Check Engine Light - Brake On/Off Sensor

[motopsycho650]

I just transplanted a '92 transmission into a '94 explorer XLT. No problems with anything except I have a check engine light that comes on about 5 seconds after I start the engine. I read the code with a flash reader, and the only code I get is a 536 "Brake On/Off Sensor."

I found the brake light switch on the brake pedal, and swapped it with the one from the '92. No change in the light. I am looking at my Haynes manual, and it shows a second brake switch in the cruise control wiring diagram.

The switch I found was on the brake pedal with a light green and a green w/ red wire going to it. The one I am looking for has a gray/yellow and a Black/yellow wire going to it (according to the diagram).

Any idea's where I might find the second switch/sensor?

 

[motopsycho650]

 

[motopsycho650]

Nothing. Well... The other issue besides the CEL is the cruise control isn't working. Since the brake switch(s) both hook into the speed control servo, would it make sense to swap those? Is this the infamos cruise control box located behind the glove box? I'm not finding it in my manual's index.

I am also looking at the wiring diagrams, and I don't see where the speed control servo gets tied into the EEC.

 

[MrShorty]

I just transplanted a '92 transmission into a '94 explorer XLT.

I get burned often enough assuming an auto, so I'll ask this time: Auto or manual transmission?

I am also looking at the wiring diagrams, and I don't see where the speed control servo gets tied into the EEC.

Speed control really isn't tied into the EEC. The cruise system and EEC share a couple of sensors (like BOO switch and VSS), but they operate parallel to each other rather than dependent on each other.

I read the code with a flash reader, and the only code I get is a 536 "Brake On/Off Sensor."

KOEO, KOER, and or CM 536? Most common cause of a KOER 536 is operator error: neglecting to press the brake after engine id is output during the KOER test. KOEO 536 indicates a break in the BOO circuit somewhere, which could also explain why the cruise isnt' working. How about your brake lights?

The wiring diagram you posted shows the BOO in the cruise control circuit. Do you also have the wiring diagram that shows the BOO switch circuit to the EEC computer?

 

[motopsycho650]

Automatic transmission

No codes durring KOEO, and no codes stored in memory. Only Code 536 durring KOER. Yes, I did press the brake pedal, turn the steering wheel, and perform the WOT test when prompted by the code reader.

The brake lights are working just fine, so I know it is not the BOO switch that is causing the problem. Something is keeping the PCM (I called it EEC before) from seeing the BOO switch.

I have the other half of the haynes diagram, but it didn't show any wires going off to the PCM. I also have a much better wire diagram from Shopkey, and it also does not show how the PCM gets tied into that circut.

 

[MrShorty]

If you are sure you pressed the brake, that does indeed suggest some problem with the BOO circuit.

The wiring diagram I have immediately available (chassis diagram from Chiltons) shows a wire branching off of the brake light circuit to the PCM from somewhere in between the BOO switch and the 3rd brake light, but it doesn't give the specific pin at the PCM. A better wiring diagram (like you might find in Mitchell at a public library) would better show us what circuit you are looking at.

At this point, it looks like we need a better wiring diagram so we can see exactly how the BOO signal gets from the switch to the PCM/cruise control amplifier. Then determine where the break is in that circuit.

 

[motopsycho650]

At this point, it looks like we need a better wiring diagram so we can see exactly how the BOO signal gets from the switch to the PCM/cruise control amplifier. Then determine where the break is in that circuit.

What I think I am seeing is is the BOO wire going directly into the cruise control amplifier. Does that wire you see come out of the cruise control amplifier and then go to the PCM? If so, it seems the amplifier is probably bad.

Being that I have plenty of "know-working" donor parts from my '92, I'm probably going to try swapping the CC amplifier this weekend. That seems like the common link between my CEL and my CC not working.

 

[MrShorty]

What I think I am seeing is is the BOO wire going directly into the cruise control amplifier. Does that wire you see come out of the cruise control amplifier and then go to the PCM?

My diagram doesn't show the PCM or the CC amp. My impression has always been that this circuit will branch off later and go to the CC amp and PCM in parallel. Meaning that a bad amp wouldn't affect the PCM part of the circuit.