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Discussion Starter #1 (Edited)
1988 Cherokee with great service brakes. No Leaks, no pulling, powerful when the whoa! pedal is stomped. Brake fluid reservoir is full and no leaks in the system.

A search of previous threads on the subject revealed the pressure sensor located just below the reservoir may stick. I unplugged it and then plugged it back in with no change...dash light still on. Any diagnostic tip for this component? I'll rap and tap on the housing it threads into this afternoon.

Is there a hand brake/E-brake position switch that could be causing the light to be stuck on? Location and access?

Thanks!

Edit:

http://www.factorychryslerparts.com...=1988&ukey_category=20302&ukey_driveLine=7389

Finally dug up a schematic of what I believe to be my hand brake lever assembly. Detail 32 is the switch, if this matches my year. Note: Some of the pieces parts on this diagram appear to go back to 1990-1991 for some of the pieces parts show, later for others.



I'll look at the hand brake switch contacts and see if they are clean and adjusted correctly.
 

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Discussion Starter #2 (Edited)
I had 15 minutes to check out the Jeep this afternoon and I unplugged the sensor on the master cylinder proportioning valve.

I shorted the connector terminals with a paper clip and that turned the dashboard brake light off. Removed the paper clip and the light comes on.

Considering the fact that the vehicle brakes perfectly and leaks no brake fluid would your opinion be that this sending unit or sensor is faulty? Or do I have another problem?

I'll see if I can dig up a diagram of the sensor.

Edit:

I think detail number 11 is the proportioning valve body the sensor threads into.

http://www.factorychryslerparts.com/showAssembly.aspx?ukey_assembly=610356&ukey_product=4520728

I read somewhere that the light can be tripped on by the sensor if the proportioning valve sticks and that rapping on the valve body can sometimes free the valve. I tried, but there's no swing room to tap the valve body. It's packed in there pretty tightly.

I can't find the sensor sold separately. Anyone know what I should search for or ask for at NAPA? Any way to disassemble, adjust or clean the valve body assembly...or just go for the hole shooting match and replace the whole proportioning valve assembly?

Here is a pic of the proportioning valve with the sensor in yellow located top/middle:
 

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Here is what I have. But the only sensor that is normally closed is the low fluid level switch. the rest of them are normally open and turn the light on with a closed circuit. Here is the diagram:
 

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Discussion Starter #4
Gms, thanks for that wiring diagram and picture. I print each and every one of the data sheets you guys post and add them to my Jeep reference notebook.

The switch on my proportioning valve (if I have that component correctly identified) must be normally closed to turn off the light? With the connector unplugged from the switch and with an open circuit the light is on. With the connector plugged into the switch...light on.

I will check the switch for continuity next!

I have to take She Who Must Be Obeyed! out for breakfast and hit a few stops afterwards, but I'll get back out under the hood this afternoon with my trusty paperclip and VOM.

NAPA and other sources only seem to sell the complete proportioning valve for around $80-$90 and no mention is made as to whether those come with or without the switch. I'm wondering if I can loosen that nut on the valve body and free up the theoretically stuck valve or if the switch, itself is just toast. Time will tell.

The factor that makes me believe it is only the switch that is faulty is that the vehicle brakes perfectly under all conditions and road surfaces. No one or two wheels lock up early. No pulling or pulsing.

By the way, I did figure out why my Craftsman VOM was giving me wonky readings when diagnosing/setting the throttle position sensor voltage. The 9V battery that powers it was down to 3V output. Doh!

I should also make it clear that it is the RED dash 'Brake' light that is stuck on and this vehicle does NOT have ABS.
 

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you can check the differential sensor at the proportioning valve and also remove the plunger to make sure it isn't stuck, but be careful cause it is under spring pressure.
 

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Discussion Starter #6
GMS, Thanks! That is EXACTLY what I was looking...an explanation on how the proportioning valve worked and why the warning light would be triggered there!

I found the plastic plunger switch to be working properly.

I found that by installing a .020" shim washer between the plunger switch and the valve body I could get the light to stay 'Off'. I know that's not a repair or fix and there is still an underlying cause as to why the 'On' light is being tripped.

It wasn't a shade tree I was working under yesterday. It was 38° in the driveway with a constant 15-20 MPH wind driving freezing rain and snow! I stopped working on it when my hands went numb for the third or fourth time.

I'll check out the valve's plunger for free movement and thanks for the warning that is under spring pressure.

Just going by how well and how evenly the brakes react to pedal pressure, I would find it hard to believe I have a frozen caliper or slave cylinder, but stranger things have been known to happen. No brake system leaks in the month or so since I've put the Jeep back in service. The master cylinder reservoir is right where it was when I first checked it when I initially inspected all fluids and changed the oil/filter.
 

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At any time prior to noticing the break light was on did your pedal bottom out ? Possibly during a sudden stop.
Try this to reset the Proportioning valve, have someone available to help you. You will need a bottle partially filled w clean break fluid, a hose that fits the bleeder valve on the front breaks and a line wrench. Loosen the drivers side front bleeder valve then tighten, place hose on valve w other end submerged in the bottle, open the valve and have your assistant press firmly on the breaks. While pedal is held down tighten the valve. This should reset the proportion valve. Refill the resovoir.
 

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Discussion Starter #9
Hooks, thanks for that procedure!

As far as I know the brakes pedal has always been solid on this vehicle. Unlike my Ford Ranger which just got 32 FEET of new brake lines installed, the Cherokee's brakes have been reliable. I've never had the pedal go soft or bottom out. I have yet to pop a line or hose on it in the years I've owned it.

While testing the lock-up of all four wheels under heavy braking on our dirt township road I did find the left rear locks up before the other three wheels.

Your procedure makes sense. Depressurize the front driver's side caliper (shortest run from the master cylinder) to allow the proportioning valve to float towards the front circuit?

I'll pull the left-rear drum and take a look a the slave cylinder for a leak and make sure the early lockup isn't being caused by brake fluid on the linings. I suspect a worn tire at this point, but I'll dig into it.
 

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Discussion Starter #10
Hooks, thanks for that procedure!

As far as I know the brake pedal has always been solid on this vehicle. Unlike my Ford Ranger which just got 32 FEET of new brake lines installed, the Cherokee's brakes have been reliable. I've never had the pedal go soft or bottom out. I have yet to pop a line or hose on it in the years I've owned it.

While testing the lock-up of all four wheels under heavy braking on our dirt township road I did find the left rear locks up before the other three wheels.

Your procedure makes sense. Depressurize the front driver's side caliper (shortest run from the master cylinder) to allow the proportioning valve to float towards the front circuit?

I'll pull the left-rear drum and take a look a the slave cylinder for a leak and make sure the early lockup isn't being caused by brake fluid on the linings. I suspect a worn tire at this point, but I'll dig into it.
 
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