| JC-4 Vacuum Pump?? [message #323858] |
Wed, 13 September 2017 21:08  |
rjw
Messages: 697
Registered: September 2005
Karma:
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Senior Member |
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While getting ready to leave for the Elkhart rally, I had a conversation with a well respected and reliable GMCer about vacuum pumps.
I had mentioned that I have installed in my coach the often recommended "JC-4 Vacuum Pump". He remarked that most of them were not very good at providing an adequate backup vacuum (for braking) in the event of an engine failure. But, that's why we have them, for an adequate vacuum source to give us brakes if the motor goes south and stops producing vacuum.
I decided to do a test to see how my brakes work without engine vacuum and how the JC-4 does as a backup. I have a 1/2 mile stretch of road near my house with little traffic. I drove there in my GMC and started out going 30 MPH, put it in neutral and shut down the motor. I pressed the brake pedal a few times to dump vacuum and then tried to stop. The result was not good and it was a good thing I didn't have to stop because I could not stop.
I did it again, except this time after shutting the motor off I turned the ignition back on (to power the JC-4 pump). I did the same thing, dumped the vacuum, but now I could hear the pump running when I pressed the brake pedal. I waited until it stopped and then I tried to stop the coach. The result was there was a slight improvement over no vacuum, but not what I had expected. I was able to stop, but it seemed not much better than with no vacuum. I repeated these tests several times with the same resulsts.
I would not want to be going down a mountain road with a toad and try to stop with the engine not running and relying on the JC-4.
My conclusion is that I am going to get rid of my JC-4 and go to something more robust.
Apparently, the problem with the JC-4 is that it shuts off before adequate (for our needs) vacuum is achieved. It might be possible to by pass the internal pressure switch to something that would allow it to pump down to the vacuum that we need to stop. Then maybe it would be OK.
My advice for those that have JC-4 Vacuum Pumps is to try my test and see how comfortable you are with it as your emergency vacuum backup.
Richard
76 Palm Beach
SE Michigan
www.PalmBeachGMC.com
Roller Cam 455, TBI+EBL, 3.42 FD, 4 Bag, Macerator, Lenzi (brakes, vacuum system, front end stuff), Manny Tranny, vacuum step, Tankless + OEM water heaters.
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