LMTV FMTV Exhaust Brake Questions?

[Mullaney]

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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Not that I know of - more specifically - my trucks do NOT turn on the brake lights when the PAC or Jake Brake is on and you back out of the throttle.

 

[hike]

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.

I keep thinking this situation is more appropriate for hazard lights, not brake lights—

 

[chucky]

I can see the military not wanting lights to come on in the wrong time or place ! But in commercial veh since early 80s has made the brake light come on when jacobs (jake) brake were in use because for those of us that have had to run harder and faster than L.E would like us to lots of us ran with the jakes turned on at night ! So we would either put the brake light on a toggle switch on the dash or pull the relay to keep the lights from coming on when we would get hit on our radar detectors were trying to drop speed fast without lighting the world behind us up at night and looking guilty so if you were meeting a radar cop your hoping he shoots someone out in front of you so you can get slowed down and stay dark ! And if he shot you he had you but if hes slow on the lock button and hes watching you you might get that speed down enough to not make them mad and if they didnt see brake lights they didnt think we were using radar detectors !

Also over use of jakes is not good for any engine so word to the wise theyre there for you to use to help not over heating brakes on steep grades .


When CAT went from A model 3406 to B model one of the main updates were to put a better top ring on the piston to hold up better to use of jake brakes way back when people were just bolting jakes to any motor without changing top rings were baking theyre motors

 

[Piatheem]

I have a 2004 a1 and my retard function stopped working. I’m trying to see how it should work so I can properly diagnose. Here is what I observed:
- the warmup feature works, I can hear it and light comes on, I’ll visually confirm if the valve moves.
- when I switch retard on while driving nothing happens, the throttle pedal is released. The exhaust light briefly flashes once.
- it did work a few times. The truck shifted into 3rd gear. Is this expected ?

 

[GeneralDisorder]

I have a 2004 a1 and my retard function stopped working. I’m trying to see how it should work so I can properly diagnose. Here is what I observed:
- the warmup feature works, I can hear it and light comes on, I’ll visually confirm if the valve moves.
- when I switch retard on while driving nothing happens, the throttle pedal is released. The exhaust light briefly flashes once.
- it did work a few times. The truck shifted into 3rd gear. Is this expected ?

Are you moving fast enough? Third gear won't necessarily be going fast enough to authorize the exhaust brake. It does stop operating below a certain speed. The engine computer and transmission computer ultimately are the gatekeepers - the engine computer has to control the exhaust brake and you have to be going X speed and you have to be in converter lock which means 3rd gear or following a locked downshift to 2nd while decelerating.

The light being off means the computer is not commanding it. If the light were on but the brake was not operating that would indicate a problem but the light being out means the computer is not currently asking for exhaust brake. So either the inputs/conditions are not correct or there is an intermittent connection in the circuits causing it to drop out. The light does not come on for warmup mode. Does warmup mode disengage if you put the truck in gear?

 

[Piatheem]

Are you moving fast enough? Third gear won't necessarily be going fast enough to authorize the exhaust brake. It does stop operating below a certain speed. The engine computer and transmission computer ultimately are the gatekeepers - the engine computer has to control the exhaust brake and you have to be going X speed and you have to be in converter lock which means 3rd gear or following a locked downshift to 2nd while decelerating.

The light being off means the computer is not commanding it. If the light were on but the brake was not operating that would indicate a problem but the light being out means the computer is not currently asking for exhaust brake. So either the inputs/conditions are not correct or there is an intermittent connection in the circuits causing it to drop out. The light does not come on for warmup mode. Does warmup mode disengage if you put the truck in gear?

That would explain it, I was going probably 30mph. The warmup does turn on and off. My RV also had an engine break, but it worked differently, it did not switch transmission and it worked at any speed, so I am a little confused by how it's supposed to operate on an LMTV.

 

[GeneralDisorder]

That would explain it, I was going probably 30mph. The warmup does turn on and off. My RV also had an engine break, but it worked differently, it did not switch transmission and it worked at any speed, so I am a little confused by how it's supposed to operate on an LMTV.

Operation of the exhaust brake is largely the responsibility of the engine computer. There are a couple options that can be changed in terms of how it interacts with cruise control for example.

The ABS computer also knows about the "retarder" and needs to be aware when it's in play so there's relay logic involving that module as well.

With reduction hubs the exhaust brake typically works down to about 10 mph. With ECO hubs it's more like 20 mph.

Try going a bit faster and report your findings.

 

[Ronmar]

Yea the TCU will drop out of exhaust braking if it senses an impending stop, the same as it drops out of lockup. a member here posted a video showing his holding ex braking(and i am assuming TC lockup) down into 2nd, but I suspect he was decelerating VERY slowly for it to maintain that situation.

 

[Piatheem]

Operation of the exhaust brake is largely the responsibility of the engine computer. There are a couple options that can be changed in terms of how it interacts with cruise control for example.

The ABS computer also knows about the "retarder" and needs to be aware when it's in play so there's relay logic involving that module as well.

With reduction hubs the exhaust brake typically works down to about 10 mph. With ECO hubs it's more like 20 mph.

Try going a bit faster and report your findings.

Spent some time diagnosing the engine break, there was a massive air leak at the pac break. The fitting was very loose. Once leak was fixed, the engine break started to engage. I have ECO hubs and it looks like it only engages if I am going faster than 28 mph.

 

[GeneralDisorder]

Spent some time diagnosing the engine break, there was a massive air leak at the pac break. The fitting was very loose. Once leak was fixed, the engine break started to engage. I have ECO hubs and it looks like it only engages if I am going faster than 28 mph.

Correct. And it stays engaged down to around 20 or less if it can remain locked in 2nd (trans computer decides).

 

[Ronmar]

Yea what General said…. put some weight on and try it down hill empty truck on level ground will slow down and drop out of EB sooner…

 

[GCecchetto]

From a post a made a long time ago in the ECO hubs thread. I highly recommend the settings outlined below.

I know this is the Eco Hubs thread, but since we had some lengthy discussion here about my feelings that an exhaust brake was critical for mountain driving when you have Eco Hubs installed I thought I would post a follow up. So, back to the exhaust brake conversation. Since I have now installed my new PacBrake, Switch, solenoid, and associated air lines, I've now gotten focused on changing the way the exhaust brake functions to make it something that is useful in a truck that is now capable of cruising 75mph on the highway.

There was a previous post that identified that there is a "target gear" setting in the TCM. I have the Allison software, but can't use it as it needs to be activated. Way too expensive to buy, $1100 and then $590 each year to keep it active. So I'll have to find a commercial Allison tech here locally to change the target gear. Where this gets exciting though is that there is even more useful changes that can be made in the ECM with CAT ET, which I also have. These improvements require that you also have the ECM setup for cruise control. Here's what I (I means my daughter who is a computer genius have done and it makes the exhaust brake function exactly how I want it to and essentially the same as the exhaust brake in my 2014 Ram.

In CAT ET you go into the cruise control menu and set the cruise control to "Latched". There is also a setting called something like "auto retard threshold" The lowest value this can be set to is 3 mph. We had to shut the truck down and restart for all of the changes to take a affect. Now with these changes you have three different exhaust brake modes based on the status of the cruise control.

Exhaust switch on/cruise control switch off - in this mode the exhaust brake engages when you lift your foot off of the accelerator pedal. The downshift to the target gear happens in this mode.

Exhaust switch on/cruise control switch on but cruise not set - in this mode the exhaust brake engages when you press the service brake and stays latched on whether you are on the brakes or not until you depress the accelerator pedal. The downshift to the target gear happens in this mode.

Exhaust brake switch on/cruise control on and set - in this mode with the cruise set when you are coasting with your foot off of the accelerator pedal, the ECM automatically modulates the exhaust brake to maintain your set speed. When your speed reaches the threshold value you set over the set cruise speed the exhaust brake engages and stays latched until your speed is back withing the threshold range you set. In my case the threshold is set at the minimum 3 mph value. In this mode, the trans does not downshift to the target gear.

Hope you find this info useful. For me this is a MASSIVE improvement in the exhaust brake function and will be even better once I have the target gear changed.