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Old 09-16-2012, 11:26 AM   #1
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Yellow Park Brake Switch

I have a gasser and would like to know how the yellow park brake switch controls braking. It does work but I don't how it does it. This is a '98 Dolphin on a Chevy 454 chassis.

Anyone know how this thing works?

Thanks,

John
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Old 09-16-2012, 11:36 AM   #2
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So you are a gasser. Do you have air brakes????? Normally on a air brake system the yellow parking brake knob when releases air into a secondary "spring chamber" on the rear air brakes that then allow the springs to apply the rear parking brakes. Also, if you lose air pressure then the springs apply the rear parking brakes at around 30 psi and lock up the rear wheels. On some National motor homes there is only a parking brake on the driveshaft just behind the transmission and the yellow knob cuts off the air to that so it is applied. You can see what looks like a brake drum that the driveshaft is connected to and is behind the transmission.
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Old 09-16-2012, 11:53 AM   #3
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Quote:
Originally Posted by Mike Canter View Post
So you are a gasser. Do you have air brakes????? Normally on a air brake system the yellow parking brake knob when releases air into a secondary "spring chamber" on the rear air brakes that then allow the springs to apply the rear parking brakes. Also, if you lose air pressure then the springs apply the rear parking brakes at around 30 psi and lock up the rear wheels. On some National motor homes there is only a parking brake on the driveshaft just behind the transmission and the yellow knob cuts off the air to that so it is applied. You can see what looks like a brake drum that the driveshaft is connected to and is behind the transmission.
No air brakes and no compressed air of any kind.

I had a Winnebago before where the brake drum on the drive shaft was set by the gear shift in PARK. I know because it it had a lot of problems.
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Old 09-16-2012, 09:16 PM   #4
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Quote:
Originally Posted by zzjea View Post
I have a gasser and would like to know how the yellow park brake switch controls braking. It does work but I don't how it does it. This is a '98 Dolphin on a Chevy 454 chassis.

Anyone know how this thing works?

Thanks,

John
Most likely that you have a GM/Chevy P32 motor home chassis with the infamous J71 Auto Apply Parking Brake (AAPB). There is lots of info about that chassis and AAPB system here: Workhorse Important Discussions

Also, if you have AAPB problems or need additional info, you should contact the acknowledged EXPERT on the J71, Roger Haag, at: oldusedbear11@charter.net His help is free and highly recommended.
Ed
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Old 09-17-2012, 08:15 AM   #5
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Originally Posted by edgray View Post
Most likely that you have a GM/Chevy P32 motor home chassis with the infamous J71 Auto Apply Parking Brake (AAPB). There is lots of info about that chassis and AAPB system here: Workhorse Important Discussions

Also, if you have AAPB problems or need additional info, you should contact the acknowledged EXPERT on the J71, Roger Haag, at: oldusedbear11@charter.net His help is free and highly recommended.
Ed
Thanks. I have a '98 and I think it's a P30 chassis. The references are for 99-2000 Workhorse which mine is not, but my guess is that in principal, they are probably the same.

I am not having any problems with Park Brake, but was curious how the Yellow knob worked in the system. It looks like it is a manual override that will lock the transmission even if you take the gear shift out of park and into reverse, etc. I will test that soon to see. I will also test it with the jacks down.

John
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Old 09-17-2012, 08:42 AM   #6
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I did the same study as you are - The attached is a document that compiles a lot of general Auto Park info. You can add to it as you study.
Update - I'll PM the attachment - I can't get it to upload.

Reader's Digest version:
Prior to 1999 GM built the P-30 chassis for step vans and the P-32 for motorhomes. In 1999 Workhorse took over the chassis and evolved it to the W series with the 8.1 chevy engine. Recently Freightliner bought the line and discontinued production.
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Old 09-17-2012, 09:05 AM   #7
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The AAPB was, as I understand it, a "fix" for the fact that the parking pawls in the GM transmission weren't strong enough to hold the weight of a typical motor-home. It doesn't work with the transmission, but actuates a drum brake on the drive-shaft.

It has some undesirable failure modes and was one of the reasons we decided to eliminate the WH chassis from our search for a motor-home which culminated in the purchase of a Ford F53-based rig.
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Old 09-17-2012, 11:33 AM   #8
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Quote:
Originally Posted by zzjea View Post
Thanks. I have a '98 and I think it's a P30 chassis. The references are for 99-2000 Workhorse which mine is not, but my guess is that in principal, they are probably the same.

I am not having any problems with Park Brake, but was curious how the Yellow knob worked in the system. It looks like it is a manual override that will lock the transmission even if you take the gear shift out of park and into reverse, etc. I will test that soon to see. I will also test it with the jacks down.

John
Workhorse bought the Chevy RV chassis business from GM in 1999 and continued to build essentially the SAME design until they stopped production in 2005.
ALL the motorhome chassis are P32, but vary by GVWR. Your model # will be Pxxx32. The first three digits are code for the wheel base.

You have the J71 AAPB because there is NO "lock" (parking pawl) inside the GM 4L80-E 4 speed tranny.

IMO, there are only two types of J71 owners...those that have had problems or those who will........so I suggest you learn all you can about the system and save the contact info for oldusedbear11@charter.net so you can get help when needed.
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Old 09-17-2012, 11:35 AM   #9
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Quote:
Originally Posted by tomwalt View Post
....................

Reader's Digest version:
Prior to 1999 GM built the P-30 chassis for step vans and the P-32 for motorhomes. In 1999 Workhorse took over the chassis and evolved it to the W series with the 8.1 chevy engine. Recently Freightliner bought the line and discontinued production.
Point of order....It was NAVISTAR, not FCCC, that bought Workhorse and has recently closed it down.
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