LMTV FMTV Exhaust Brake Questions?

[Ronmar]

On the Allison, it enters lockup in our 3rd gear when turbine RPM approaches input RPM. It stays locked up 100% of the time all the way up thru 7th and back down thru 3rd. It drops out of lockup in 3rd most likley when it senses the downshift to 2nd is iminent.

The main thing the pacbrake interface to the trans does is to preselect 4th gear(our 5th?) if you are in a higher gear...

 

[Mullaney]

On the newer FMTV trucks with exhaust brakes, does the Allison lock up when the exhaust brake is applied? I have two trucks with Pacbrakes and they are great, but with manual transmissions.

How effective will adding a Pacbrake work with an unlocked Allison?

.
Not really sure about locked or unlocked Allison, but the transmission definitely helps slow down the truck. The Allison starts down-shifting as it gets down to the proper speeds to help reduce your speed...

 

[tennmogger]

On the Allison, it enters lockup in our 3rd gear when turbine RPM approaches input RPM. It stays locked up 100% of the time all the way up thru 7th and back down thru 3rd. It drops out of lockup in 3rd most likley when it senses the downshift to 2nd is iminent.

The main thing the pacbrake interface to the trans does is to preselect 4th gear(our 5th?) if you are in a higher gear...

Exactly what I needed, thanks Ron.

 

[coachgeo]

On the Allison, it enters lockup in our 3rd gear when turbine RPM approaches input RPM. It stays locked up 100% of the time all the way up thru 7th and back down thru 3rd. It drops out of lockup in 3rd most likley when it senses the downshift to 2nd is iminent.

The main thing the pacbrake interface to the trans does is to preselect 4th gear(our 5th?) if you are in a higher gear...

For everyone's clarity...
true for A1 and A0???. (as long as A0- does not have have PTO in Allison TCU thus can't link pacbrake to it?)

 

[Ronmar]

True for every Allison... If that input is programmed for PTO(or something else/not programmed like mine is), it cannot of course provide the preselected downshift…

The lockup function, 100% LU/engine hard coupled to the transmission in 3rd and above, is a normal part of the Allison shift programing. This is a recent revelation for me, I used to believe it lifted lockup during shifts, but that was in error. As soon as it sets lockup, it maintains it continuously until, I believe, it senses that the 3-2 downshift is imminent.

if your TCU is not programmed to activate the preselected downshift, you can simply do it by hand, but the pac brake will still be effective…

 

[Mario]

The only thing I don't like about my exhaust brake is that there is a significant time delay to accelerator input.
My 1083A1 goes into limp mode and I can't really accelerate for good 30-40 seconds after switching exh brake switch off.

 

[Ronmar]

The only thing I don't like about my exhaust brake is that there is a significant time delay to accelerator input.
My 1083A1 goes into limp mode and I can't really accelerate for good 30-40 seconds after switching exh brake switch off.

Well that sounds weird. Do you get immediate throttle response when you step on the accelerator while the switch is still turned on?

On the A1/A1R, the ECU is the source power for the exhaust brake. I would say the ECU only provides that power when the TPS/pedal is in the idle position. The 24V provided by the ECU goes to the rocker switch and when you select ex brake, that 24v is sent to the ex brake relay(coil and common contact pin 30) and to the TCU EX brake request input. The TCU provides a path to ground to energize the ex brake relay. Pin 87A(de-energized pin) on the ex brake relay is the output to the ex brake Via the other contacts in the rocker switch Set to the ex brake position. The TCU, if it authorizes the ex brake sends a signal to the ABS, and opens that ground path to the ex brake relay, de-energizing it and sending the ECU ex brake power to the ex brake solenoid turning on the ex brake.

 

[Mario]

Well that sounds weird. Do you get immediate throttle response when you step on the accelerator while the switch is still turned on?

On the A1/A1R, the ECU is the source power for the exhaust brake. I would say the ECU only provides that power when the TPS/pedal is in the idle position. The 24V provided by the ECU goes to the rocker switch and when you select ex brake, that 24v is sent to the ex brake relay(coil and common contact pin 30) and to the TCU EX brake request input. The TCU provides a path to ground to energize the ex brake relay. Pin 87A(de-energized pin) on the ex brake relay is the output to the ex brake Via the other contacts in the rocker switch Set to the ex brake position. The TCU, if it authorizes the ex brake sends a signal to the ABS, and opens that ground path to the ex brake relay, de-energizing it and sending the ECU ex brake power to the ex brake solenoid turning on the ex brake.

Yes. I can hear the engine trying to spool up, but it's retarded. After about 30 seconds, it automatically comes out of the retard mode and all is back to normal.

 

[Ronmar]

Wonder if your butterfly is sticking? Do you get a check engine light? What does “limp mode” mean? Regardless of wether the switch is in the ex brake position, as soon as you switch it off or touch the accelerator, the ex brake butterfly should immediately swing open…

got a webcam or wireless camera? You could put a cmer down there to monitor its response time when in operation…

 

[Mario]

Wonder if your butterfly is sticking? Do you get a check engine light? What does “limp mode” mean? Regardless of wether the switch is in the ex brake position, as soon as you switch it off or touch the accelerator, the ex brake butterfly should immediately swing open…

got a webcam or wireless camera? You could put a cmer down there to monitor its response time when in operation…

Let me see if I can remember to take a quick video next time I'm out driving around.

 

[Wingnut13]

Lubricating the pac brake is a yearly maintenance requirement. I forget the reference, but it’s super easy to get too!

 

[Keith Knight]

I finally made it back home and working on trying to wire the exhaust brake to the brake lights on my 2003 M1078. I’ve been in the fuse panel trying to track down the exhaust brake relay that activates the solenoid. The relay labeled exhaust brake only comes on for a moment when it is first engaged. So I can’t use that relay. Or is the exhaust brake solenoid only required momentary power to switch on? Something is keeping the exhaust brake light idiot light in the dash on the entire time? Any help would be appreciated.

 

[Ronmar]

The ex brake light on the A1 receives the same power the ex brake solenoid valve does, so light on means brake engaged. Like I wrote above, That power originates at the ECU. and passes thru K16 contacts and also powers its coil. But it passes thru the de-energized contacts. So relay off = ex brake on. Either the ABS or ECU can disable the ex brake by providing a ground to the relay coil and energizing it and removing power to the valve and idiot light. And of course the ECU controls the source power, and should remove it as soon as it detects the throttle has been moved off idle.

if you turn on ex brake, it should engage, and release if you step on the accelerator pedal. ABS should take it offline if it detects wheel lockup, and the TCU will probably disable it when you drop out of lockup(somewhere in 3rd gear?).

 

[Keith Knight]

Thanks again ronmar, your amazing skills at reading these schematics are far superior to mine.
I’ve been looking at the schematic last night and today and I’m still having a difficult time understanding what’s going on. My simple question (if in fact it is simple) is, is there 12v+or- or 24v+or- coming from relay K16 somewhere that is on when exhaust brake is actively engaging so I can connect a relay to it, that will in turn energizer the brake lights? And if so would it be on 1, 2, 3, 4 or 5 terminal on K16.

 

[Ronmar]

Should be the de-energized relay contact, looks to be pin4 on that relay. It will be 24V.

I have not seen an ex brake config setup to operate the brake lights before...

 

[Keith Knight]

Thanks RONMAR. After searching the internet I get a lot of conflicting answers to that subject. Some motorhomes have it, some fire trucks have it but most trucks / Semis do not. Some say it’s legal some as it’s not!
My opinion is it’s safer for me to let someone know I’m slowing down abruptly. Especially because I’m already the slow vehicle with tailgaters. I don’t want to give them any excuses.

 

[Keith Knight]

Sorry to bug you again, for testing the wires purposes it would be easier to do while parked versus driving and looking at a volt meter while the exhaust brake is working. Would I get the same voltage on pin 4 just by hit the warm up switch to on?

 

[Keith Knight]

Well I hear the solenoid latch when warm up is turned on but K16 relay does not light up momentarily and no voltage on pin 4. So I’ll have to get a passenger and go for a test drive to test the theory.

 

[Ronmar]

Correct, the solenoid valve power when in warmup comes from the neutral relay and only passes thru the switch. Contacts.

If the ex brake light is coming on there is power on K16 pin4(87A on the drawing K16), as that is its source. The light power taps off before the warmup/ex brake switch contacts(right hand side of the above drawing)...

 

[Keith Knight]

Once again RONMAR you amaze me with your skills at reading wiring schematics. Just finished the test drive and you are correct pin 4 has 24v+ during ex brake engagement. Now to connect a 24vdc relay and hook it up to the brake lights. My initial thought was to apply 12+ to the trucks brake light relay coil but I'm afraid that would back feed and act as if I hit the break pedal and disengage the ex brake. So my second thought is connecting the 12+ from the new relay contact directly to the brake lights. Or would that also back feed some how that I'm not seeing?