aw4 shifting issues

[coolwind57]

I've had limited time to look into this so far but I'd like to go ahead and toss this at you guys to see if I can quickly remedy this with your experience and knowledge for the next time I can afford a few minutes.

 

My 89 AW-4 Automatic (paired with 4.0L) began shifting erratically and within a few minutes stopped messing around and refused to budge past 1st gear.  I had to drive 3 miles back home in 1st.  This occurred only 2-3 minutes into the drive.  I parked and didn't look into it for a few weeks.

 

Today, I got into it and ran it.  Same thing, worked great for a couple minutes and began shifting funny--like it was searching or something, then it lost it's ability to leave 1st gear.    I got home and reset the transmission shift cable (using that little button reset method found near the throttle body).  I checked the fuse that likely goes to the TCU--yellow wires, in-line 10amp fuse at passenger foot well.  The fuse and wiring looked great, btw.

 

Next thing I did was pull the fuse and attempted to drive.  Without fuse, it reeeeaaaalllly didn't want to move at 1st gear and the truck crept along.  I replaced the fuse and it moved via 1st gear normally.

 

What's the next check?   Maybe TPS?  Should I swap in a new one?  Any other troubleshooting you can think of to narrow this down?  Sound like a bad TCU maybe?

 

It's my understanding that the AW-4 is overall pretty hearty.  Appreciate your thoughts.

 

 

 

[coolwind57]

I've got a Standard Motor Products TH67 (not the inferior TH67T) TPS on order that I found at a great price, so that's what I'll replace next to see if that'll take care of the problem.  Maybe I'll find some time in a few days to go test the old one.  If this is not the issue, then at least I'll have a backup TPS on hand for the future. 

 

just an update for those interested in following this issue with me.

[llhat]

22 hours ago, coolwind57 said:

Next thing I did was pull the fuse and attempted to drive.  Without fuse, it reeeeaaaalllly didn't want to move at 1st gear and the truck crept along.  I replaced the fuse and it moved via 1st gear normally.

 

What's the next check?   Maybe TPS?  Should I swap in a new one?  Any other troubleshooting you can think of to narrow this down?  Sound like a bad TCU maybe?

 

It's my understanding that the AW-4 is overall pretty hearty.  Appreciate your thoughts.

with the fuse pulled you 'should' have been able to manually shift the transmission to 1, then move toward D (it skips 2nd and goes to 3rd) you will not have overdrive or TC lockup...  I drove mine around for a good while manually shifting [it would try to start off in 3rd gear, then would 'go' when i moved the lever to 1]

 

According to a recent thread just down the way,  read it even though the title of the thread seems not related...

The TPS may be a decent trial, but you 'can' test the output ( I think analog meter is preferred to see sweep and any dropoffs)

 

otherwise look at an older thread i started..(it is about 7 pages back !) 

it had a LOT of tips from Ohm  and tests to wiring....

Ultimately 'my' issue was the TCU, but there were a LOT of red herrings along the way...  including the NSS... 

 

I do wish for you expedient repairs!

Edited March 16 by llhat
post2

[75sv1]

Yes, pulling the fuse requires manually shifting. I did a few trips with this. Like 400 to 600 miles one way. Watch out, you have to brake or slow down to down shift. If your trans operates manually, then its an electrical problem. I would change the fluid a few times. If not pull the pan, clean the pan and magnets.

      Now electrically,  the AW4 is controlled by 5 or more electrical components. The TSP being one of them.  There are three solenoids in the AW4. They can be checked by a plug up by the trans dipstick. They should check 10 to 16 ohms. Yours should have one speed sensor. It can be checked. It is a Hall effect sensor. As I know it, you have to use an analog ohm meter. Then rotate the shaft manually. It should blip on and off. Also, the ECM, NSS (might need cleaned and greased (lithium grease). Make sure your battery is good. The Trans ECU also. And there is the brake light switch. Maybe check it. It might be corroded. Replace if needed. 

     Mine was a '98. So two speed sensors. I replaced the solenoids, put in a shift kit. Cleaned the NSS (range selector). It was the out put speed sensor. 98 and up have two speed sensors.

[coolwind57]

Yesterday before work, I was able to properly pull the ECU fuse and test drive in each gear successfully.  The moment I plugged that fuse back in, the shifting issues returned. I suppose that makes this an electrical issue and likely not mechanical. 

 

I appreciate all the help, fellas.  I look forward to having some time to dig into this issue this weekend. 

[coolwind57]

On 3/16/2025 at 4:32 PM, llhat said:

with the fuse pulled you 'should' have been able to manually shift the transmission to 1, then move toward D (it skips 2nd and goes to 3rd) you will not have overdrive or TC lockup...  I drove mine around for a good while manually shifting [it would try to start off in 3rd gear, then would 'go' when i moved the lever to 1]

 

According to a recent thread just down the way,  read it even though the title of the thread seems not related...

The TPS may be a decent trial, but you 'can' test the output ( I think analog meter is preferred to see sweep and any dropoffs)

 

otherwise look at an older thread i started..(it is about 7 pages back !) 

it had a LOT of tips from Ohm  and tests to wiring....

Ultimately 'my' issue was the TCU, but there were a LOT of red herrings along the way...  including the NSS... 

 

I do wish for you expedient repairs!

Thanks for sharing the info.

 

Today, I replaced my TPS with a new TH67 and with spray electrical contact cleaner, cleaned the two trans connectors near the trans dip stick.  No corrosion and pins looked fine.  I then set the TPS output to 83% of the input and took her for a spin.  and....No-Go.  Still not shifting past 2nd gear.

 

Unplugged NEG battery cable, and looked for corrosion/issues with battery cables.  All good.  Replugged NEG and tried again, with fantasies of the trans actually shifting.  No such luck.

 

I replaced my NSS maybe two years ago, so I should be fine there.  Layed down and did a visual inspection of wiring on both sides of transmission and all looked well.

 

I suppose based upon your results, a TCU may be my next step.  I can really respect all of the wire-shooting you did with Ohm's guidance but I think I'll go with an R&R of the TCU and see what happens.

[llhat]

think that may be the key there...   though some of the tests we ran on wiring helped narrow it down

I did have a problem with NSS,  but replacement only got me back up lights in reverse

there's some TCU on evay, but many are not sure "IF" they work...  I got lucky on one source there.

If you find a good source,  share "after" you are pleased.  it may not be a bad idea to have a spare

 

there's a thread here about one fellow that found a bad resistor in his TCU and replaced it.

 

 

 

[coolwind57]

On 3/21/2025 at 7:09 PM, llhat said:

think that may be the key there...   though some of the tests we ran on wiring helped narrow it down

I did have a problem with NSS,  but replacement only got me back up lights in reverse

there's some TCU on evay, but many are not sure "IF" they work...  I got lucky on one source there.

If you find a good source,  share "after" you are pleased.  it may not be a bad idea to have a spare

 

there's a thread here about one fellow that found a bad resistor in his TCU and replaced it.

 

 

 

Thanks my friend.  I think a speed sensor on the tranny is sometimes the issue for some guys. 

I was not expecting $150+ for a TCU!  Man, I hope this is the issue.

[llhat]

yeah, i had to "buck it up" too but it sure is nice not manually shifting this auto transmission....

 

not sure what cost would be for a  bench technician to test the TCU board components,

some report age on solder joints is an issue, as well as failed transistor/resistors or other board level components.

 

the speed sensor is also a 'testable' part... 

[coolwind57]

Well, a "it works" TCU from E-bay didn't solve the issue unfortunately.  I'm gathering thoughts on what to do next.  I may fool with the NSS next, I suppose.  Give it a good cleaning and lube and reinstall. 

 

My truck acts the same with either TCU...starts off great in D.  But doesn't shift.  i pull the in-line fuse and I can shift throughout each gear manually. 

 

That in-line fuse tested good, btw as did the 7.5a fuse in the fuse box, driver's side. 

 

TPS is now new and had no issues getting it adjusted.  Numbers looked good.

 

Any ideas?  I'm so limited in time so shooting wires would be my unfavorable last option....just was hoping it was something I could discover relatively easily.  Am I overlooking something that's easier than shooting wires?

[llhat]

is it possible the evay TCU was also defective...  that was my concern also with any purchase there.

 

in my quest, i "knew" my NSS was not proper, as there were no reverse lights.  I bought a new one on evay (taiwan mfg) and it worked.

 

Your situation was different than mine,  You stating 'starts off great in D, no shift'  and My situation was D was a high gear start "and" no shifts.

 

so sounds like if you are starting off with D and D is giving gear ONE,  either logic in TCU (through resistors, etc)  or a Solenoid problem inside the transmission

Have you checked resistance on the the solenoids?  it 'can' be done at the connector under hood near transmission dipstick, or at the TCU connector... (see my older thread with advice from Ohm...

 

i do see where you state manual shifting is possible, so on the electronic side is the path.

[coolwind57]

47 minutes ago, llhat said:

is it possible the evay TCU was also defective...  that was my concern also with any purchase there.

 

in my quest, i "knew" my NSS was not proper, as there were no reverse lights.  I bought a new one on evay (taiwan mfg) and it worked.

 

Your situation was different than mine,  You stating 'starts off great in D, no shift'  and My situation was D was a high gear start "and" no shifts.

 

so sounds like if you are starting off with D and D is giving gear ONE,  either logic in TCU (through resistors, etc)  or a Solenoid problem inside the transmission

Have you checked resistance on the the solenoids?  it 'can' be done at the connector under hood near transmission dipstick, or at the TCU connector... (see my older thread with advice from Ohm...

 

i do see where you state manual shifting is possible, so on the electronic side is the path.

I have not checked resistance of the solenoids.  I'll try to accomplish that this weekend.  Just sucks that there seems to be no shop to bench test those TCUs. 

 

i kind of assumed the trans solenoids were good when I was able to manually shift with the in-line fuse removed...but I suppose there may be electrical/wiring issues going on, so I get what you're saying. 

 

Thanks for you help, man.  I'll report back when I get some troubleshooting time in the next few days. 

[llhat]

with the fuse removed, the solenoids are null and void,  the shifter changes the manual valve to direct flow directing internal fluid flow, otherwise the solenoids fire to perform the shift.

 

you pull the lever down to 1-2 and with a good tcu it will shift to 2 on its own, otherwise, it will stay in first gear.

 

agree about 'someone' to get to board level on that box for testing and repair,  there is a thread on here about someone replacing a resistor on board.

he reported most failures have been solder joint failures probably as an effect of age and heat.

 

I found a seller on evay that stated 'rebuilt' tcu,  but they were not inexpensive at all...

 

then there's the complete alteration to do toggle switch application of power to the solenoids to manually switch,  getting overdrive and converter lockup...

but one has to remember to unlock the converter at stoplights.. 

there's writeups on this process also...  IIRC it bypasses the TCU.

 

@Ohm really helped me with tracing out the connectors, etc..  

 

it is frustrating trying to 'fix' but once it is...........  betta  

[coolwind57]

2 hours ago, llhat said:

with the fuse removed, the solenoids are null and void,  the shifter changes the manual valve to direct flow directing internal fluid flow, otherwise the solenoids fire to perform the shift.

 

you pull the lever down to 1-2 and with a good tcu it will shift to 2 on its own, otherwise, it will stay in first gear.

 

agree about 'someone' to get to board level on that box for testing and repair,  there is a thread on here about someone replacing a resistor on board.

he reported most failures have been solder joint failures probably as an effect of age and heat.

 

I found a seller on evay that stated 'rebuilt' tcu,  but they were not inexpensive at all...

 

then there's the complete alteration to do toggle switch application of power to the solenoids to manually switch,  getting overdrive and converter lockup...

but one has to remember to unlock the converter at stoplights.. 

there's writeups on this process also...  IIRC it bypasses the TCU.

 

@Ohm really helped me with tracing out the connectors, etc..  

 

it is frustrating trying to 'fix' but once it is...........  betta  

Oh cool.  I am getting a better understanding now, thanks.  I'll get the ol' multimeter out hopefully tomorrow and go over what you and Ohm talked about to test those solenoids.  If they test good then I'll take my NSS off and clean/lube.  If that doesn't work, then perhaps I'll probably be sitting on two busted TCUs unfortunately.  That would really suck.