Need help with M936A2

[kd0ezq]

We have a 1990 M936A2 wrecker we use for off road/heavy recoveries along I70 in Kansas. Our unit was working fine until recently, we use it to recover a truck that had went off the interstate and into a field a distance from the pavement, we recovered the truck successfully using the wrecker and everything was fine until it was time to head home… after putting the wrecker back into road gear it killed itself immediately after putting it into drive. It would go in reverse with no issue, but when going forward, it will kill itself.
After consulting with a few local people we were told the torque converter was locked up and causing the issue. We have found there is very little local knowledge about this truck. We have googled and searched through the forum and have checked the button on the end of the Tcase lever. We have voltage to the button, and voltage out when pressed, we have voltage to the solenoid on top of the t case and can hear it switch when button is pushed. In the normal state, air passes through the solenoid and stops when the button is pressed. The solenoid on the transmission has 24v to it at all times, and has been checked for operation. The engagement lever on the left side seems to be working normally. And all operations on the wrecker work properly. But still the truck dies when put in gear. Sometimes immediately, sometimes delayed, but it will not move. The drivelines begin to turn then it kills itself.

we have searched for schematics to trace the wiring and air lines and haven’t found anything other than the TM manuals which we’ve checked and rechecked to be sure of operations. We are at a loss and needing help.
Does anyone have an idea what to check next. We are to the point of considering paying to have the truck shipped somewhere to be repaired.

thanks for your help

 

[98G]

This has come up here before . I'll see if i can find the link with the answer before Wes @simp5782 gets here and explains it in detail.

I don't remember the details of the solution.

 

[Floridianson]

Was not there a problem like that if the button on the high / low leaver stuck in the pressed position?

 

[kd0ezq]

Was not there a problem like that if the button on the high / low leaver stuck in the pressed position?

we have checked the button, we get 24v to the button and through the button when pressed…

 

[simp5782]

[/QUOTE="kd0ezq, post: 2503524, member: 31637"]
we have checked the button, we get 24v to the button and through the button when pressed…
[/QUOTE]
Negative. 24v should be thru the button at all times. And no voltage with the button depressed. It's ground side switched.

Hold the button in and see if it will stay running when you put it in gear

 

[kd0ezq]

[/QUOTE="kd0ezq, post: 2503524, member: 31637"]
we have checked the button, we get 24v to the button and through the button when pressed…

Negative. 24v should be thru the button at all times. And no voltage with the button depressed. It's ground side switched.

Hold the button in and see if it will stay running when you put it in gear
[/QUOTE]

tried holding the button and still does the same thing…
One thing we are unsure of is there is what I would term as a limit switch on the front side of the transfer case that would be triggered off the range lever ( H-N-L). .. it has two air lines running to it, we get no air to that switch and it doesn’t seem to ever be triggered by the washer on the linkage unless the lever would be in Low, is this the switch that locks the front end in?
We seem to have constant 24v to the lockup solenoid on the transmission, ( it is grounded at the transmission) and to the solenoid on top of the t case, but that solenoid does close when the silver button is pushed in because it stops air going through the lines it’s hooked to.

 

[simp5782]

That is the low range engagement washer the washer activates the linear valve so that it goes 6x6 when in low range automatically.


With your rear pto lever disengaged and engaged movement do the brakes do the honk to apply the spring brakes? Do this without the parking brake being on.

 

[kd0ezq]

With your rear pto lever disengaged and engaged movement do the brakes do the honk to apply the spring brakes? Do this without the parking brake being on.

yes, if we pull the lever on the left side of the seat forward, it sets the parking brakes, same as pulling the park brake handle

 

[simp5782]

yes, if we pull the lever on the left side of the seat forward, it sets the parking brakes, same as pulling the park brake handle

Take the transfer case pto linkage loose at the transfer case and ensure the bracket is shifted all the way out and you have full range of the engagement lever. Some trucks got a valve put on that linkage that locked the torque converter up. A 936a2 sold In Texas last year that was doing the same thing and once the linkage was freed up and off the transfer case pto it worked fine.

I'll try to get around one today to look for it. I believe it was a pressure switch that was put on the air brake application valve. I've only ever seen one with it cause I don't see many A2 wreckers.

The valve on the side of the transmission isn't a torque converter lockup valve. What it does is it puts the transmission into 5th gear to make for ease of shifting especially when moving