LMTV FMTV Exhaust Brake Questions?

[Keith Knight]

I have a 2003 M1078A1 the button labeled warmup/off/retard.
I’ve just realized hope wonderful that switch is when switched to retard. That is how you activate the exhaust brake. I’ve never driven a vehicle with this feature and love how well it works and will save my brake shoes from excessive wearing especially on down hill descents. But my true question is does anyone know if when it is turned on and after I take my foot of the throttle the exhaust brake activates and the light in the dash comes on that says “EXHAUST BRAKE” when this is occurring does it also illuminate the brake lights? I just don’t want to get rear ended. Also note that as soon as I press the brake or accelerator the light goes off and everything resumes normal operation.

 

[Mullaney]

I have a 2003 M1078A1 the button labeled warmup/off/retard.
View attachment 892511I’ve just realized hope wonderful that switch is when switched to retard. That is how you activate the exhaust brake. I’ve never driven a vehicle with this feature and love how well it works and will save my brake shoes from excessive wearing especially on down hill descents. But my true question is does anyone know if when it is turned on and after I take my foot of the throttle the exhaust brake activates and the light in the dash comes on that says “EXHAUST BRAKE” when this is occurring does it also illuminate the brake lights? I just don’t want to get rear ended. Also note that as soon as I press the brake or accelerator the light goes off and everything resumes normal operation.

.
Exhaust Brake doesn't light up your brake lights.

 

[Keith Knight]

Well that sucks, how did you figure that out?

 

[Mullaney]

Well that sucks, how did you figure that out?

.
Only because the brake lights don't light up when several of us are rolling down the road together.

 

[GeneralDisorder]

Well that sucks, how did you figure that out?

Schematics. But you could make it work the brake lights if you tapped into the brake light switch circuit and borrowed the exhaust brake solenoid signal - might be a relay involved to ensure you don't pull too many amps from the exhaust brake circuit. I would have to consult the schematics on the exact configuration but it would definitely be possible.

It would potentially light up the brake lights in warmup mode as well. But technically you are not supposed to drive the truck with it in warmup so I'm not really seeing the harm in it.

Note that the engine ECM is involved in exhaust brake operation - when you turn on cruise control the exhaust brake only operates when you press the brake pedal.

 

[Keith Knight]

Thanks, My mind was already going down that path of how I could make that happen.
I’ve also discovered the second part is true. When cruise control is on a slight tap on the brakes disengages the cruise and starts the exhaust brake operation. I’ve also found if if I turn the cruise control to off it will also start the exhaust brake operation. It works very well. Too bad they didn’t think to apply brake light operations to it. But if there is a will there is a way.

 

[Ronmar]

I have driven several vehicles with ex brake and none have operated the brake lights.

No ex brake vehicle I have ever driven has ever lifted the EX brake when you press the brake pedal, only when you press the accelerator... you should be able to use the two brake modes together...

I will.have to look at the diagram but if I recall, warmup interacts with the neutral circuit...

 

[Ronmar]

I have driven several vehicles with ex brake and none have operated the brake lights.

No ex brake vehicle I have ever driven has ever lifted the EX brake when you press the brake pedal, only when you press the accelerator... you should be able to use the two brake modes together...

I will.have to look at the diagram but if I recall, warmup interacts with the neutral circuit...

Yep, when you switch the switch to warmup, it sends power provided by the neutral relay directly to the ex brake solenoid and the dash light…

 

[NY Tom]

I have seen buses with extra yellow lights that come on with the exhaust brake.
Nice feature for other drivers that are aware enough to know what that means.
Never saw it on a truck before. Probably because most pull standardized trailers that would not have this lighting.

 

[Third From Texas]

Never saw it on a truck before. Probably because most pull standardized trailers that would not have this lighting.

Me neither.

And keep in mind that exhaust braking is illegal in a lot of US cities, towns, bypasses, etc near a population. Such zones are typically well marked. Typically.

I don't think the laws stipulates between semis, RVs, on military trucks.

 

[Keith Knight]

Thanks I’ll keep that in mind.

 

[Mavcaster]

Me neither.

And keep in mind that exhaust braking is illegal in a lot of US cities, towns, bypasses, etc near a population. Such zones are typically well marked. Typically.

I don't think the laws stipulates between semis, RVs, on military trucks.

I think maybe you’re getting confused with jake brake. The exhaust brake shouldn’t be loud like a jake brake that uses valves for compression braking in the engine.

 

[Ronmar]

You probably should clarify that a little. Jake brakes are illegal/restricted in many places because they are obnoxiously noisy when they start popping their pressure relief(BAA, BAA, BAA,BAA, BAA)… the signs I am used to seeing are worded something like this: “Unmuffled Jake brakes prohibited beyond this point“ Helps to keep our sleepy little logging towns still sleeping when the log trucks start rolling…

An exhaust stream brake like the pacbrake makes no real additional noise As you are simply restricting the already contained and muffled exhaust. And since you are not even adding any fuel(vehicle is noisier while accelerating)… They are fitted on many production diesel pickups(50 state legal). Last job I had we had 2 new 2500HD trucks with duramax and allison drivetrains. You could not tell from outside the vehicle and it made no additional sound inside when it was online, but you could sure feel its effect. Ran it ll the time, hardly ever touched the brake pedal…

 

[Ronmar]

I think maybe you’re getting confused with jake brake. The exhaust brake shouldn’t be loud like a jake brake that uses valves for compression braking in the engine.

Wooah… Stereo…

 

[Mavcaster]

Wooah… Stereo…

We’re on the same wavelength yeah. I also have a Duramax with turbo brake and it’s great. Only sound it makes is when you tap the accelerator and you can hear the vanes open back up.

 

[Third From Texas]

Thanks for the correction.

 

[Keith Knight]

That’s the experience I’ve seen. I hear it inside the cab but I’ve been watching for evidence of how load it is to people near by on side walks or bikes any time I’ve been near someone they never reacted as if there was no increase in noise. I kept expecting them to turn their head as if I blew the horn off but no reaction is a good thing. Thanks for the info, now I need to get to wiring the brake lights up to it.

 

[tennmogger]

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?

 

[Wingnut13]

For the little bit of time I’ve used mine, and lightly loaded it was very effective.

 

[Keith Knight]

I just finished my conversion to a full blown motorhome and with full diesel, full fresh water, full 50 gasoline tank for generator back up and all of our gear. I weighed in at 26,800 lbs and I have just driven about 1,100 miles in 10 days from Florida to central Georgia and it works great only need to use the brakes when I don’t judge it properly but I’m getting better each day.