BAC Forum
General Boards => Technical Support => Topic started by: Lonne Mays on September 21, 2019, 01:35:42 AM
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I'm trying to troubleshoot the exhaust brake (Pacbrake) control wiring on my 1999 Contessa (Magnum Chassis). With the ignition off, I can cause the Pacbrake solenoid to function by applying +12V to the wire labled "exhaust brake" (which is hooked to the wiper terminal (30) of the relay labeled "Endurance Brake"). With the ignition on and the exhaust brake panel switch turned on, there is always +12V on this wire, but the Pacbrake solenoid is not activated because there has to be another relay in series with this wire that is controlled by either the CAT 3126 ECU (the ECU will use this hidden relay to prevent activation of the exhaust brake solenoid unless the throttle is at/near zero (no fuel being injected into cylinders) and the engine RPM is above 900rpm) or the Allison WTEC transmission ECU. I am trying to find the location of this ECU controlled relay in the exhaust brake wiring circuit. Does anyone know where this relay is? Does anyone have a schematic or wiring diagram of the exhaust brake circuitry for this Magnum chassis (or any Beaver of or near this vintage)? I could reverse engineer the schematic if I can just physically locate the relays involved, but I have only been able to find the "Endurance Brake" relay and the "ABS Retarder" relay (both in the front wiring bay). Hopefully someone knows where the other exhaust brake circuit relays are located and can point me in the right direction. I have attached a jpg file showing what Cat says should be typical wiring for exhaust brake control, but the wiring I'm seeing on my Beaver appears to be much different.
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I have run a couple of wires to the coil labeled "Endurance Relay" and monitored it by a meter while driving the coach, but it does not activate under any condition whether I have the exhaust brake switch on or not. Setting the exhaust brake switch ON does activate the transmission downshifting when I brake, so at least that part of the system is working. Tomorrow, weather permitting, I will climb under the back of the coach and trace the air line from the exhaust brake to locate the actuating solenoid, and from there start tracing the wire that goes to the solenoid to see if I can discover where they hid the relay that is controlled by the ECU. Wish me luck.
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Lonne,
It might be easier to use a wire toner hooked to the switch to trace where the wire goes. I'm convinced that the relays are in the electrical bay. Just need to ID them.
Good Luck,
Steve
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Hi Steve,
Good idea on using a signal injector and tracer on the line (I actually have one, so I will use it). I have checked and identified all the relays in the front bay, and the only ones associated with the exhaust brake are the "Endurance Brake" and "ABS Retarder" relays. The former supplies +12V from the wiper (30) to the normally closed contact (87a) which feeds a wire labeled "12V signal to ECU". The coils of both of these relays are hooked to the same control line so that when the "Endurance Brake" relay coil is energized the "ABS Retarder" relay coil is also energized.
Everything I have researched from websites detailing Cat's recommended wiring circuits and Pacbrake's recommended wiring circuits suggests that there must be relays to coordinate the Pacbrake operation so that Cruise Control control and ABS operation are disabled during Pacbrake operation, and that either the engine ECU or the transmission ECM will prevent the Pacbrake from energizing unless the RPMs are sufficient and the throttle is at zero.
I will let you know what I find out as I trace the wires.
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Lonne, On my '05 Patriot, the cruise controls are part of the 'smart wheel'. There is a 'smart wheel' relay box in my front run bay and according to the schematic posted here, the smart wheel (steering wheel) controls the cruise, headlights, wipers, etc all through this relay box. The smart wheel, of course, is just an update to what we used to call a 'clockspring'. You might have an issue with the clockspring, which used to use metal wipers on a foil trace as contacts. Now a clockspring is just a ribbon cable with connectors at both ends.
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Attached is a picture of the layout in my front electrical bay. The wire labeled "exhaust brake" goes to a relay labeled "endurance brake". The coils of the endurance brake and ABS retarder relays are in parallel. I've done a lot of research on the Pacbrake and Allison Transmission websites, and the information I've found says that it is the Allison WTEC transmission that actually controls the Pacbrake, and that with the ABS system installed, the transmission ECU coordinates all control of the Pacbrake and ABS systems. (This would explain why the "Endurance Brake" relay and "ABS Retarder" relay coils are controlled by the same line.)
I'll continue my troubleshooting and tracing out of the wiring to locate all the relays and/or switches in series with the Pacbrake solenoid. Once I have located them all, I'll be able to find out where the control signal is being dropped.
P.S., I've also discovered that the mystery ABS switch on my dash enables me to retrieve the WABCO ABS system diagnostic blink codes, and have found and downloaded the instructions for this (I'll post these in a new post, as I'm sure I'm not the only one looking for this info).
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Lonnie:
I think I can help you. I have a one page "A" size diagram titled "Exhaust Brake", Dwg # PTE04, released on 2/25/99 for a Patriot.
I am amazed at how far you are getting with your troubleshooting. I happen to be an EE and this schematic/wire #'s/component diagram seems to have many things that manage that pac brake.
You are welcome to call me (408) 515-5957 and maybe I can just talk you through the interfaces which are many including the Allison ECU and the CAT ECM, Endurance Brake, the Allison VIM and ultimately power finds its way to the dash exhaust light and the exhaust brake solenoid. Oh yes, the brake switch and cruise control are also involved.
I don't know how to get a copy to you easily and quickly but I can shoot a photo, put it on my pc and attach it and send it to you. I want to be sure if I send you this page that it is large enough to easily print out on a "A" size sheet of paper.
Remember, your coach will likely be somewhat different but I'll bet not by much. I'm 37 feet and I think you were longer so the brake circuits could be different as well.
Larry Fritz
98 Patriot for past 20 years
GO HUSKERS from Nebraska
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Thank you, Larry Fritz, for the Exhaust Brake circuit schematic from the 98 Patriot. This looks like it is very similar to the wiring in my 99 Contessa. I have taken the photo Larry sent me via phone text, and have annotated it with the location of the K8, K9, and K6 relays (Endurance Brake, ABS Retarder, and Cruise Control). This is going to help me a great deal in my troubleshooting.
P.S. I have modified the drawing to show the as-built wiring I have traced out so far in my 99 Contessa. There are a few significant differences with the Patriot, like the fact that the dash Exhaust Brake light is driven directly from the 205 line wire, and so does not actually indicate that the exhaust brake solenoid has actuated. I am still trying to find the physicall location of the "ALLISON VIM" relay, which actually controls the Pacbrake (exhaust brake).
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I have located a wiring schematic for an Allison WTECIII TCM and VIM, and have annotated it (with color and labeling) to show the exhaust brake (Pacbrake) associated circuitry. I added annotation to indicate how my 99 Contessa is wired. The TCM (Transmission Control Module) and VIM (Vehicle Interface Module) are both located in the shift console built onto the wall and floor to the left of the pilot's seat.
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Lonnie, May have information of interest with you pac brake problem. Your engine ECM control
module when program to latch mode, the engine brake stays enabled after service brakes are
released. Direct immediate pressure on the brake pedal latches the engine brake in the on position
the engine brake will remain in the on position until the accelerator pedal is depressed.
When Exhaust Brake Mode is Programmed to manual mode, the engine brake operates independently
of cruise control status. The engine brakeoperates with the application and the release of the service brake in the coast mode. The engine brake turns on with the application of the release of the service
brakes in the latch mode. The engine brake stays on even when service brake are released.
Have wire circuit schematic for exhaust brake that is powered by relay circuit. Will send to Steve
If he can post. Dave Atherton Retired Cat Mechanic
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Hi Dave, Attached is the exhaust brake wiring diagram (from a similar vintage Patriot to our Contessa), which I have modified to reflect the wiring differences I have traced out so far for our 1999 Contessa. I have also added to the drawing an LED indicator (on the actual line that goes out to the exhaust brake solenoid), and a manual override button I am adding to enable me to activate the exhaust brake even if the transmission's VIM relay is disabling activation.
I have been able to verify that setting the exhaust brake switch to the ON position deactivates the cruise control. When I am driving downhill and put the exhaust brake switch ON, the transmission will downshift to a lower gear as soon as I take my foot off the accelerator, which slows the vehicle some. With the LED indicator I added I will be able to verify whether or not the exhaust brake has actually applied. (Until I can confirm that the exhaust brake solenoid is getting its signal, I am not convinced that I'm getting the exhaust brake activation, as I do not get the amount of speed reduction I was hoping for on long steep grades.)
I also added a freewheeling diode/surge suppressor to the solenoid drive coil circuit, just in case Beaver did not (as I don't see one on the drawing). Adding a diode across a solenoid coil will add a few milliseconds to its drop-out time, but will prevent high voltage spikes from being generated when the coil current is commutated.
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Hooray! I have confirmed that my exhaust brake (Pacbrake) is working properly. The LED indicator I installed showed me that the exhaust brake solenoid, and thus the exhaust brake itself, is actuating when it should.
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Looks like there was never a problem with my exhaust brake circuitry. I started this in-depth investigation of the exhaust brake circuit because I was expecting much more effect from the exhaust brake on long 7% grades. With the exhaust brake switch ON, and going down such long grades, I have found that I have to keep the motorhome in 2nd or 3rd gear and the speeds under 40, and still need to use the service brakes (braking moderately hard for 3-5 seconds, then let the speed climb up 5 to 6 mph before stepping hard on the service brakes again for three to five seconds). Therefore I was not convinced that the exhaust brake (Pacbrake) was actuating when going down these long grades.
I have tested and observed the Pacbrake swing closed when the exhaust brake solenoid line is energized, and now know that the Pacbrake is actuating when it is supposed to. I guess that I was just expecting too much effect from the Pacbrake, i.e. that I would not have to also use the service brakes to keep my speed under control when going down long steep grades.
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Lonne,
I had a Pac brake in my 01 Contessa, a Jake in my 00 Marquis and an exhaust brake in my 07 Contessa. All kicked the tranny into 2nd when they were active. Some of the members (mainly w/ C12s) had the brake reprogramed to drop to 4th instead of 2nd to avoid the high RPMs caused by the drop to 2nd since CAT recommended not reving the C12 above 2100 (?). If yours doesn't drop to 2nd, it may have been reprogrammed.
Steve
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Our 2004 Marquis would drop to 4th gear from 6th gear when the Jake Brake actuated. We had the transmission reprogrammed at the Pasco, WA Kenworth shop to not shift down with the Jake Brake actuation. Now it stays in 6th gear until I manually shift it down. On our first trip (away from the firm where we purchased the Marquis in 2012), as we crested the Santiam Pass West of Sisters, OR, it shifted automatically from 6th to 4th and I thought the engine blew up. It wasn't supposed to shift to 4th from 6th if doing so caused a too-high-of-RPM event. I immediately shut off the Jake Brake, shifted up to 5th, and after reducing speed shifted to 4th... and proceded down the grade in my control.
After the reprogramming, I don't have to worry about that happening again! As a retired airline pilot friend of mine said, software should never be allowed to override the pilot's control of the plane. That's what was happening to the Boeing 737 Maxi airliner and it caused many deaths.
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On my trip last week (my first trip into the Blue Ridge mountains) I used the exhaust brake extensively. I hardly ever touched the brake pedal. I am impressed with how well it works.
Mine activates when you let off the throttle pedal. The Aladdin shows the "Gear selected" as 4 (I think) but transmission wont downshift unless the road speed is in the proper range for the downshift to keep engine from over revving.
So I would have the transmission shift pad in 6th gear, let up on the throttle and the engine brake would start slowing coach - but most of the time it did not downshift. If I needed a little more speed reduction I would step on the brake, and then it might downshift. Our east-coast mountains are just hills to you west coast guys.
Also, I think it goes from 6th to 5th then to 4th (doesn't go straight from 6 to 4th) even if the "programming is set for 4th gear", which makes sense to me. I cant recall any harsh downshifts.
I'm not sure if it skips 5th or not. I'll have to pay more attention to that on the next trip.
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Lonne, Glad you confirmed all is as it should be, thanks to your LED. I'd like to have a dash indicator to tell me my cruise control is activated; If I switch screens on the Aladdin, yes I get a Cruise On indication, but a glance at an LED in the dash would be safer.
Steepest grade for us so far was Teton Pass, between Jackson Hole Wy and Victor ID. 10% grade. I did at one point, have to brake and downshift to slow, all the while using the Jake brake in HI (6 cyl) position. That brought us down to 15 or 20 mph toward the bottom (east side) of the hill, the tight corners required a reduction in speed, not the grade.
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As info.
I went down a 14% grade from Torrey, UT on UT SR-12 to Escalante, UT (around 65 miles). There are no guard rails, easily 800+ drop offs from the curbside wheels, and a white knuckler for 40 miles of the trip. I think it took us 3 hours. Hundreds of switchbacks. Barely touched the service brake the entire way down. Kept it at 25mph the best I could most of the time. My Pac brake downshifts to 2nd gear when it is engaged.
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I have found it much easier and quicker to use the Aladdin screen for the engine and transmission vital statistics, plus the cruise control settings (on/off and selected speed setting). The Aladdin screen is in plain sight to the left of the steering wheel is not blocked by the steering wheel as are the dash gauges. It's the same with my Silverleaf screen; it shows several engine and transmission statistics, plus my twelve tire TPMS pressures... and the Silverleaf screen is in plain sight to the right of the steering wheel. Also, the Silverleaf screen shows a green, yellow, or red letter "J" when the Jake Brake is engaged, depending upon the level of braking. I rarely change the screens from where they're set after I start a day's drive.