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Exhaust Brake Coasting Relay

Posted 11-27-2015 at 02:27 PM by ladagobago
Updated 11-28-2015 at 08:32 AM by ladagobago

The down shifting of the trans can mask an improperly operating EB cylinder action. By observation you can determine the piston / PAC operation is normal by the deceleration of the MH. Down shifts when the circuit is activated are gauged by the distance it takes to stop using telephone poles as a measure. Typically they are spaced 100 feet apart. At 50 mph we can slow to 20 mph in about 250 feet with the PAC. note: (it was cheaper to get it, the piston, at Freightliner than a Cummins)

Several good reasons to install a brake relay
Freightliner engine braking or exhaust braking switches are found on the dash board and are DSDP switches. It is either ON or OFF. There associated lights to indicate on the instrument panel when it is on. Freightliner installs this manually operated switch and the driver must select to either have it ON or OFF by moving the switch from one position to another.

It was my desire to have an AUTOMATIC CONTROL of the exhaust brake over the MANUAL CONTROL. When you only have the factory option here is the sequence of operation. With the switch off, there is no activation of the exhaust brake when the brake pedal is depressed. With the switch on, the exhaust brake activates as soon as you remove your foot from the throttle pedal. This effectively slows you down faster than needed when all you wanted was to stop accelerating or coast to a slower speed and that feature is not available.. Often it happens when you really do not want the exhaust brake activated - so you turn the switch off and now when you need it – the exhaust brake switch is off and so is the activation. BANG – CRASH OR FUMBLE FOR THE SWITCH.

If you have your MH in auto cruise the effect of the switch on is even more alarming since if you gain speed going down a slight hill, the engine brake will be activated and try to slow you down and so now your driving tempo and temper is at it’s limit so you again are tempted to turn of the dash switch.. Every time you turn off the switch you have less driving safety because you have BYPASSED the braking capacity of the system.

Once you have made the decision to implement the Brake Relay interface and followed the directions below you will have completely automatic engine braking. Now when you remove your foot from the throttle pedal your MH will coast down the road. When you select cruise control and are going down the hill you will coast. As soon as you decide use the brake pedal the exhaust brake circuit will active and you will get the full force of braking effort and normal downshift you would expect. One very nice extra to my circuit is the flexibility of having a foot switch to bypass the relay an revert to standard exhaust brake activation. There are times when you want full continuous brake assist.

If the road is slick follow the manufactures recommendations and turn off the dash switch to completely deactivate exhaust brake operation.

The control of the PAC is by way of the Dash Board Switch for ON/OFF operation. With the switch in the ON position the operation of the PAC engagement becomes frustrating as it engages as soon as you begin to decelerate as in removing your foot from the accelerator or in cruise when starting down a grade. This may cause you to have to turn off the dash switch to reset your cruise or regain your original speed. In either case some just prefer to turn it off. The next scenario is that you want to have it on to stop fast and reach down to turn it back on and at night that might just not be easy. It might even be TOO LATE !

Rather than constantly reaching for the control switch I have created my own version of PAC/CRUISE control with a few low cost parts and some wire modifications. Once installed you will never want to go back to the factory method, BUT if you want too do so, you can with the depression of the foot switch. It is accomplished with nothing more than a two splices and a simple relay.

Although similar retail product was available a few years ago, the product cost was $79 as it can be put together for less than $15. There are a variety of relays that can be used, I used an end of line fire alarm relay, 12-24 vdc, 3 amp contact plus I purchased a foot switch (like the old fashion head light dimmer or starter switch, (on/off) so I never have to touch the dash for normal operation.

Operation is -with exhaust brake on and when letting off accelerator pedal, coach will coast. If in auto cruise down hill coach will coast. To activate PAC relay - touch brake pedal and release and PAC activates and begins down shifts. Of course, if this is an emergency stop - keep your foot on that pedal hard.

If you decide not to slow down or stop simply depress accelerator and PAC turns off till next event. If you were in auto-cruise speed control, simply pull the cruise switch to resume previous speed. What a wonderful addition and all from a spare relay.

Below is the drawing for modification, of the dash mounted exhaust brake switch allowing for smoother driving experience but still having the option of an instant exhaust brake activation.

�� let up on pedal and coast, trans will not down shift and PAC will not operate, the dash light will on. Press on accelerator pedal to continue. Going down hill vehicle will coast unless you wish to slow down.
To Slow down - Depress brake pedal and relay will make contact and activates PAC and Downshift
�� If ready to continue acceleration simply depress fuel pedal and PAC will release till next event.
�� If you were in cruise then depress fuel and pull the cruise switch to resume previous speed.
�� Alternative is to depress the foot switch (optional) and will return to normal PAC operations.
�� Normal may be preferred in some traffic conditions.
�� The computer will not allow your rig to have both brake and fuel at the same time.
This arrangement is for a 99 Freightliner Winnebago and may not function the same in other MH.
(created by Jerry , 99 Chieftain, )
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File Type: pdf Here is the setup for the PAC brake.pdf (149.7 KB, 238 views)
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  1. Old Comment
    Randm Musing's Avatar
    With the switch on, the exhaust brake activates as soon as you remove your foot from the throttle pedal. This effectively slows you down faster than needed when all you wanted was to stop accelerating or coast to a slower speed and that feature is not available
    That is exactly the frustrating "feature" that I have. As soon as I get my rig operational, that was one of the first things that I intended to address - by developing just the setup that you have. But since you have led the way .... Thanks.
    Posted 08-27-2017 at 11:34 AM by Randm Musing Randm Musing is offline
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