M1123 starting issue

[BananaRay]

I have an M1123 that I purchased a year ago. I have been changing fluids, checking hubs, cleaning grounds, and trying to get road worthy now that I have a SF97 and have retired. Cleaning the grounds behind the gauges allowed the fuel gauge to work again. I put new batteries in it when it arrived. It had been starting beautifully and running smoothly until the other night. While the truck was in park with the motor running, the transmission warning light started flashing Morse code. I turned off the truck. The next day, with pen and paper in hand to try and decipher what code I had been seeing, I tried to start the truck. I turned it to run and nothing. The wait light does not illuminate, there is no usual clunk of a relay, and I have no lights, either dash or headlights. I dug through old posts on here, Google, and G503. Today, with a printed copy of TM 9-2320-280-20-1 in hand and a multi meter I started trying to find the root of the problem. I have 12.8 volts at the run switch, across the starter positive and negative lugs, and on each battery when disconnected. There are 25 volts from the positive on the front battery to the negative on the rear. No current on the start position of the ignition switch. I replaced the switch with a keyed one when I first received the truck. Tomorrow, I will put the original HMMWV switch back in it and try again. I checked Ohms across the shunt, and it was good. The same with the battery wires. The TM and I will become the best of friends in the coming days. The 12 volts showing up instead of 24 is frustrating. Ideas?
Thanks,
Ray

 

[ToddJK]

Put the original switch back in? Sounds like the ignition switch went bad. If it was that switch, I would just invest in a keyed battery shut off switch so you don't have those same issues again.

 

[Coug]

A lot of "truck won't start" stories on here begin with "I installed a keyed switch"
It's probably the number one modification that causes issues.

Definitely remove it and go back to the stock switch. The keyed ones won't hold up to the amps being put through it.

If you are lucky the switch will take care of the issue.

If you aren't you might be looking at replacing the control box. One of the guys on here sells an aftermarket one that fixes all the issues with the ones the military used.


I second what ToddJK said. A battery cutoff switch is a much safer method to use for adding security/disabling the truck than a keyed switch. Added bonus is you won't have to remove the passenger seat to get to the battery connections every time you need to unplug any electrical connections (the control box HATES having anything related to it unplugged while the batteries are still connected)

 

[ToddJK]

A lot of "truck won't start" stories on here begin with "I installed a keyed switch"
It's probably the number one modification that causes issues.

Definitely remove it and go back to the stock switch. The keyed ones won't hold up to the amps being put through it.

If you are lucky the switch will take care of the issue.

If you aren't you might be looking at replacing the control box. One of the guys on here sells an aftermarket one that fixes all the issues with the ones the military used.


I second what ToddJK said. A battery cutoff switch is a much safer method to use for adding security/disabling the truck than a keyed switch. Added bonus is you won't have to remove the passenger seat to get to the battery connections every time you need to unplug any electrical connections (the control box HATES having anything related to it unplugged while the batteries are still connected)

From what I've seen, mostly the switches on Amazon/Ebay, they are just like you said, they aren't meant to handle a high constant amperage, ones I saw specified 20-30 amps, but the description always mentioned 12 volt,not 24, and from what I understand, the amps are cut in half from 12-24, so a switch at 12v is 20 amps, that's only 10 amps at 24v. That's the way I always understood it. Both of my disconnects are 75/150 amp constant flow and surge rates to 200/400 at 24/12v.

 

[Mogman]

No headlights has nothing to do with the ign. switch, although all power to the headlights and run switch go through the PCB box.
I would start by checking voltage from the neg and pos term on the battery to the engine block with the headlights on to make sure it is not the main ground lead (should be zero and 25V), I would then check power in and out of the PCB box next.
Do all tests with a load (headlights on) once you figure out why you have no headlights the rest will probably work.
EDIT, it should be zero volts from the neg terminal to the engine block, 25V from the pos terminal, use the actual terminal not the battery connector when doing this test.

 

[BananaRay]

Update. I charged each battery overnight. The front battery went up to 12.76 and the rear to 12.91. The voltage across the two went to twenty-six volts. I removed the keyed switch and put the meter to it. There is continuity between battery and ignition with the key in run position. There is continuity between ignition and run with the key turned to start. There is no continuity between ignition and run or battery and run. Truthfully, I do not know what those readings show me. I removed the FMP keyed switch and replaced it with the original military switch and reattached the batteries. The truck started like new and the headlights work. The transmission warning light still has a dull flickering while the truck is turned off but works as it is supposed to when starting the truck. Time to track down the source of the flicker. Thanks for the support.
Ray

 

[Mogman]

I would say you still have an issue, replacing the run switch has nothing to do with the headlights, if they were not working when it would not start and they are now, again nothing to do with the run switch as the headlight power does not go through the run switch.
You should have some load when measuring the battery voltage, unloaded battery voltage actually means nothing.
Harbor freight has a cheap battery load tester, not the finest tool in the world but it gets the job done everyone should have one, $20 and it puts a 100A load on the battery (only one at a time!).

 

[BananaRay]

I will get a battery load tester. There has to be something else going on with the lamp flickering when the truck is turned off. I still need to follow up with checking the power in and out of the PCB box. Thank you for the suggestions. Also, thank you for being specific in your instructions. We all do not have the same skill sets. I do not have very much experience with vehicles. I disassembled critters in a meat market for the last 50 years. Once I took them apart, I did not have to reassemble them.

 

[Mogman]

Here is the troubleshooting guide for the transmission, it may be helpful on the telltale trans light.
That may or may not be related to the total power outage you experienced.

 

[Coug]

From what I've seen, mostly the switches on Amazon/Ebay, they are just like you said, they aren't meant to handle a high constant amperage, ones I saw specified 20-30 amps, but the description always mentioned 12 volt,not 24, and from what I understand, the amps are cut in half from 12-24, so a switch at 12v is 20 amps, that's only 10 amps at 24v. That's the way I always understood it. Both of my disconnects are 75/150 amp constant flow and surge rates to 200/400 at 24/12v.

To have the same power output at 12V you cut the amps in half at 24V.
Amps times Volts equals Watts.
12V 10 amps is the same as 24V 5 amps.
That's just the power ratings though. It doesn't directly correspond to the power being used in things like switches and relays and whatnot.


You can pretty much always use a higher voltage rated relay/contactor/switch in a lower rated job, but can't use the lower rated voltage item in a higher voltage application even if it's rated for the same amperage because higher voltage causes high levels of arcing during connect and disconnect cycles.
The contact points aren't designed to handle the higher voltage, so they burn up, causing issues.

Even if the switch is rated for 20 amps at 12V, 10 amps at 24V can destroy it due to the arcing.


The amperage rating usually stays the same no matter what voltage you are using. a 10 amp rated switch at 24V can still only handle 10 amps when used at 12V.
I've seen lots of relays that have the same rating for 12V DC, 24V DC, and 120V AC currents.

As for the disconnects, they are typically not operated under a heavy load, except in case of emergency, so arcing isn't as big of an issue most of the time.

 

[ToddJK]

To have the same power output at 12V you cut the amps in half at 24V.
Amps times Volts equals Watts.
12V 10 amps is the same as 24V 5 amps.
That's just the power ratings though. It doesn't directly correspond to the power being used in things like switches and relays and whatnot.


You can pretty much always use a higher voltage rated relay/contactor/switch in a lower rated job, but can't use the lower rated voltage item in a higher voltage application even if it's rated for the same amperage because higher voltage causes high levels of arcing during connect and disconnect cycles.
The contact points aren't designed to handle the higher voltage, so they burn up, causing issues.

Even if the switch is rated for 20 amps at 12V, 10 amps at 24V can destroy it due to the arcing.


The amperage rating usually stays the same no matter what voltage you are using. a 10 amp rated switch at 24V can still only handle 10 amps when used at 12V.
I've seen lots of relays that have the same rating for 12V DC, 24V DC, and 120V AC currents.

As for the disconnects, they are typically not operated under a heavy load, except in case of emergency, so arcing isn't as big of an issue most of the time.

That makes sense. My understanding of electrical is not as much as it should be. I appreciate you clearing that up for me!

 

[Autonomy_Lost]

Yep, its amps that burn things up not watts. This is why circuit breakers are rated in amps. Wire is also rated for amps based on what guage it is. It's pretty common for wire to be rated for 600 volts, and its the same amperage rating whether its carrying 1 volt or 600 volts.

In fact this is one of the advantages of a 24v system over 12v, you can get more power through the system for a given amount of amps (or less amps for a given amount of power)

 

[jake20]

Watch out for "new" switches from the big vendors as well, I've been having quite a fun time with issues they've had. The start switch is by far the most innocent seeming but one of the most important things to function properly based on my wallet's adventures.

My thread has details regarding what a good start switch should act like when you test it with a multimeter (note: starter removed and the two large gauge positive cables are connected) post #86: https://www.steelsoldiers.com/threads/humvee-starter-flexplate-teeth.210931/page-5


As far as your flickering trans light goes, I encountered this recently on another truck. My is to check the engine speed sensor and re-seat the wire. This was causing a very inconsistent flickering, rather than a consistent code on a friend's truck. When we checked the trans codes, we only got a "no codes present" code.