Couple issues with M920

[Hammer]

Well, after working over the truck and driving it some, I found a few quirks.

First thing, the dash lights work when I pull the light switch out to the running lights position.
But when I pull it all the way out to turn the headlights on, all the dash lights go off. Even the shifter lights.
Also, it doesn't dim the dash lights when they work. Wasn't sure if this switch did the dimming function or not.

I swapped the tires out to the 16r20 Michelin tires. Now I need to correct the speedo.
Is there a known adapter for this?
And where do a find a key to open it?

The headlight dimmer switch is not doing well. Anything special about these? Or will any standard floor switch work?

My voltage gauge drops voltage fast when I turn anything on in the cab.
This kind of sounds like bad grounds, or something other than a large drop in voltage. Just one dash fan drops the voltage by over a half volt.

What is the fuel button supposed to be for about 400 HP?
My truck does NOT feel like it is even close.

All of my heater controls are stuck except the fresh/recirculate.
I think the fan motor is seized up, but I haven't pulled that apart yet.
Are all the cables generic that push pull the controls? They look like it.

My PTO light on the dash doesn't always work.
My PTO cable is bent, so it will engage the PTO, but not disengage. I have to climb under it to manually disengage.
The PTO light works sometimes and not others. The wiring looks fine, etc.
Is it a pressure switch, or a mechanical switch?

 

[Elideclan2]

I have a m920 as well, with many of the same issues. I hope you get responses

 

[silverstate55]

My M920 is a 1980 model and has almost the same exact problems as yours.

The only possible cure I have found is with the headlight switch; it can't effectively handle the load of all the lights on, especially when dash lights are dimmed. Previous threads mention to leave it on all the way lit or dimmed completely. A relay or two will help lessen the amperage going through the switch. Someone installed a separate switch for clearance lights on my dash, but removed it and left a jumble of wires behind the dash. I found the clearance lights wire and reinstalled it to a switch in the added hole (on my truck the clearance lights were self-grounding to the cab). This worked and now my dash lights, headlights, and clearance lights work just fine.

Some links to help get you started:

M916-M920 Tech and Tips

Artisan's M916

Photos of my M916

Relays

915 Headlight switch wiring?

Headlight switch for M915A1


Sorry but I don't have answers for your other questions. Most of the answers to my immediate issues (truck is in storage until I sell our house/property in spring) were found with searches in both 5-ton forums (5-ton & Hot-Rod/Mods) with M915, M916, and M920 as keywords. Many of these problems are spread across the M915/916/917/918/919/920 range, as some issues with electrical and transmissions are common in early models. A lot of the threads also contain links to other archived threads that have answers/solutions.

Good luck and please post your results & solutions.

 

[JDToumanian]

I'm not real familiar with an M920, but when you refer to a key to open the speedo, I assume the truck has a tachograph? (tachometer, speedometer, odometer, clock, and hour meter all in one)

If it's a tachograph key you need, they're all the same and readily available:

https://www.ebay.com/itm/New-MILITA...transporter-8x8-M1070-VDO-others/263005550376

Jon

 

[Hammer]

For the headlights themselves, I will either install a relay setup like I have on many other vehicles. Or install LED headlights (might still run the relay anyways.)
This will eliminate the power draw on the switch, wiring, etc. I like doing this for most of my vehicles.

I have read through a few of those posts, but they are LONG and it takes some digging to get just the facts, part numbers, etc.

 

[silverstate55]

For the headlights themselves, I will either install a relay setup like I have on many other vehicles. Or install LED headlights (might still run the relay anyways.)
This will eliminate the power draw on the switch, wiring, etc. I like doing this for most of my vehicles.

I have read through a few of those posts, but they are LONG and it takes some digging to get just the facts, part numbers, etc.

But they're worth taking the time to read through...you'll pick up a LOT of other info about your truck that way. I have a lot of them in a separate Subscriptions folder.

Most of your electrics will be 12-volt; the starter, slave cable bracket, and military trailer plug in the back of the cab are about the only 24-volt items on the truck. So the rest is easily swapped/adapted/replaced if necessary. Been looking at heated 12V LED headlights for Jeeps on ebay myself.

 

[Hammer]

I agree, there is a TON of information in those threads.
I need to sit down for a couple hours, go through the threads with a fine tooth comb and make an actual list of items, part numbers, etc. from those posts for a quick reference.

 

[silverstate55]

Some more links for your perusal:

Needed assistance, New M920 owner Ele problem

M917/M916/M920 Wiring Diagrams

CBR Metalworks M920 hauler 'Mad Max'

Allison M916 + Mk48 Cab = 6x6 "FEMTT"

Look for anything by member M920 (Soni). He's been through these series of trucks pretty thoroughly and his threads are worth subscribing to.

 

[Hammer]

I will replace the headlight switch tomorrow.
I will run a relay for the headlights, even though I will have LED headlights for this truck in a couple weeks. I still prefer to keep the headlights on their own power, and just use the switch to signal the relay.
Next step is to do a relay setup for the clearance lights. That seems to be the other power draw.

I just got my new LED overhead clearance lights in today!
I will also do LED turn signals and taillights to really drop the power draw for lights on the truck.
What flasher works on these trucks with ALL LED lights?

 

[silverstate55]

I replaced the turn signals & rear brake lights on my truck with commecially-available LED units, and the stock flasher seems to work just fine for me.

 

[Hammer]

Interesting! I figured they would flash way to fast, or not at a speed that works.
Guess I will find out soon enough.

 

[Hammer]

Replaced the overhead clearance lights with the LED ones, very nice difference!

Replaced the headlight switch. The original harness must have melted, and there were random colored wires just plugged into the switch. So I had to do a lot of trial and error, but all the dash lights work and dim, headlights work, etc. Yeah!

My floor headlight dimmer switch is bad. Unless I SMACK it, it can sometimes stick with NO HEADLIGHTS! Not a fun thing at all.

I have new LED front turn signals that I will put on soon.
I have new LED headlights on, and those make a nice difference as well. My stock headlights looked like two candles with mirrors...

 

[Hammer]

Sheesh, old thread.
I replaced the headlights, headlight switch, floor dimmer/switch. Those all work great now.
I have LED front turn signals, overhead clearance lights and new taillights. All commercial over the counter stuff.
Turn signals work great now!
Even when running the lights and signals on a commercial trailer.
The only NON LED lights are the overhead rotators and the rear work lights.

I have went through a couple of the dash fans now though. They just don't make them worth anything anymore! I like having those. They work great for defrosting the windows, or just keeping you cooler.