Possible ABS disable switch project for First Gen

So we really are back to the original, but now with more confidence. Thanks guys, appreciate it. I'll write it up and take pics, so we have a good reference on TW!

 

As far as power source I'd use cig lighter or power outlet

 

Yep, that was my plan barring any better option.

 

OK, looking at this again this morning as I write up my plan of attack for when I return. I think I found an inconsistency in the FSM:

ABS wiring diagram. Note that:

• pin 7 (EXI) - Diff Lock detection SW
• pin 14 (EXI2) - L4 detection SW

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But then, in the Terminals of the ABS ECU that @mechanicjon pulled up for us, they flipped them:

• pin 7 (EXI) - L4
• pin 14 (EXI2) - Rear Diff

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My guess is that the table is correct (or at least, that going after pin 14 is the right way to go given discussion from 2008 that I linked in the original post and the fact that the locked diff is what disables ABS).

 

just have to check each wire for the signal action per shifter position

 

OK, I just spent a bunch of time writing up my steps to test and then apply this mod when I have access to my truck and something stuck out as wonky (besides the swapping of pins 7 and 14 in the FSM, which is relatively easily testable so not a huge issue).

Here's the bit where I'm now stuck:

The FSM states that when the rear diff is locked, that pin 14 expects "below 2.0V" and that when the rear diff is unlocked, it expects 10-14V.

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So then, it seems to me that we'd want the "below 2.0V" (rear diff locked) which would mean connecting the other side of the switch to ground, not to +12V.

Thoughts?

And, what's the resistor doing for me in the circuit? I mean - it's reducing current when the switch is closed (ON), but it's not like it changes the voltage - (either +12v/ground) - so why do I need it?

 

The circuit might act like a head light with no ground. Power will go thru the light giving the ground a +voltage. Maybe that makes the ecu thinks its an open circuit. When the power is turned off the voltage would drop below to volts. You need to test voltage at switch to ecu when switch is open. I think the resistor is there to just lower the voltage a bit. keep it under 14volts.

 

A headlight with no ground? Headlights are all grounded...so I'm not understanding you there.

And a resistor doesn't change voltage, it just changes current flow...or am I missing something?

 

Yes head lights are grounded. I was making an example of a situation with a bulb with no ground. power will go in and come out as a +12volts. technically yes on resistor slows current flow

 

In my experience. When you see an input on an ecu and the input on the ecu is for a switch and that switch whether NO or NC completes or incompletes the circuit to ground tells me the sensor input on the ecu is looking for a continuity to ground.

Also in my experience, I've seen enough bad information listed in my 04 FSM that I wouldn't put any voltage to any PIN of my ecu unless I was COMPLETELY sure exactly what I was going or had a spare ecu.