Comanche Stuck in 3rd

[SequioaMancheBoogers]

Hello, first post, before I discuss the issues, I've tried my best to look at most of the forum for a solution.

I have an 87' Comanche, 2WD automatic, 4.0 inline 6.

I was at the gas station, pulled in just fine, got my dranky drank, and when I went to pull out, reverse worked fine, put it into D, and I had no power whatsoever. I pulled my TCU out, and I still had the same issue, I have to manually shift it from 2-1 to D. And when put it in D, it starts off in 3rd, doesn't up or down shift.

Reading online, I've seen a couple of things as far replacing the TCU, replacing that TPS, stuff like that. I have a new TCU lined up hopefully, however I just replaced the TPS after diagnosing the last one. When the TPS was relaxed, it was reading 4.8 volts. Switched out the new one, and the open throttle position shows me 4.8V.

I guess my first question would be, How do I diagnose the TCU with a multimeter? How do I know if my new TPS was installed correctly? And is my transmission just shot? I've been after this for months, my mechanical knowledge extends to replacing an alternator on my dad's comanche, and an actuator, so I'm learning from scratch here. I'd appreciate any feedback, tips, etc.. Thank you in advance.

[pizzaman09]

Howdy, welcome to the forum!

I'm no expert the automatics but I figured I'd add my two cents.

 

This doesn't sound like a transmission mechanical issue to me as it seems to shift through each of the gears manual with no problem.  

I believe there is a selector switch on the side of the transmission of most automatics of that vintage that is actuated by the shift lever.  I would strongly suggest looking at the condition of that switch before going through the effort of replacing the tcu.

[Ωhm]

[SequioaMancheBoogers]

That was the first thing I checked!

[SequioaMancheBoogers]

Thank you pizzaman09, do you mind elaborating which side? I apologize if it seems like I need my hand held, truth of the matter is, I do.

[Ωhm]

One more fuse to check. TRANS (7.5A).

 

 

[SequioaMancheBoogers]

Checked that one as well!

[jpnjake]

Here’s some information about how the aw4 works, by default without a tcm hooked up you will have 2 gears, you will have first gear in 1-2 and in 3 and overdrive you will have 3rd gear, definitely an issue with your tcm, getting power to the tcm, or it could be a problem with the wires that control the solenoids in the valve body either way it’s some sort of wiring or module problem, most likely not a transmission problem 

[SequioaMancheBoogers]

1 hour ago, pizzaman09 said:

Howdy, welcome to the forum!

I'm no expert the automatics but I figured I'd add my two cents.

 

This doesn't sound like a transmission mechanical issue to me as it seems to shift through each of the gears manual with no problem.  

I believe there is a selector switch on the side of the transmission of most automatics of that vintage that is actuated by the shift lever.  I would strongly suggest looking at the condition of that switch before going through the effort of replacing the tcu.

Okay, looked under there, found the selector switch, how do I go about troubleshooting this?

[SequioaMancheBoogers]

2 minutes ago, jpnjake said:

Here’s some information about how the aw4 works, by default without a tcm hooked up you will have 2 gears, you will have first gear in 1-2 and in 3 and overdrive you will have 3rd gear, definitely an issue with your tcm, getting power to the tcm, or it could be a problem with the wires that control the solenoids in the valve body either way it’s some sort of wiring or module problem, most likely not a transmission problem 

Appreciate your suggestion! Can you point me in the direction of like a troubleshooting guide? I've looked at my TCU inside and out, and so far I don't see any burnt out capacitors/terminals upon visual inspection. Are there any good wiring diagrams? I've tried to look them up before but all I see are Ebay ads for people selling their's.

[pizzaman09]

11 minutes ago, SequioaMancheBoogers said:

Okay, looked under there, found the selector switch, how do I go about troubleshooting this?

I'd personally take it off, clean it up with some electrical contact cleaner and put it back together.  I really don't know anything about the AW4 transmission, my truck is a manual.  However I've worked on some similar vintage automatic Fords and cleaning up that switch helped it work much more repeatably.

 

I suspect jpnjake is likely on the right path with tcu power.

[SequioaMancheBoogers]

1 minute ago, pizzaman09 said:

I'd personally take it off, clean it up with some electrical contact cleaner and put it back together.  I really don't know anything about the AW4 transmission, my truck is a manual.  However I've worked on some similar vintage automatic Fords and cleaning up that switch helped it work much more repeatably.

 

I suspect jpnjake is likely on the right path with tcu power.

Yeah I'm actively looking on here for a solid TCU troubleshooting guide. I appreciate the both of you for y'alls help! I'll look at both in the morning! While I was under there, I noticed one of the connections was cracked, not sure if it's holding continuity or not, but it doesn't look right so I'll end up replacing it anyway. Thanks again!

[cruiser54]

Go to www.cruiser54.com and complete Tip 10 first.

 

Have you ever refreshed the ground at the dipstick tube stud per Tip 1 on my website? 

Here's an excerpt.

 

All the components listed below ground at the dipstick tube stud:

Distributor Sync Sensor, TCU main ground, TCU “Shift Point Logic”, Ignition Control Module, Fuel Injectors, ECU main ground (which other engine sensors ground through, including the Oxygen sensor, Knock Sensor, Cruise Control and Transmission Sync signal. All extremely important stuff.

 

It's likely that your TCU failed or the fuse socket is bad under the dash. You had voltage at both sides of the fuse when you checked it?

Sometimes the socket itself is corroded. 

 

I'm going to once again encourage folks to start snagging TCUs at the junkyard. 87 to 90 interchange. 88 and later shift nicer due to different programming bur most people won't even notice the difference in reality. 

[jpnjake]

I don’t have any guides that come to mind, I can check my wiring book at home, but other than that it’s just the advice I have.
First thing I’d do is unplug the tcm and check for 12v with the key on to see if it’s getting power, if it’s not then your problem is that, if not it’s either the tcm itself or the wiring after the tcm 

[SequioaMancheBoogers]

6 hours ago, cruiser54 said:

Go to www.cruiser54.com and complete Tip 10 first.

 

Have you ever refreshed the ground at the dipstick tube stud per Tip 1 on my website? 

Here's an excerpt.

 

All the components listed below ground at the dipstick tube stud:

Distributor Sync Sensor, TCU main ground, TCU “Shift Point Logic”, Ignition Control Module, Fuel Injectors, ECU main ground (which other engine sensors ground through, including the Oxygen sensor, Knock Sensor, Cruise Control and Transmission Sync signal. All extremely important stuff.

 

It's likely that your TCU failed or the fuse socket is bad under the dash. You had voltage at both sides of the fuse when you checked it?

Sometimes the socket itself is corroded. 

 

I'm going to once again encourage folks to start snagging TCUs at the junkyard. 87 to 90 interchange. 88 and later shift nicer due to different programming bur most people won't even notice the difference in reality. 

I will give her a go, thank you man!

[cruiser54]

[SequioaMancheBoogers]

So I have a TCU from a 90's jeep that I just got off market place. However, MY tcu said 86, although my jeep is a 87 (might have been marketed as such, and assembled in 86) and the TCU I purchased today, says 91. I've tried the connector for it, same pins, but there's different grooves. Will this work if I swap connectors? They look the same, they run the same voltage, I just don't want to install it and then @#$% something else up. 

[SequioaMancheBoogers]

3 hours ago, cruiser54 said:

Also, I looked/cleaned/ all the connectors, no change. I got this new TCU relatively cheap, not really crying about it. 

[cruiser54]

Must be 87 to 90.

 

Notice the color of the plastic plug on the 2 TCUs. I bet they're different color.

[SequioaMancheBoogers]

15 hours ago, cruiser54 said:

Must be 87 to 90.

 

Notice the color of the plastic plug on the 2 TCUs. I bet they're different color.

Yeah, it's teal. It does the same thing though, no? Would I be able to swap the connectors? Same voltage, same company.

[SequioaMancheBoogers]

40 minutes ago, SequioaMancheBoogers said:

Yeah, it's teal. It does the same thing though, no? Would I be able to swap the connectors? Same voltage, same company.

Trick question, it is NOT the same thing. I just forced it in there, now when I put it into D it'll start in 1st, but won't automatically shift. So I definitely need a another TCU.

[cruiser54]

wrong TCU for your application if the color differs. 

[gogmorgo]

TCU pinouts are different for ‘91+.
There’s a NAXJA thread out in the internet somewhere called “Everything you want to know about the AW4” or similar. Might be linked in the DIY master thread. It’ll go over what you need to do to make different year components work together. 

[SequioaMancheBoogers]

1 hour ago, gogmorgo said:

TCU pinouts are different for ‘91+.
There’s a NAXJA thread out in the internet somewhere called “Everything you want to know about the AW4” or similar. Might be linked in the DIY master thread. It’ll go over what you need to do to make different year components work together. 

I have a multimeter, 3D printer and a 3 speed manual jeep, I'll see if I can figure that out. Thank you!

[cruiser54]

I have the correct tCU in my parts shed.