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I have a drivetrain from a 92 f250 with a 5.8 and E4OD swapped into a classic f100 and I have a question about the brake on/off switch.
The torque converter lockup behaves wierd on this thing and I suspect a problem with the BOO switch. At cruising speed (say 50+mph), light throttle, the TC stays unlocked. If you lay into the throttle to say 60-70% it locks up but the second you ease off, it unlocks. I have a feeling it's related to the BOO input to the pcm. What I want to know is: is the BOO switch just a simple 12v trigger that's activated with the brake lights or a 5v sensor trigger or what? How's it actually work? I get the feeling it's seeing the brake as always on and it preventing lockup under normal circumstances or the lack of a switch (don't think it was ever hooked up on this truck) is causing it to default to a 'brake-on' kind of mode. Either way, I need to figure that issue out and get it working properly and that may entail adding a BOO switch to the system.
Your idea has possibilities but speaking of missing sensors... what are you using for speed input to the PCM? This signal is manditory for correct trans operation, the '92 had a VSS sensor in the rear axle that fed the PSOM in the gauge cluster and that in turn sent a speed signal to the PCM, your f100 has none of these parts unless you also swapped over the rear axle, dash cluster, and all associated wiring.
The BOO will cause the exact symptoms that you have.
I can't recall the specifics of how that circuit works. On stock trucks, if you replace the bulbs with LEDs it pulls less current and the PCM thinks the brakes are always on. The same thing happens when the third brake light burns out, the PCM thinks the brakes are always on. So maybe the PCM is monitoring the ground side of the circuit?
Your idea has possibilities but speaking of missing sensors... what are you using for speed input to the PCM? This signal is manditory for correct trans operation, the '92 had a VSS sensor in the rear axle that fed the PSOM in the gauge cluster and that in turn sent a speed signal to the PCM, your f100 has none of these parts unless you also swapped over the rear axle, dash cluster, and all associated wiring.
The truck was swapped onto the 92 chassis so the entire driveline, and associated sensors, are intact.
For the VSS, the dash cluster was disassembled and the small module on the back of the speedometer was hooked up and mounted up under the F100s dash.
The transmission shifts perfectly normal except for the odd TC behavior.
Originally Posted by Mark Kovalsky
The BOO will cause the exact symptoms that you have.
I can't recall the specifics of how that circuit works. On stock trucks, if you replace the bulbs with LEDs it pulls less current and the PCM thinks the brakes are always on. The same thing happens when the third brake light burns out, the PCM thinks the brakes are always on. So maybe the PCM is monitoring the ground side of the circuit?
Sounds like it's a little more complicated than a simple 12v trigger. I'm gonna have to do some research I guess.
Is there a feed and return side to the BOO switch on the ECM? It could certainly monitor current draw that way.
Pretty sure its 12v switch, open brakes off, closed brakes on.
Did the 1992 come with cruise control?
There are redundant brake sensor switches.
On a 1994, there is a switch at the pedal and one on the master cylinder.
On the 1992, there is one on the pedal, and I beleive another near the proportioning valve on the frame.
I think you need to check your switch on the pedal, probably has a constant 12v, and does not open.
Pretty sure its 12v switch, open brakes off, closed brakes on.
Did the 1992 come with cruise control?
There are redundant brake sensor switches.
On a 1994, there is a switch at the pedal and one on the master cylinder.
On the 1992, there is one on the pedal, and I beleive another near the proportioning valve on the frame.
I think you need to check your switch on the pedal, probably has a constant 12v, and does not open.
I've read a lot of problems people have had with the BOO circuit, and TC lockup issues like mine, when installing LED lights or when a brake light goes out indicative it's activated by current draw. I'll have to study some wiring diagrams to figure it out.
Is the entire brake light circuit powered by the BOO feed from the ECM? That would certainly explain how LED's or a burnt bulb could confuse the BOO/TC lockup function in the ECM.
I actually didn't do the swap so I'm unsure if the source truck had cruise.
Originally Posted by wwhite
You are saying you do not have one?
How are your brake lights activated now?
What did you do for the overdrive button?
How about the CEL? any codes?
I do not have a BOO hooked up, that I'm aware of. Some wire tracing/probing will be in order; maybe a 12v source was wired in incorrectly making the ECM think brakes are always on.
The brake lights are through the original f100 wiring. All the engine/transmission/ ECM functions are through the 92's wiring harness with power feeds from the f100 to power it all up. Anything beyond power train function on the 92's harness are just open circuits.
There is no OD button. It's basically 'always on' with no way to turn it off.
I do have some codes including a BOO switch missing/inoperative (have to look up the actual verbiage on that one) as well as some EGR/Air injection codes because of deleted emissions stuff.
Thanks for all the wiring diagrams.
Best I can tell it really is just a 12v trigger set by the brake switch.
Ought to be easy to Integrate it into my trucks wiring.
for the lockup? The brake circuit has to have a functioning 3157 (load wise) x2 when braking
a 12 volt led brake lamp wouldn’t work for example
Looking at the wiring diagrams it's hard for me to visualize how the BOO circuit is reading the load from the brake lights.
Adding a load resistor would be simple enough but where to put it???
Originally Posted by AuroraGirl
that button serves important function btw
you would be wise to integrate it. It’s a diagnostic indicator and functionally significant esp trailering.
Neither of those functions is especially important on this particular truck at this time as its really more of a pleasure vehicle (remember this is all in a 67 F100). I do plan on eventually hooking one up though; I've just got 'bigger fish to fry' at the moment.
Looking at the wiring diagrams it's hard for me to visualize how the BOO circuit is reading the load from the brake lights.
Adding a load resistor would be simple enough but where to put it???
At S236,above the BOO switch,is the feed for the stop,turn,hazards.
I think you'll be fine with a switched 12v signal.
it also compliments with a manual transmission computer im pretty sure
A manual transmission computer will not shift a 4R70W. The trans will only have fourth gear with an unlocked converter and reverse available with a manual trans computer.
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