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Old 02-03-2013, 01:18 PM   #1 (permalink)
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How do I test a vacuum pump?

The truck in question is a 93 W350. The problems are:
1. A/C, heater and defrost controls don't work. No matter where the setting is, the air blows out the dash vents. Pushing heat or defrost doesn't change anything.
2. Heater control valve doesn't operate properly. Pushing the A/C button should close the valve, heat/defrost should open it. The valve stays closed no matter what has been selected (A/C, heat). I unplugged the vacuum line to it which allowed the valve to open for heat.
3. ABS dash light on. I don't know if this could be a vacuum problem or not. I just unplugged it behind the glovebox.

Now the kicker is, the brakes work perfectly, no issues whatsoever. So I'm trying to eliminate the vacuum pump as the problem. Looked at a bunch of threads but couldn't find any on testing the pump. My problem might be a bad A/C, heat, defrost control panel...or it might be something else entirely. Any and all suggestions are much appreciated.
Thanks, Bill
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Old 02-03-2013, 01:40 PM   #2 (permalink)
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If you have power brake assist you've already eliminated the pump. There are a couple of methods of diagnosing from here. You can start at the controls and check for vacuum there . Harder to get at but will confirm that you don't have a vacuum source for the controls. Or you can follow the line from the point it crosses the firewall back to it's it's connection to the vacuum source. Or you could follow the vacuum line from the pump and look for the point that the accessory line taps off.

Steve g
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Old 02-03-2013, 01:50 PM   #3 (permalink)
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Quote:
Originally Posted by Steve g View Post
If you have power brake assist you've already eliminated the pump. There are a couple of methods of diagnosing from here. You can start at the controls and check for vacuum there . Harder to get at but will confirm that you don't have a vacuum source for the controls. Or you can follow the line from the point it crosses the firewall back to it's it's connection to the vacuum source. Or you could follow the vacuum line from the pump and look for the point that the accessory line taps off.

Steve g
Thanks Steve. I had a hunch that with the brakes working, it probably wasn't the pump, but I wanted someone that knew for sure to tell me. These vacuum lines that go into and out of the control panel are super small (as I'm sure you know). I've unplugged a couple of them from the actuators by just dropping down the glovebox door. I can't feel any vacuum on them, but like I said, their so small I don't know if I would be able to. I bought a replacement panel from a bone yard (out of a 92), I guess the thing to do is just take out the dash face and try to see if anything obvious jumps out at me.
Bill
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Old 02-03-2013, 01:58 PM   #4 (permalink)
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I'd actually start at the other end. If you can't feel vacuum at any of the openings at the control valve end of the lines the problem is going to be at the other end. Start under the hood and follow the hose from the pump to the booster and check all the takeoffs there. You should be able to see the small hard line somewhere near there.

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Old 02-03-2013, 02:16 PM   #5 (permalink)
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OK, I'll do that. Am I going to actually be able to feel vacuum on these small lines? Is there a single line that goes from the booster into the panel itself? I've looked under the dash and I do see a bundle, almost like a harness of vacuum lines. I assume there is a single source coming from the booster. I'm sure this will make more sense once I get into it. Thanks again.
Bill
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Old 02-03-2013, 02:30 PM   #6 (permalink)
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Yes, you will be able to feel vacuum in the small lines (unless your fingers are a lot more calloused than mine) and yes, there is a single line that is the vacuum source. It is a hard plastic line and often gets man handled and broken under the hood. I'm sure once you start looking for it this will become very clear.

Steve g

Edit. The other section that frequently gets broken is the t fitting itself.
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Old 02-03-2013, 02:41 PM   #7 (permalink)
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Can't thank you enough for this information. Going to watch the Super Bowl today, might tear into it tomorrow. I'm just glad it's not the pump. I know you can get rebuild kits from Gould (I think), but I've never tore into one of those either. If I get into it and have questions I might PM you if that's OK. I'm hoping it's pretty straight forward though.
Bill
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Old 02-03-2013, 04:41 PM   #8 (permalink)
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Rebuilding the pump is straight forward and not bad at all .
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Old 02-03-2013, 05:02 PM   #9 (permalink)
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Mines probably OK, but where do you go to get the kit? Never know when that might go out.
Bill
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