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The feared brake Switch


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Just purchased a used brake switch. I have an intermittant brake light and associated ABS light which I had hoped to clear. Problem is, the used switch is totally different than the one on my car. The switch I purchased has a black plastic body with 3 pin connector molded into the body. The switch on my car looks to be an aftermarket unit. It has a pigtail about 8 inches long with a 3 pin black connector at the end of it. The body of the switch is much smaller in diameter than the one I bought and appears to be Stainless Steel. it has a white lable on it the reads .... Engineering and 5000psi max on it. Can't read the underside of this switch.

Anyone have a picture of the correct switch? Any idea what is mounted to my car now?

Thanks,

Bob

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Thanks Jim,

I will position a mirror this weekend to read the rest of the label. Think it may be an Omega Engineering switch. I can read Samford on the top of it. Judging from what I read on their website, it may be adjustable. That would solve my problems I think. We'll see....

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Bob,

It would be great if you could get the info from your switch if it works. Are you having any other brake issues besides the lights? You might try swapping the switches with the used one you found and see if that clears up the problems.

1995 S4s

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Thanks for the picture - that is the one that I bought.

I was able to read the lable on the one that's presently installed on my car. It is an Omega switch and it is adjustable. Here's a picture of it and the link for Omega. I will attempt to adjust it this weekend. Looks to be $135 interesting no?

.PSW-500_m.jpghttp://www.omega.com/ppt/pptsc.asp?ref=PSW-500&nav=

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Hi Bob

I may be missing something here :popo: but what you are showing( in your pic and the origanal fit in Olafs post) is a pressure switch,that is attached to the brake pump accumulator assembly ,this switch has no effect on your actual brake lights.Your switch on your brake pedal would cause brake light failure AND cause the ABS computer to show a steady yellow ABS fail light ,because your car knows it is slowing (wheel speed transducers) but has not had a message(input) from the brake pedal switch telling it . I had the same problem on my S4s ,so may be an idea to check the switch first,mine had just been kicked out of adjustment.

Nick S4s

Edited by fflyingdog

Simplest things first.

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My problem is with the red tell tale brake light on the dash and the yellow ABS tell tale. Sometimes my car works fine and neither of them come on. Sometimes I get I get the red brake light for a period of time after braking then it goes out when the pump stops cycling. Other times the red light doesn't go out so I get the yellow ABS light to come on with it. Sometimes I get the red light from the get go and it never goes out - also accompanied by yellow ABS. It acts like the switch is letting the pressure drop too far before turning the pump on hense the tell tales.

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OK, I have played with the adjustment on my Omega switch with the following results:

  • With the switch adjusted to open at maximum pressure, the pump will run continuously with both the telltale ABS and Brake lights lit
  • With the switch set to open at minimum pressure, the pump cycles at every brake application. The brake tell tale comes on when the pump is running, and goes out when the pump stops. The ABS tell tale doesn't light.
  • Adjusting inbetween, I get a range performance from pump cycling every brake application to pump cycling every 7th brake application. When set for the higher brake applications, the ABS tell tale will come on and set due to the extended time the pump runs to recharge the system. In all cases the brake tell tale come on when the pump starts and goes out when it stops.
  • The best setting seems to be around a pressure where the pump cycles after every 3 or 4 brake applications. At this setting, I never get an ABS tell tale, but as stated above, get a brake tell tale when the pump is running. Brake performace, including ABS function works well at this setting.

I think I understand what may be happening with the brake tell tale. The factory switch like the Omega swith has a common lead and a normally open switch along with a normally closed switch (3 pins). On the Omega switch, both switches activate at the same time; the one that provides the ground signal to the pump closes and the one that provides low pressure signal to the ECU opens - hence the brake tell tale anytime the pump runs. Looking at the wiring diagram, I beleive the factory switch has different set points for pump ground and the low pressure switch such that the low pressure switch opens at a lower pressure than the pressure where the pump cycle switch closes. Anyway, that's the theory.

If I'm right, I could eliminate the brake tell tale by permanently grounding the low pressure wire such that it never sees the switch open. I would lose any indication of a low pressure condition, but I think I could feel the low pressure condition in the brake operation. Otherwise, I could just live with the brake light coming on when the pump runs. I think I will run this way, at least for awhile.

So it looks like the Omega switch can be used as a replacement for the unabtainable GM switch with this small drawback.

Bob

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Sounds to me like the Omega switch does not work in the parameters needed for the system to operate properly. It should close at 2200 psi to start the pump and open at 2700 psi to stop the pump. Anything outside of that and you're going to get warning lights and error codes in the controller.

You really need a pressure gauge to see what is actually happening with the system. I'd be very careful about forcing the system to operate outside of the parameters that it is designed for as the consequences could be dire...

1995 S4s

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You're right Jim, a pressure guage would be a great help. Unfortunately, I don't have one in this pressure range so I'm setting up the switch based on pump run times and pedal cycles described in the workshop manual. I know what setting cycles the pressure above the pressure relief valve, so I think I can get in the ballpark. The Omega data sheet specs the switch deadband at 345psi for the PSW-572 switch that I have, so that should be in fine if the pressure is set about right.

Anyway, I will keep you posted on the performance of this system.

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  • 3 months later...

Hi everyone,

This switch has come up several times on various posts. Steve Taylor at the Elan Factory in Melbourne has come up with a solution. He has been able to source two switches. A low pressure switch and a high pressure switch. Has split the wiring for the original and has a setup now that is being passed by an engineering firm in Melbourne. Steve has had the setup on a car in his shop for awhile with everything working fine. Steve also can supply a test rig for $150 AUD.

I will keep everyone informed as I have just gone through my ABS system (I'm up for an accumulator), but more than likely somewhere down the track my switch will fail as well.

I do not know what the setup from Steve will cost as yet, but will keep everyone informed.

Bob,

I didn't know that an SE had ABS??

Michael.

All we know is that when they stop making this, we will be properly, properly sad.Jeremy Clarkson on the Esprit.

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I wish it had the previous non-ABS, but alas it carries the dreaded Delco Moraine system. I think ABS broke in around 92my.

My Omega switch is working well. With the adjustment I made a few months back I no longer get the yellow ABS telltale light. The red "brake" light does come on when the pump cycles (every 3rd or 4th brake actuation) but extinguishes as soon as the pump stops. Not as good a factory switch that works properly, but definately better than one that doesn't. The dual switch system should solve brake light issue, though it's not that big of a deal. Maybe even a feature; I can tell that the pump's running even with the stereo turned all the way up - ya it's a feature, that's it!

Bob

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