1992 Master Cylinder Pressure Differential Switch

[Joe L.]

Has anyone ever had problems with the above referenced switch? Specifically, I do not think that the switch, itself, is supposed to be exposed directly to brake fluid. However, it seems that brake fluid is somehow getting into the switch orifice. This is on a NEW Delco master cylinder with a transferred switch that never had this problem on the original master cylinder..

[Gary C.]

Re: 1992 Master Cylinder Pressure Differential Switch

Joe,

The only problem I've had with my '92 ZR-1 - switch alarms if fluid's barely down, like 1/8 of an inch.

Topped off reservoir and the DIC brake alarm went away.

Gary
....

[Joe L.]

Re: 1992 Master Cylinder Pressure Differential Switch

Joe,

The only problem I've had with my '92 ZR-1 - switch alarms if fluid's barely down, like 1/8 of an inch.

Topped off reservoir and the DIC brake alarm went away.

Gary
....

Gary------


Just to be clear, are you referring to the low brake fluid switch or the brake differential pressure warning switch. The fluid level switch is mounted on the side of the reservoir. The differential pressure switch is mounted on the lower, right side of the master cylinder.

[Gary C.]

Re: 1992 Master Cylinder Pressure Differential Switch

Joe,

Apparently, it was the switch inside the master cylinder that was very sensitive to the fluid level and required it to be filled to the rim or it tripped the DIC Service ABS, Service ARS lamps.

Gary
....

[Joe L.]

Re: 1992 Master Cylinder Pressure Differential Switch

Joe:

There's an exploded view of the master cyl in Section 5A of the 1992 Book 1 FSM.

To me, it appears that there is a groove in the proportioning valve / differential valve spool. (Same part performs two functions.) When the differential valve is centered, the switch is in the groove. With a pressure differential, the spool moves, the switch moves out of the groove and contact is made to turn on the light.
The switch groove is between two o-rings. To me, there shouldn't be fluid at the switch port. The o-rings should seal the cavity that the switch screws into on each side of the port.

I'd be happy to go out and remove the switch and see if there's fluid in the port in my cyl. I have the same part. But my 92 is in Bakersfield, 150 miles away. It will be a couple of weeks before it comes back to Los Angeles.

Dave-----


Thanks very much. I think my problem has been resolved. Apparently, the new Delco master cylinder I had purchased was defective out-of-the-box. I obtained another of the Centric brand and it had no problem with brake fluid entering the switch cavity. Actually, I really think that both of the master cylinders came from the same manufacturing source but for whatever reason, the Delco was defective. Most likely, the o-rings you mention were defective or assembled improperly. It was probably the only one made that was defective and I got it!


By the way, you don't want to need that switch. It's GM discontinued and extremely difficult to locate. As far as I have been able to determine there is no aftermarket replacement. The same switch was used on C4 master cylinders through 1994. It is threaded for M12X1.0. For 1995-6 the switch changed to to 3/4" thread of some unknown thread pitch. The switch appears functionally the same as the earlier although its external configuration is somewhat different but uses the same connector as earlier. From what I have found, that switch is also discontinued and virtually impossible to locate. No aftermarket replacement for it, either.

Both switches are really very simple devices.

[Joe L.]

Re: 1992 Master Cylinder Pressure Differential Switch

Joe,

Apparently, it was the switch inside the master cylinder that was very sensitive to the fluid level and required it to be filled to the rim or it tripped the DIC Service ABS, Service ARS lamps.

Gary
....

Gary------


That sounds really weird. I don't understand how fluid level could affect the pressure differential switch.

[Joe L.]

Re: 1992 Master Cylinder Pressure Differential Switch

It doesn't. There are two switches. Pressure Differential and Low Fluid Level.

Gary and Joe; you are both confusing the two switches and their functions. Joe is talking about Pressure Differential. Gary is talking about Low Fluid.

The pressure differential switch only influences the BRAKE light in the cluster.

The low fluid level switch "grounds" the BRAKE indicator but is also an input to the EBCM (ABS/Traction Control). A "low fluid" condition will cause the EBCM to illuminate the SVC ASR, SVC ABS, and ASR OFF indicators in the 'Uh-Oh' panel. It also sets a Code 83 in Module 9.

One of the first issues I had to deal with in my 92 was the brake lock-up that Joe experienced with his 92. I replaced the master cyl.

On one of the first drives after resolving the brake problems, I accelerated hard on a green light and four warnings came on simultaneously. It was night time, so it was dramatic. I wondered "WTF caused all this at the same time? All I did was get on it." I cycled the key, and all the lights went off. I queried the CCM for codes and 9.H83 was present. "Low brake fluid". I looked, it was, and I added some. No more light-show. But it was at probably 2/3 low. Not an 1/8".

Joe: Please report back with the results of the master cyl change-out. Did it cure the brake lock-up?

Dave------


Yes, my problem has been corrected. However, I replaced both the master cylinder and the booster. I really believe that the master cylinder was the problem but I figured I'd do both at the same time. One of the reasons I replaced the booster is that I noted that there are a lot of the "metal" replacements on the market and rebuilt originals have gotten very hard to find. So, I surmised that these boosters must be somewhat problematic and even if mine is not a problem now, it might be soon. It so happened that I had a Cardone rebuilt original so I used it. I also plan to have my original booster rebuilt as a spare. The GM new originals have been discontinued for a long time.

I also plan to "autopsy" the master cylinder to see if I can discern what failed. I realize that it might not be discernible but I don't have a lot to lose but a little time.

[Justin S.]

Re: 1992 Master Cylinder Pressure Differential Switch

I'm curious what the functionality of this switch is?

Does it control or just measure?

Were these switches the result of the advent of ABS?

[Justin S.]

Re: 1992 Master Cylinder Pressure Differential Switch

Dave,
I don't know what happened to your response but thanks for your thorough explanation.