M939 Jake Brake and Allison Questions

Arny

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Hi all,

I'm planning to equip my M923 A1 with a Jake brake and did a lot of reading and researching the last weeks.

Still I could not find answers to all of my questions.

Can anyone tell me something about the lockup of the tranny?
The Allison Manual says that the Transmission has a lockup in 2nd, 3rd, 4th and 5th gear - can someone definitely confirm that?
The Point is when I have a look at the TMs I only find a description of a Solenoid responsible for the 5th gear lockup.

I want to know that because I need to get the Signal "lockup" & "not locked up" from somewhere, since the Jake should only be working on lockup of the gears.

Did anyone finally equip his M939 Series truck with a Jake brake?

Thanks.

Arndt
 

sandcobra164

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Arndt,
I've put many a thousand miles on this type of truck. Once the converter locks in 2nd gear, it stays locked until you slow down to about 10 mph. When you first start accelerating, 1st gear is converter driven, you'll feel the 1-2 shift and shortly after you'll feel another "shift". That's not 3rd, it's simply the torque converter locking up. It stays locked the rest of the time you're above 10 mph, should work fine with Jake Brakes. You'll have to provide the signal from a switch that you control manually, I can't think of a way that the truck can do this automatically but I'm willing to hear another opinion.

Joseph
 

WillWagner

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Easy peasy, wire it from keyed B+ source to the pump switch, out of the pump switch to the Allison pressure switch installed in the lock up port on the trans body, IIRC, Jake sells one too, I will see if I can dig up the p/n, to the 3 position switch and from there to the brake housings. I have a manual, I will scan the diagram and see about the Jake p/n. IIRC, the Allison switch p/n is 3590.

Do it like this and the brakes will shut off when the throttle is depressed or the lock up pressure goes away, that is usually around 1000 RPM + or -. No need to keep turning them on and off.
 

WillWagner

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Here....

Oh yeah, the part numbers for the switch is...Jacobs, 003951 and Cummins, 3871305. OR a 100 PSI, NO switch. Allison never said the use of the off the shelf, generic switch was OK to use, but we used them all the time, sorry, do not have a generic p/n for one, it has been WAY too long!

NO meaning Normally Open...with no pressure in the lock up galley, the switch is open, an incomplete circuit. When the galley is pressurized above 100 PSI, the switch closes, meaning the circuit completes and sends the 12/24 volts to the 3 position switch.
 

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Arny

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Wow great! First of all thank you very much for your answers and the wiring Diagramm.

I'm actually planning to use a small foot-switch like the one for high beam to turn on the jake.

So if I got that right there is already a switch on the fuel pump I can use and there is also this pressure Switch on the tranny already (marked "P" in the TM?)?
No Comes the stupid question: If I wire the jake Switches with the fuel pump switch and the pressure Switch - this would only work if the circuits that are already using this Switches have fuses in their ciruit which can handle the current needed for the solenoids of the jake, right?

Is it not common to link the jake also with the stop light?Unbenannt.png
 

WillWagner

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None of the switches/wiring are there, you need to buy, install and wire it. The brakes you have are likely 12 volt, switches don't matter, but you will need to buy 24vdc solenoids for the brakes. I can scan a foot pedal diagram for you later tonite.
 

wcuhillbilly

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Or,,,, wire up a reducer to supply 12vdc to the Jake circuit wiring, and a limited number of 12vdc accessories. This is how I power my jakes. as I don't want to be hunting up an expensive 24vdc Jake solenoid. mine is a smaller reducer for 30A

yes.... Will is the man to go to for the Cummins know how...
 

Arny

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Easy peasy, wire it from keyed B+ source to the pump switch, out of the pump switch to the Allison pressure switch installed in the lock up port on the trans body, IIRC, Jake sells one too, I will see if I can dig up the p/n, to the 3 position switch and from there to the brake housings. I have a manual, I will scan the diagram and see about the Jake p/n. IIRC, the Allison switch p/n is 3590.

Do it like this and the brakes will shut off when the throttle is depressed or the lock up pressure goes away, that is usually around 1000 RPM + or -. No need to keep turning them on and off.
That means that the pressure switch will only give a signal if the tranny is locked up after shifting from 1st to 2nd, right?
It cannot detect if there is a shifting in progress e.g. back from 4th to 3rd?
I assume that if I keep the Jake brake engaged during a shift the brake pads in the tranny would wear out sooner - or am I wrong on this one?
 

74M35A2

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Will is a bad a$$.

No brake pads in the trans, but you probably mean clutch discs.

You can compression brake with an unlocked converter, it is just not as effective, but will still supply some additional braking power.

Yes, converter locks up partially through 2nd gear, and stays that way all thereafter. There is a pseudo-shift during 2nd gear, that almost feels like a full gear change, yet is not. This is the converter locking up. You can demo it by doing a drive in 2nd.

You could wire it up through the TC lockup switch and brake light switch in series if you want full auto operation simultaneously with brakes. Could do it on a multi-position switch. On, off, auto.
 
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Arny

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Just wanted to let you know I finally finished the jake project this year and retrofitted my M923 A1 with two Jacobs 25b heads on cylinders 1&2 and 3&4.
Took some time and modifications but in the end it worked well.
Anyway I couldn't have done it without Will's help - many thanks again! :)

Put in a foot switch to engage it and also another switch to shut it off completely on the shift cover. Used a 30amp fuse and went directly on the battery.

DSC_1733.jpgDSC_1734.jpgDSC_1736.jpgDSC_1977.jpgDSC_1978.jpgDSC_1989.jpg
 
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dmetalmiki

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Some great knowledge here. Could I impose and ask, Is it possible to have that Allison Transmission " Cease Driving" (as in (Freewheel)) Through operation of the hydraulics, By mechanical means, Like a pedal, (Existing clutch pedal).
My (Converted) Diamond T 980, Has that Allison Gearbox coupled to a Detroit engine and still retains the original 3 Speed Transfer Case, Which I would rather like to be able to 'Change ratios on the fly'.
 
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