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LMTV FMTV Exhaust Brake Questions?

Keith Knight

Keith Knight

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Yes the ex brake disengages when the brake pedal is depressed.
After doing real world testing I applied +12v to the k10 relay coil during ex braking and it did back feed and disengaged the ex brake. I then tested and found a single wire on plug J308 pin 7 that when you apply +12v directly to it the brake lights illuminate and the blinkers still work like normal.
An interesting side effect, just so you know the brake lights are working while the ex brake is engaged the left and right indicator lights in the dash illuminate. Super happy 😃 hope this helps anyone else interested.
Great thanks to RONMAR who was extremely helpful in figuring all this out.
Here’s a drawing of how I wired it up.
IMG_9105.jpeg
 
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Ronmar

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jpelc said:
Question: does M1078 A0 have or can have exhaust break?
Click to expand...
It does not, but it can. The issue is on the earlier A0 with the WTEC 2 transmission, the transmission auxiliary function used for ex brake is already assigned/wired and most likely the TCU is programmed for PTO function. I have not confirmed that PTO programming on mine, but I have confirmed that it is NOT programmed for ex brake control, as it does not energize the proper output signal when the ex brake signal is applied.

Now you can simply add a pack-brake butterfly valve and it will work as the torque converter is locked in third gear and above 100% of the time Normally. You would need to downshift manually to maximize its performance, and you would need to add a throttle position switch to disengage it if the pedal comes off idle.

To do it properly you would want the TCU reprogrammed so that the aux input would tell the trans to engage the ex brake, switch to the more agressive downshift schecule with a target low gear it is wanting to step down to, and to monitor the installed TPS to only enable the brake valve if the pedal is at idle. The TCU would then control the brake valve actuation and downshifting automatically…

In 98, they started putting the WTEC 3 into the A0, and I know one person who has added an EX brake and his TCU appeared to already be programmed for the ex brake function, but he may have just gotten lucky…

i have thought that it might be handy to be able to force converter lockup in conjunction with 1st or 2nd gear for steep off-road hill descents, particularly with Eco-hubs. You would most definitely need to release it before you tried to bring the vehicle to a stop, so something on a momentary switch…
 
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Klaus

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My truck is a 98 A0 with WTEC 3. What exactly would be involved in adding an exhaust, or a PAC brake? Assuming the WTEC 3 is already configured.
 
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GeneralDisorder

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Klaus said:
My truck is a 98 A0 with WTEC 3. What exactly would be involved in adding an exhaust, or a PAC brake? Assuming the WTEC 3 is already configured.
Click to expand...
I would call PAC brake and see what their latest kit is, but It has been covered in one of Sean's video's about 4 years ago:

 
R

Ronmar

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Klaus said:
My truck is a 98 A0 with WTEC 3. What exactly would be involved in adding an exhaust, or a PAC brake? Assuming the WTEC 3 is already configured.
Click to expand...
Basically accessing the vehicle interconnect wiring similar to what is seen in this WTEC2 diagram. Pacbrake would have the exact wiring config. I saw it not long ago, let me see if i can find a link. This diagram shows using a VIM relay, and you don't have a VIM, so you will need to add a relay…

IMG_3869.png
 
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Klaus

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Thanks for the info Ron and General. I will see what PacBrake have to offer these days and add to the thread what I find out. Looking forward to Sean's video on how he did his install.
 
coachgeo

coachgeo

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Ronmar said:
I just procured a PRXB4, so looks like I will be working this out on my WTEC2:)
Click to expand...
So our 3116 is Full Marmon ? which PRBX model did you get? Understanding I got from PacBrake tech was that the PRBX workings is most about the balance dance between spring, orifice size and check valve matching the engine's exhaust pressures... Then of course the way it mounts and exhaust size. (ours is 4"?)
 
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coachgeo

coachgeo

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coachgeo said:
So our 3116 is Full Marmon ? which PRBX model did you get? Understanding I got from PacBrake tech was that the PRBX workings is most about the balance dance between spring, orifice size and check valve matching the engine's exhaust pressures... Then of course the way it mounts and exhaust size. (ours is 4"?)
Click to expand...
side note.... cause of pacbrake not partnering as nice with the programming in the Wtech II on FMTV's.... one thought have had was to move the whole shifter/keypad to an armrest built onto the dog house .... kinda create a Star Trek Captains chair (just aged myself) with the shift pad right in reach instead of up on dash. That way one can get full use of the PacBrake since it looks like our programming wont down shift her. Problem solved.. just flick open a clear cover made to sit on the keypad and down shift yourself. No need to unbuckle seat belt to reach the dash corner to do that.

PS- also will make it easier to shift between 1st and Reverse when trying to rock the truck out of a Stuck situation... Been there Done That
 
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Ronmar

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The 3116, 3126 and C7 all use the same 4” full Marmon turbo outlet. Yep, its a balance between flow, orifice size and spring. The one I fell into is actually a model intended for a C7 so will probably have to do a little tuning As the 3126/C7 has ~10% more displacement/flow At a giver]n RPM. I suspect they handled this small difference with a spring/adjustment difference. In the end they get aligned to A. Have no more than 50RPM drop when engaged at idle, and B. To remain below a max back-pressure figure at full RPM application. There is a test port on the valve housing to make the measurements. The tech bulletins from Pacbrake show 15PSI of back-pressure max for earlier serial number 2BK series(road) 3116 engines, and 55PSI max for the later serial numbers. They show 55 PSI max for the 3126 and 40PSI max for the 3126B/E and C7 engines. What I am still working out is the max pressure setting for our 7AG series(Agricultural?) engines used in the LMTV…
 
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tennmogger

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Following up on one of the original questions about whether brake lights come on with exhaust brake: i listen a lot on CB when on road trips. I heard a couple of CB'ers laughing about the 18 wheeler descending a long grade with his brakes on continuously. They were laughing that his brakes would be toast before he got to the bottom. But he was listening and told them his brake lights come on when his exhaust brake was applied, and he was not dragging his brakes. So some 'big trucks' do activate brakes with Exhaust brakes.
 
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