Engine warning light when PAC brake used

[Jim Houghton]

On our first trip with our 2003 Contessa I noticed the engine warning light would illuminate when using the PAC brake going down a long hill. The warning light will go out as soon as the accelerator pedal is pushed. Only thing that seems unusual is oil pressure seems high on gauge during operation and will actually go up when brake is operating. Any thoughts or concerns?

[Larry Williams]

Is the exhaust brake engaged light also coming on? Maybe that wire is hooked to the wrong light.

[Jim Houghton]

Yes the brake light is on and you can feel the PAC brake working. The warning light comes on after about 30 seconds of activation. If I turn the exhaust brake switch off the warning light will go off after about ten seconds. This corresponds to the oil pressure going back down also. Not sure if related or not. Will have to check oil pressure seems high above 75 cruising but goes up on gauge to about 90 when brake is in use. This may be my light issue or a bad sending unit. I may need to get engine SN and give Cummins a call and see if maybe missed a TSB or has another issue.

[Larry Williams]

There are two exhaust brake lights, one to say the switch is on and one to say the brake is engaged. I assume you meant the engaged light when you said brake light.

[Joel Ashley]

How's your coolant level in the surge tank, Jim?  It may simply be nothing to do with the brake, but the coach angle on a slope may drift low tank fluid enough past the level sensor to trigger the Check Engine lamp, esp. when the pac brake kicks in.  When you hit bottom and the accelerator, the fluid moves back over the sensor.

Joel

[Jim Houghton]

Yes, I do mean the engage light in reference to the brake both are on. I will check the fluid level but it looked just under half once we arrived. Started to get dark and wanted to get everything hooked up. How about the oil pressure if it is high will that also trip a light?

[Jim Houghton]

The coolant level is just above the cold line on the tank. I also checked the engine oil and it is right on the full line. I'm going to try and lube the Pacbrake since it doesn't look like it has seen any lube in awhile and see if it responds different on the way home.

[Dan Child]

Jim,  

What does the tachometer do when the PAC brake is used?  The most common cause, for the check engine light to come on, is that the transmission automatically downshifts to fourth gear when the PAC brake is activated and over-revs the engine.  We would all like to think the computers would prevent this from happening, but it is a very common problem on many brands of motorhomes and engines (both CAT and Cummins).  If this is the problem, an authorized Allison shop can reprogram the transmission computer to not shift down when the PAC brake is activated and leave the shifting to you.

In regards to the oil pressure, I would take it to a certified Cummins service center.  Most Cummins engines that I've had run much lower oil pressure - in the range of 40 - 50 psi.  I have never seen a diesel run as high as you are seeing and my guess is that you have a faulty oil pressure sending unit, but that is truly a guess.

I hope you figure it out and look forward to learning what the solutions turn out to be!

[Jim Houghton]

Unfortunately as the tac goes it fluctuates and is erratic. Did that when purchased. The Aladin shows the trans going to 5th when pacbrake engaged. Nothing sounds over-revved but it's hard to hear that as we know. Making a list of things to fix and trying to prioritize. Engine oil pressure being first along with tach issue. Previous owner said tac did that since they purchased in 2012. Most other issues are small and will be easy fixes. So far enjoying our first trip with exception of the weather.

[Roy C Tyler]

Jim
The oil pressure gauge on my Cummins shows the same as yours.  When I first start it, it runs 75 or higher.  Going down the road it can run from around 60 to 90. After I had the oil changed last time, it does get down to 30 at idle at a stop light but still runs in the 75 range.  The Aladdin shows 55 all the time. It never changes.

[Jim Houghton]

Roy,
Yours is the same as mine right down to the Aladdin system. I would think the Aladdin should report accurate. The Tac on the Aladdin is stable within a few RPM. Forgot that one in reference to the over rev question. It does not show any indication of an over rev when using pacbrake. I'm going to hook up an actual pressure gauge to verify the oil pressure. If it's right then it may not be a factor at all.
Jim

[Larry Williams]

Jim
The oil pressure gauge on my Cummins shows the same as yours.  When I first start it, it runs 75 or higher.  Going down the road it can run from around 60 to 90. After I had the oil changed last time, it does get down to 30 at idle at a stop light but still runs in the 75 range.  The Aladdin shows 55 all the time. It never changes.

My Aladdin shows 55 all the time too. I just assumed that was the failure mode. The dash guage varies.

[Gerald Farris]

There are several models of coaches, especially with a 3126 that do not have an oil pressure input to the ECM (engine control module). On these coaches the ECM data buss reads out a fixed reading at all times.

Gerald  

[Jim Houghton]

Ok on the trip home despite the fact that the Aladdin did not want to cooperate I would have to agree that it may have been an over-rev issue after all, as stated by Dan. It does shift down when activation of the Pacbrake occurs. I did not let it run away as I did on the way out and did not get an engine warning light this time. As far as the oil pressure it idles at around 30psi but shows around 80psi at rpm. I plan to check actual pressure and replace sender if it is warranted. The Aladdin will be a new issue to deal with. Thanks for all the input it got me to look at it more closely and determine the cause and adjust driving habits accordingly.