Can anyone give me the specs on testing a vacuum pump on an OBS 97 diesel? I'm looking for how much vacuum should be in the system during operation and how long it should take for that amount of vacuum to be built up.
I'm guessing that the rebuilt pump that I just put in my truck is no good since the brake light is coming on intermittently, but I'd like to confirm my thoughts before I take it out and bring it back to the parts store for a replacement. Basically, when the truck is cold, the brake light is off. Once she warms up, the light comes on and pretty much stays on. And once the light is on, if I repeatedly pump the brakes, I can evacuate the vacuum out of the system faster than the pump can build it. Basically, I can pump the brakes until the pedal gets hard. I'm pretty sure that I shouldn't be able to do that (and I'm not pumping them like a madman either - just nice and steady).



Thanks for reading and any advice you can give.

 

The spec is 18 in hg. The only other thing that runs off the vacuum is the heater controls so make sure none of that is leaking. If your vacuum isn't at spec I would isolate the booster and the heaters and see if it changes.

 

Thanks for the quick reply!
I originally was thinking that it was bad parts, but after thinking some more I figured that it could be something else causing the problem - thus the further testing. The last pump was showing the same signs - brake light on and off randomly - until the bearing locked up and blew the pump to pieces. Now it's showing similar symptoms, so I'm thinking a little more investigation could be in order.
So for the 18 in hg, can I just hook my vacuum gauge up directly to the pump to confirm that it's outputting at that level?
Thanks again!

 

yes you can do that.

 

You might want to check out your vacuum pump switch and all the hoses / connections to that manifold ..

 

Thank you, I will definitely take a look tomorrow in the daylight.

A little more info -
So I just did a quick pump down check when the brake light was not already on... And it's almost impossible to get the brake light to come on by doing a pump down test. The brake pedal is rock hard long before the brake light comes on so I'm thinking that there's a different reason that the brake light is coming on. I am going to do a vacuum test when the brake light is off vs. with the light brake light on and see if there is a difference in the reading.
If anyone has any ideas of other things to check when the brake light is on, feel free to comment.

 

Is your park brake pedal all the way in the up position? I know that sounds silly but my light will blink on and off if the pedal doesn,t come all the way back up this is due to worn out park brake cables I some times have to use my left foot to bring the pedal up all the way so the light will not do this.

 

I would be VERY happy if that was the case. I will check and let you know, but I'm doubtful. I know that once it comes on, it has stayed on even after using the parking brake. Yeah I'm one of those guys who uses the parking brake regularly in an automatic.

 

Yeah I'm one of those guys who uses the parking brake regularly in an automatic

Yeah me too I have a high Idle controller and the brake has to be set for it to work.

good luck and post what you find

Walter

 

A vacuum test today when the truck was cold showed that she pulled about 25 in hg. I'm thinking that's good enough, but I'm going to have to test after the brake light is on just to confirm that the vacuum pump is working properly. I'll post again after I perform the test.

 

Ok, so I was able to perform the test with the brake light on and the vacuum gauge had the same reading as when the light is off, so I'm going with something else being wrong with the system. While the light was on, I also tried unplugging the wire from the parking brake, which didn't change the light status. So now I'm thinking that maybe it is the vacuum switch (in the picture above). Can anyone give any advice on how to isolate / troubleshoot the vacuum switch?
Thanks again.

 

Unplugging the three wire electrical connector to the vacuum switch should disable it.

 

Ok very good. I'll give the at try and let you know the results.
Thanks!

 

So it seems that the vacuum switch is the problem. Anyone know if that's a dealer only item?
Thanks for the advice so far!!

 

Anyone have any idea where I can find a place to buy this online? I've looked in the various sponsor's pages and I'm coming up short. Is it possible that this is called something other than a vacuum switch?
Thanks in advance!