting/connecting the boxes? Also, where did you source the "Green Smart Start" boxes, and do they have the two connectors on the front of the box, inside the cab?

 

[DHAN12435]

Thanks for the replies. I purchased my boxes from the hummerpartsguy. He says they are directly purchased from AM General and not NOS. Yes power was removed. The boxes are suppose to be the latest version available. They are just as pictured above.. So going through and comparing the voltages I basically did not find any difference between my good working unit and the one not providing power to my glow plugs. I did some resistance checks and since I did not have any glow plugs connected 575A would obliviously show very high resistance. I decided for what ever reason to connect all the glow plugs on Humvee 2 and see what happened when I applied power. My concern was originally to verify the glow plug cycle first so not to burn out any new plugs.

Bingo, apparently the new style boxes will not send out power to the glow plugs unless it sees a certain resistance available on the line. Da, seems so simple now right.. The power applied on the new boxes is about 20 volts for 10 seconds and then 5 volts until the engine warms up. Doing Humvee 1 now. Will see if the same results are present. Humvee 1 is being a bit harder to work on seeing that it appeared to already of had new glow plugs installed when in service (I imagine because the box was always putting out 24v when in the run position) so most of them are melted at the end. Having to basically bust each one and fish out the remaining part after pulling the injector.. What a drag...

 

[Retiredwarhorses]

Thanks for the replies. I purchased my boxes from the hummerpartsguy. He says they are directly purchased from AM General and not NOS. Yes power was removed. The boxes are suppose to be the latest version available. They are just as pictured above.. So going through and comparing the voltages I basically did not find any difference between my good working unit and the one not providing power to my glow plugs. I did some resistance checks and since I did not have any glow plugs connected 575A would obliviously show very high resistance. I decided for what ever reason to connect all the glow plugs on Humvee 2 and see what happened when I applied power. My concern was originally to verify the glow plug cycle first so not to burn out any new plugs.

Bingo, apparently the new style boxes will not send out power to the glow plugs unless it sees a certain resistance available on the line. Da, seems so simple now right.. The power applied on the new boxes is about 20 volts for 10 seconds and then 5 volts until the engine warms up. Doing Humvee 1 now. Will see if the same results are present. Humvee 1 is being a bit harder to work on seeing that it appeared to already of had new glow plugs installed when in service (I imagine because the box was always putting out 24v when in the run position) so most of them are melted at the end. Having to basically bust each one and fish out the remaining part after pulling the injector.. What a drag...

24volts till the GL timer expires, then about 4-5volts for 2 mins, this is basically after glow, or what they refer to
as "glow plug tip temp control". In the old EESS the lights would pulse and the volt meter would dip for 2 min...
you have have at least 1 GP plugged in to test...if you find the box does not STOP providing 24v once the wait lamp
goes out, shut the truck off...then you won't burn up your plugs. Check for voltage on IIRC wire 2a or 3a after start up should have 9-14v's, if not....the GP timer won't know when to turn off.

 

[DHAN12435]

Awesome info retiredwarhorses. I ended up putting the box that was on the working Humvee back on with the plugs working and its still good. Thought it was toast earlier because I didn't have any glow plugs connected. Im guessing something is stuck in the bad controller box as i have 12 volts on the mentioned wires. Thanks again.