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1990 AW4 Reverse Issue


NAXJA Forum User
Hi all, I have been lurking through all the AW4 reverse question threads with great wonderment and a bit of confusion. Now that I am an official member... I too have a 1990 I-6 with AW4, both of which have been recently rebuilt. This is a Jeep that I am assembling from a multitude of directions, engine from one vehicle, trans from another, harness from a third, so I do not have historical data on all components, namely the TCU. I have verified the NSS is making all the correct connections and I have completed most if not all of Cruiser's recommendations on grounding. I have not yet taken care of the two splices in the wiring loom, but will do so shortly.
With the TCU connected (either one, I have two at the moment) I have forward gears, but when I initially shift to reverse (Wheels off the ground) the tires begin to nudge in the correct direction but then nothing. Typical as all others, it's as if it is in neutral.
With the TCU disconnected reverse seems to work correctly. I have not yet done a driving test as I was hoping to do so with the transmission working automatically. I will probably just do the manual shift driving test so I can continue to break in the engine properly. Then go back to the TCU issues.

So - my question is: What is the TCU doing or not doing that messes with the trans ability to shift into reverse properly? the NSS for reverse, only seems to have an effect on the backup lights. but it must be doing something else to cause it to not function in reverse when it is connected... All the threads I've seen that reference this problem seem to Peter out into other members asking, "so did you get this fixed?", which then is met with radio silence from the OP.

Does the TCU only operate the three solenoids? Tonight or tomorrow first thing I will chick the signal from the TCU to the wires going to the solenoids. I did'nt have the correct pinouts this morning - The signal to the TCU is correct for all NSS positions. Its a head scratcher if you ask me... Any thoughts? have I missed a thread? BTW - you guys are awesome - I had a rough idle issue that was a simple mistake on my part but your thoughts set me straight!! Maybe this is likewise.
Reverse not working almost always indicates a mechanical problem, but you're saying it only happens with the TCU connected?

First gear, reverse, neutral, and park should all have solenoid 1 on, and solenoid 2 off. The solenoids don't matter in reverse, which is why having the TCU connected shouldn't be making a difference.

Does it drive normally in forward with the TCU connected? The throttlebody to trans cable is connected and installed correctly? Enough dex/mercon (not atf4+) installed? Is the shifter cable adjusted properly (thinking you might be in neutral or park and not reverse)? Try disconnecting the NSS and see if that makes a difference (it shouldn't).
Another question - What year trans and xfer case? There was an output shaft speed sensor change in 1998 which shouldn't affect reverse, and a change in the output shaft spline count in 1991.
Thanks! its all 1989/90 stuff - 21 spline. I checked the output of the TCU and it has power to the number 1 solenoid wire. The RENIX tester from Nick in Time says it is in first gear too - I assume it means the solenoids are such that it would indicate 1st gear.

I have not actually tried to drive it yet - rear tires are still off the ground for testing. I might just do so in the morning to see if it is a mechanical issue. I went back over the schematics and can see no reason, as you say, that the TCU would effect reverse. All I know is when its connected the tires don't spin and when its disconnected they do.

Could it have anything to do with the torque converter unlock switch from the brake pedal? I do not have one connected at the time. My pedal hanger does not have the bracket for the switch so I will have to make one.

Yes, full of DEx/mercon TV cable seems to be adjusted correctly. Would that, could that have anything to do with reverse?

The tires do rotate forward with the TCU connected. I suppose a road test is the only way to see if they will actually move the vehicle, but they take some effort to stop them from turning.. Will post the results of that test tomorrow. I've seen a few threads on this and other forums with a similar issue, but they never seem to get resolved, or at least written about when they are..
The brake switch disconnected would prevent signaling the tcu to unlock the t/c, which it wouldn't be doing in reverse anyway. You might want to connect the switch and just tie it out of the way for now, which would prevent lockup instead

I actually have a relay triggered by the normal brake switch, since the remix tcu wants grounded instead of power in brake application.
Good news... Well, if bad news can be good news, then this is good news. I went for a test drive, first with the TCU connected and it seems to shift through the forward gears normally. Reverse is still a no go. I then disconnected the TCU and reverse works. However, first gear is not working so well in this configuration, 3rd and Drive seem fine. Hard to tell as I don't wan to get too crazy with a new engine/trans.

The good/bad news is, when I got back to the garage reverse was no longer working with or without the TCU connected. I guess that tells me the TCU is fine and that it needs to go to the shop that rebuilt the trans for repair. that's actually good because he can worry about getting it right instead of me tilting a windmills I can't see.

But first I will finish the paint work... Will check back in when all is said and done.

Thanks for all the effort people put into this site, both managing and especially those posting with real advice. I was on a forum once, when I was rebuilding an old Land Rover Series 2a and everyone's answer was, "I'd put a 200TDI in it", even when I asked about something completely not engine related!! :)

Yeah, you get that a lot in other sites. On a spitfire forum I was talking out other possibilities of engine swaps since my budget wouldn't allow the 13b I really wanted; they wouldn't let up on insisting that I put a miata engine in it, basically calling me an idiot if I didn't. Not even pointing out the fact they are not as cheap as they thought or that I didn't want to go that way would not dissuade them.
ah, that's a good idea with using the regular switch - maybe easier than adding a bracket to hold the plastic one.

I did it because I forgot to grab the bracket and didn't want to drive back. I had a small relay in my electronics parts bin. As I recall, you want to wire it so power from the brake lights closes the contacts for the trans switch, which pulls the voltage on the pin at the TCU to ground..