Simple O/D wiring, A-518 - no computer

["Mad Max"]

Been trying to come up with a simple wiring schymatic for the O/D on my A-518 that bypasses the PCM and is easy to install, and I think I've about got it. After lots of thread chasing and pings to several folks, and after a real good conversation with Alan ("cerberusiam"), I think I've got it - So here's the plan:



Three switches - the O/D 2-prong, a hobbs pressure switch, and a kill switch. 12v power to the O/D, ground coming out going to one side of the hobbs, the other end going to the kill, the other end of which goes to ground. All connected and 'enabled'... at the specified pressure/speed... the circuit will ground/close thus engaging the O/D. But, flip the kill switch (on-off switch in the cab) and that opens the circuit, disabling the O/D. It couldn't be any simpler, and it completely bypasses the TPS, the PCM, the XYZ, the R2D2 and - and the C3PO... . and all the other maddening factory problem areas there are in a 1st gen wiring harness .



Now, doing it this way guarantees that the O/D will engage regardless of load, temperature, throttle, time of day, day of the week, weeks in a year or alignment of the friggin' planets. So, the driver will need to be gentle on the O/D during warm-up and under big loads... ... and I'm that way anyway so no problem. But it also guarantees that the O/D will not disengage when it's not supposed to... ever... and that is good. If any of you have driven a 1st gen with an auto you know how all-of-a-sudden infurating the O/D can be #@$%! - well this bypass should be the hot ticket, and it'll be easy... . and cheap



And hey Chipster - thanks mucho for the hobbs switch tip amigo - that's about as slick as it gets!



Stand by for the full wiring diagram and testing.



- Sam

 

["Mad Max"]

very cool - Overdrive is working and the 'plan' works fantastic, posted in 2 parts.



Part 1:



Here's the schymatic:



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Here's the step-by-step:



First, on the 518 unbolt the transfer case shifter and linkage (if so equipped). The shifter linkage and bracketry is 100% smack in the way of the port . I had to hack the shifter bracket like so:



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Any 518 governor port will function the same. The 618's will not because of the lock-up converter - the pressure varies. Locate this pressure port on the transmission.



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Call up Summit Racing and order up this nice adjustable Hobbs switch (from the Holley NOS lineup no less). It is adjustable from 25-50 psi . $45 bucks later it's in the mail.



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Then grab a 90* 1/8 pipe tee, some teflon thread tape, and make it all look like this:



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Pt II on next post...

 

["Mad Max"]

Pt 2:



Okay, next is controlling the thing. I wanted the O/D to engage/disengage automatically, and also have a manual override. At governor pressure 'x' (proportional to vehicle speed), the hobbs switch closes and, in this case, completes the ground circuit that closes the O/D switch on the transmission (see schymatic), and engages the O/D. But, I wanted manual control of the ground circuit up in the cab at the shifter. Found some great switches and decided to do some surgery on the Hurst tee handle :-laf -



Had to make a big honkin' 3/4" hole for the switch



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Plug in the two wires and press in the switch...



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... but then of course the diff locker got jealous and needed its very own switch too



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Thread the wires through.....



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and here's what it looks like - two switches, ready to go, nice and neat



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Installed...



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Took it out for a spin, got it adjusted, and it's locked in to engage right at 40 mph and it hits every time. I've no idea what the corresponding psi is that equalls 40 mph, but with 3. 54 gears and 36" tires it's diff'rnt than most others' will be. Slow down below 40 and it disengages, speed up and it hits again, then hit the shifter button and it disengages clean. Works great, and for the few miles I've driven it it has worked perfectly, and, I can adjust the engagement psi/speed . Very cool, I like it. PCM... not required.



A big thanks to all of you that have helped figure out the tech side of this conversion. I really appreciate your inputs and thoughts. Thanks fellas.



- Sam