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Brake pump or pressure switch? 1991 V6


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My pump does not come on at all... Have checked all relays and also their connections. I get no lights... I am thinking the pump is shot or the pressure switch is shot. I turned on the key and pulled the plugs... using a volt meter, I checked for power on the 5 prong pressure switch... found power on 2 blades. On the pump I found low voltage .45v on both blades.

Any thoughts? Is there any easy way to test either pump or pressure switch e.g. way to bypass wiring and just fire up the pump?

the next question... where would I get either of these parts?

P.S. I did buy a new accumulator.. the old one was shot (worked for about a week, then intermittent, now nothing... wondering if I had a weak pump for awhile and not nothing?

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My pump does not come on at all... Have checked all relays and also their connections. I get no lights... I am thinking the pump is shot or the pressure switch is shot. I turned on the key and pulled the plugs... using a volt meter, I checked for power on the 5 prong pressure switch... found power on 2 blades. On the pump I found low voltage .45v on both blades.

Any thoughts? Is there any easy way to test either pump or pressure switch e.g. way to bypass wiring and just fire up the pump?

the next question... where would I get either of these parts?

P.S. I did buy a new accumulator.. the old one was shot (worked for about a week, then intermittent, now nothing... wondering if I had a weak pump for awhile and not nothing?

Not going to ask a guy that can test voltage if you checked all the fuses as I expect that you already did that.

Multiple things could be wrong, lets start simple and work our way up the chain. You could be air locked, that is these front lines don't bleed easy without a vacuum bleeder and you might need to bleed the brakes again as your accumulator could be full air.

You could have a bad connect at the relay, two of us here have found spade lug female connect loose in the brake relay block and that makes for a false good test as it pulled up and into place when the relay was pulled out and slid down when relay was pushed back in, test good but doesn't connect good. Pull relay block free from fender and with relay's in place gently pull each individual wire to make sure it's actually connected.

If first opening rear brake line with key on and slight pressure on brake peddle doesn't get the pump running and your connects on the relay block are all good you can pull the pump relay and using a thick paperclip jump the relay connects on the block or go right to testing pressure switch. Pump needs 12v to run and .4 is phantom / bleed back / floating voltage and you can ignore it for now.

Pressure switch is a make or brake, after checking relays pull plug connect and check for continuity, open/closed contact. Use wiring diagram to identify contact to test by wire color to contact. Personally I would force feed the 12v before chasing the switch contacts, this is a personal choice of where to look first if you haven't already found the problem.

These pumps motors are tough little suckers, high pressure pump short duration motor and they usually burn a fuse or feed wire before you cook a motor. Not to say they last forever, but a bad pump after accumulator change usually runs the motor without making full pressure or leaks back through the pump and system won't hold pressure and you get a no fluid loss system leak.

If you have gotten this far and it's still not working the .4 at the pump gets a closer look and a whole new set of questions.

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I bled the rear brake... the pump goes on. It is intermittent (does not run continuous)... with key on and someone applying pressure on the pedal, should the fluid come out of the rear bleed valve at high pressure or just flow out? I am wondering if the accumulator I bought is bad? cant really tell.

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I bled the rear brake... the pump goes on. It is intermittent (does not run continuous)... with key on and someone applying pressure on the pedal, should the fluid come out of the rear bleed valve at high pressure or just flow out? I am wondering if the accumulator I bought is bad? cant really tell.

Can't see what you see, but it sounds like everything works.

Front brakes do most of the work and get most of the pressure, proportioning valve limits pressure to rear. Rear brakes should flow continuous with light pressure on brake peddle, continuous pump run or surging with light pressure on peddle would be sigh of trouble.

In metaphoric description that might make some sense of this; the accumulator works like a rechargeable battery. Charge it with fluid and it holds a charge until needed, use it and it discharges, use it enough and it needs a recharge. A good accumulator should charge quick and hold a charge equal to 12 normal stops before it's dead, run it down and like a battery on a float charger the pump kicks and in a few seconds, like a quick charge battery, the accumulator should charge and hold enough pressure for another dozen stops. Sounds like pressure switch, pump and accumulator are working, now lets find out how well.

Bleed out the front brakes, small piston means a dozen short spurts from peddle push to move the same amount of fluid that you would expect to see with the "common most other cars vacuum assist master cylinder" and even with a vacuum bleeder getting air from the top of the line to the bleed valve takes time and there is no quick trick bench bleed for a new ball. One thing to check when you first start bleeding out front lines is how hard you have to push brake peddle when bleeder valve is open, do this with your finger tips on the peddle and use your fingers to compare left to right side pressure. If you have any thought that one side bleeds easier than the other now would be a great time to replace the flex brake lines.

If you have a vacuum bleeder set it for low and slow, one pint every couple of hours and you're going to have brakes like new. While bleeding slow put a drop of liquid hand soap on the bleeder threads about every 30 minutes or so to help keep air from sneaking back in and if everything else is working you should be pleasantly surprised by the difference this makes in your brakes.

Edited by Digger914 (see edit history)
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I am really thinking it is electrical. New Accumulator from TC Parts, applied power to the pump and it ran no problem... I applied power to pin #4 (12v) on the pressure switch, and the pump switches on while applying the power. I tested my relays and think one is bad... so replacing that tonight.

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Anyone know where I can find the orange relay (the middle relay)?

Though I haven't actually gone shopping for one I did have reason to suspect my orange relay for about 30 seconds before finding the bad wire connect to the relay. Part number might be hard to chase but it looks to be a standard run of the mill relay and before you go shopping the electronics suppliers take a shot with having your local NAPA store match it up with what they stock.

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I am now on to the brake pressure switch. If I energize pin 4, the pump runs... however when it is hooked up the pump does not run... when I bled the rear brakes, the pump will switch on/off/on/off but only for a second each way once I start getting fluid out... I would think it would run more constant to build pressure.

When just sitting there, the pump wont run when I turn on the car... it will run just while I push the peddle, but only while pushing down and only for a second (it should continue to run until it builds pressure).

Anyway to test the pressure switch? Can I test voltage on any pin/combination of pins to see if it will switch?

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You can sort of test the pressure switch, there is special equipment for testing hydraulic pressure and if you had it you could test and know for certain, I don't have it and doubt that you do either. We can make the brake system give us rough guess for some simple testing of switch operation by running the pump and checking for make and break switch contact.

Obviously you have an electrical problem and hopefully it's just one.

Always check for good ground that should be brown wire on pin one (1). Nothing works like it should without good ground.

Pin four (4) is voltage to energize pump relay, feeding the juice that makes the pump work and as it does we will abuse it later.

The power that feeds the switch comes from the ABS controller and you've already said that you found power on pin two (2) so we know that some power power goes to the switch and that should be your brake warning lamp looking for ground.

Real power comes from the ABS Controller and it should default for power to the pump in any fault mode if it knows that pressure is low, it also appears to kill pump power if the system has no fluid and a flaky fluid level switch could give you the pump run you've described. So could several other things.

With the break system pressure discharged you should have continuity between pin (4) and only one other pin and it should be the one that gets power from the controller.

Check continuity between (4) and all the other pins, because that .4 you found at the pump earlier could be bad ground, or switch contact, or brake fluid has seeped into contact area and everything is shorted and this is what your ohm meter will tell you. Make a chart of what points make contact with no pressure and then we can abuse the pump.

I've said this before in other postings and I will say it again because it is such a tripping point, if every strand but one of a multi strand wire is broken it will ohm good and display voltage on a meter. Use a simple light tester when looking for power, if you can't light a 12v bulb your voltage means nothing. With key on and the pump unplugged and the switch plugged in, the light bulb tester should be bright on both sides of the contact point.

Now force feed the pump power, for safety sake do this in 10 second intervals, then pull the pressure switch plug between runs and check continuity against the chart you made, when you can hear the pump make it's slow down sound do it once more and call it done. Shouldn't need to run more than 40 seconds total to answer your switch question and with this you should have a chart that tells you brake warning light on for low pressure, ABS warn for low pressure and if your switch works like?, maybe even like it should.

If your switch doesn't work and it's not shorted it can be removed and cleaned, if it works after cleaning go ahead and bleed the brakes, if not it's time for a new, or good used switch.

If your switch does work, it's contacts or controller and with no warning lights, .4v at the pump and intermittent operation MY guess is bad ground before bad controller.

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  • 3 weeks later...

It is a bad pressure switch. When I jump the connector (pins 3-5) the pump runs. However, when I look for continuity pins 3-5 on the switch, I get nothing. I am assuming that I should have continuity for low pressure and that should kick on the pump... no continuity, bad pressure switch. Now where do I find a pressure switch?

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from the 1990 part book showing the master cyl/booster:

1 4464 006 1 RESERVOIR PACKAGE

2 4464 030 1 SPACER, Brake Booster Mtg.

3 4464 365 4 BOLT, Spacer Mtg.

4 4463 982 1 GASKET, Brake Booster Mtg.

5 4464 001 1 ACUMULATOR PACKAGE

6 4464 008 1 CAP, Reservoir

7 4464 002 1 HOSE PACKAGE, High Pressure

8 4464 012 1 ACCUATING PACKAGE, (Included Valve Block) ***

9 4464 009 1 HOSE PACKAGE, Low Pressure

10 4464 017 1 SEAL PACKAGE

11 4464 003 1 BOLT, Banjo

12 4464 011 1 PUMP PACKAGE

13 4464 005 1 SWITCH PACKAGE, Pressure Warning

14 4464 004 1 INSULATOR PACKAGE

15 4464 007 1 BRACKET PACKAGE

16 4464 010 1 CLIP

17 4464 013 2 SENSOR ASSEMBLY, Frt. Wheel ***

18 4464 014 2 SENSOR ASSEMBLY, Rear Wheel ***

19 4464 015 1 SPACER PACKAGE, Sensor

20 4464 016 1 CONTROL MODULE ASSEMBLY

21 4464 029 1 BRACKET, Support

4464 028 1 BRACKET, Frt. Wheel w/ABS Sensor Support (D)

item #13????????????????????????????

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from the 1990 part book showing the master cyl/booster:

1 4464 006 1 RESERVOIR PACKAGE

2 4464 030 1 SPACER, Brake Booster Mtg.

3 4464 365 4 BOLT, Spacer Mtg.

4 4463 982 1 GASKET, Brake Booster Mtg.

5 4464 001 1 ACUMULATOR PACKAGE

6 4464 008 1 CAP, Reservoir

7 4464 002 1 HOSE PACKAGE, High Pressure

8 4464 012 1 ACCUATING PACKAGE, (Included Valve Block) ***

9 4464 009 1 HOSE PACKAGE, Low Pressure

10 4464 017 1 SEAL PACKAGE

11 4464 003 1 BOLT, Banjo

12 4464 011 1 PUMP PACKAGE

13 4464 005 1 SWITCH PACKAGE, Pressure Warning

14 4464 004 1 INSULATOR PACKAGE

15 4464 007 1 BRACKET PACKAGE

16 4464 010 1 CLIP

17 4464 013 2 SENSOR ASSEMBLY, Frt. Wheel ***

18 4464 014 2 SENSOR ASSEMBLY, Rear Wheel ***

19 4464 015 1 SPACER PACKAGE, Sensor

20 4464 016 1 CONTROL MODULE ASSEMBLY

21 4464 029 1 BRACKET, Support

4464 028 1 BRACKET, Frt. Wheel w/ABS Sensor Support (D)

item #13????????????????????????????

#13 is the 3 way pressure switch. It controls the pump on and off circuit and the ABS light on the dash.

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