High Idle switch

[Nick and Joan Peros]

We have a 1999 Beaver Marquis with a Cat C-12 engine.  During our 4 year ownership, the engine has an idle speed of about 700 - 750 rpm.  As such, backing the coach causes heavy brake use to keep it from surging ahead, which in turn bleeds the air in the brake system.  Very frustrating and not fun.  I have inquired with the local Cat dealer if the idle speed can be adjusted and am told it cannot however the info I read is that the C-12 has a preset idle of 600 rpm.   

Is there a high idle switch that possibly is engaged on this coach?  I have read some coaches have a high idle switch but I either don't have one or cannot find it.  Can anyone help on this?

PS: I see in the owners manual there is a provision for high idle using the cruise control switches.  I never use cruise control however is it possible the idle has been set to 700 using the cruise control? 

[David T. Richelderfer]

Our coach, a 2004 Marquis Sapphire, has the Cat C12 505 HP engine.  It does have a manually settable "high idle" function that is manually set UP and DOWN by using the Smart (steering) Wheel cruise control buttons.  I usually set the engine's idle speed up to 900-1000 RPMs shortly after starting the cold engine to facilitate air pressure buildup.  The high idle setting is immediately released by hitting the cruise control OFF button or touching the foot brake.  I don't remember now what the low idle setting is but the 600 RPM number sounds like a good guess for our coach.

[Nick and Joan Peros]

Thanks for your reply.  Our 1999 coach has the cruise control on the left column.  I am wondering if the idle speed is not returning to 600 rpm due to a faulty resume switch.  The coach is in the shop now for other work but I will check it out when I pick it up.  Thanks again.

[Fred Brooks]

   Nick,
The coach is probably at 600 rpm and the tach is off 100 rpm. The C-12 is controlled by the ECM (electronic control module) and is very protective of its parameters. We all deal with the same condition using excessive air when maneuvering in idle conditions such as a campground parking spot. I keep an eye on the air gauge when I am in that situation. As soon as it gets down to 80psi, I pause and put in neutral and throttle it at 1000rpm back up to 120psi. When was the last time you serviced the air dryer and filter? If you do want to use the fast idle feature that Richard was talking about. Turn the cruise control to the on position, raise the rpm to the desired level (1000 rpm) then engage the resume switch one time. To cancel it just step on the brake pedal. Hope this helps, Fred

[Carl Boger]

Nick,

If your coach is like my 98 patriot you will have to turn the Cruise Control on and then hit the  set button to set it to high idle.  After that you can use the resume button as long as the Cruise Control has not been turned off. 

Like Fred was stating be careful with your air pressure when parking or using the brake often.  I have been there done that where I suddenly had no brakes since the pressure dropped to much.  Now it is slow and steady, applying the parking brake and shifting to neutral in needed to rebuild pressure.  Its one of those things you have to learn, and after scaring myself  I started to learn quickly.

[Nick and Joan Peros]

Thank you Carl, Fred, and David.  I appreciate your replies so much.  As for the question: "When was the last time you serviced the air dryer and filter? " I have not serviced them but will do so now.  Much to learn here.  This is our first RV and we have learned as we progress.  I drove the coach back from the service shop back to our storage place.  At idle, it ran a solid 700 rpm.  600 rpm would have been so much more comfortable.  It is nice to know others have experienced the same thing.  Again, thank you!!

[Eric Maclean]

Nick
With a short search I came across a thread talking about C12 idle speed and alternator charging issues caused by a 600 RPM idle it appears that Cat has deliberately set the idle too 700 to cure this problem.

http://beaveramb.org/forum/index.php?topic=5505.0

Hope this helps
Eric

[Dave Atherton]

Nick, there is a caterpillar service bulletin about your problem on alt not charging battery’s.
Yes increase rpms to 725 rpms will correct the charging problem. Dave Atherton Retired Cat
Mechanic

[Joel Ashley]

But Nick still has a control issue maneuvering at low speed, like backing into a site with a jumpy throttle.  Is there an alternator that charges at 600rpm idle so he could adjust it down?

[Lee Welbanks]

NIck, My 06 PT with the C13 idles at 700 can't do much about that. Have you done a brake bleed down test, sounds like you have some air leaks on your service (applied) side if you are loosing air that fast.
Build up your air pressure to cut off (120) parking brake released, engine off, apply your brake foot pedal medium and see how fast you loose air pressure after you apply the pedal you should not loose more than 2 psi per minute.
On my coach every air hose from the valves to the brake cans leaked, every one of them because they (Monaco) used that crappy braided air hose..
https://emergencyresponse.spartanmotors.com/wp-content/uploads/2019/09/MJM-2019-DOT-air-leak-down-test.pdf