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'91 Dynasty, dreaded limp mode...

17K views 39 replies 11 participants last post by  ka9yhd 
#1 ·
Hi guys,

So I recently bought a '91 Dynasty (3.3/4speed), did a full tuneup and the correct fluid/filter change using atf+4, then drove it home (150 miles) with no problems. Today I had to change the coolant bypass hose, and disconnected the battery just because, and then after reconnecting, left to drive to work, and it upshifted into 2nd and stayed there. Before anyone suggests I need a new trans, as I know the reputation of these A604's, anything I can do to remedy this? Since these are electronically controlled, I thought perhaps disconnecting the battery did something, as I had no issues prior to this other than occasional harsh 1-2 upshift when cold. Any tips on what to look for, where to look for it, etc. would be greatly appreciated. Thanks.
 
#3 ·
I don't have a scanner to do so. However, after posting this, I disconnected the battery for about a half hour and then reconnected it, drove home from work and it shifted perfectly. Hit 4 different stop lights, and each time it did what it was supposed to as far as upshifting/downshifting, with not so much as a harsh shift...but the last stoplight, I took off and it went to shift into second, revved up like it was in neutral, then hard shifted into 2nd and went into limp mode again. So this is why I'm confused..it shifted perfectly 3 different times and then does this the 4th time? FYI my TCM is indeed the old style, and I'm wondering if replacing with the updated '92 version would fix this.
 
#4 ·
If the problem is low fluid pressure, an updated transmission computer will deal better with that. But without scanning it, you won't know if the new computer will help.

Some faults occur have to occur once and the transmission goes into limp home mode. These are hard faults. Other faults are soft faults and have to occur several times before the transmission goes into limp mode. The fact it drops out of gear concerns me.
 
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#7 ·
Any tips on how/where to scan and check for codes? It's just odd to me that it shifts fine and then suddenly has a problem, but with resetting, will shift fine again a few times.

The 1992 TCM was different than 1990-1991. I'm not saying that this is your problem, but smooth sided TCMs have been obsolete for about 20 years.
There may also be internal transaxle wear issues.
The finned TCM with the latest and greatest software version is desirable.
TSB 18-24-95 covered this. See p. 4 for the correct TCM part # for your application. Looks like 4796122:
http://www.robskorner.info/faqs/tsb/18-24-95.pdf
I read somewhere else on here that '92 model TCMs are the only newer/updated versions that can be swapped in, unless you can find a '91 and older one thats been replaced..and that TCM # 4796124 would also work ('92 minivan, finned version with newer software)?
 
#5 ·
The 1992 TCM was different than 1990-1991. I'm not saying that this is your problem, but smooth sided TCMs have been obsolete for about 20 years.
There may also be internal transaxle wear issues.
The finned TCM with the latest and greatest software version is desirable.
TSB 18-24-95 covered this. See p. 4 for the correct TCM part # for your application. Looks like 4796122:
http://www.robskorner.info/faqs/tsb/18-24-95.pdf
 
#8 ·
No, you can't swap a newer TCM. They are specific to a year/range. In 1993 they began using reflashable TCMs (a finned box instead of a plain box). However, you cannot retrofit these in an earlier car.
However, Chrysler offered a reflashable TCM for the earlier years. What you need is a finned TCM with software 4796122. The software should be shown on a decal affixed to the finned TCM box. If you are looking in a junkyard or eBay, the donors are 90-91 3.3 and 3.8 powered AC (Dynasty, short New Yorker), AY (Imperial, NY Fifth Avenue) or minivan. 3.0 powered cars won't work as a donor.

The only way to get the codes from the TCM is to have a dealer or some other shop with the proper scanner to access the TCM and pull the codes.
Unfortunately I've experienced the "it drives OK for a while after a rest" type problem. In my case it never freewheeled though. The fact it losses all gears is not good. A total electrical failure or whatever sensor failing should still leave second and reverse functional. It certainly would not hurt to try a newer 4796122 TCM, especially if you can find a used one cheap as if can still be used even if you have to have the trans worked on.
 
#10 ·
So I've gotta find a '90-'91 with the updated TCM...

Sidenote: Today I drove to work and the car had reset itself it seems, as I did not disconnect the battery this time, just parked it last night..and today it shifted just fine other than occasional kinda rough 1-2 upshift, but not consistent, other times it shifted 1-2 smoothly..and drove to work just fine, not going into limp mode at all. So I'm very confused...perhaps an updated TCM will solve this new issue?
 
#12 ·
Yes, if it has that part number that is the latest software for your car.

Rough shifts are often a sign of low pressure.
The new TCM may shift oddly at first until it it learns your transmission. On these early ones there is no quick learn with a scan tool so you just drive it.
 
#13 ·
Yes, if it has that part number that is the latest software for your car.

Rough shifts are often a sign of low pressure.
The new TCM may shift oddly at first until it it learns your transmission. On these early ones there is no quick learn with a scan tool so you just drive it.
Sounds good, I'll give it a shot and see if it helps the problem. Thanks so much for the input and help! What causes low pressure? Leaky seals I assume? It only seems to shift rough sometimes, not always, and its always the 1-2 upshift. Again, it drove close to 200 miles home to Lake Tahoe from Northern CA with no issues or any shift problems, so it's so bizarre to me that its acting this way so suddenly.
 
#14 ·
I had work done to my trans a few years back and the mechanic was intent on filling the trans with more fluid than recommended. The trans had all kinds of issues with shifting until I reduced the fluid level. I believe with my trans the fluid level has to be at operating temps, cycled thru all the stops, and then check.

Perhaps another level check if not done properly?
 
#17 ·
The problem is this latest issue sounds like it could be a sensor however the post earlier indicated the transmission freewheeled which is not an electrical fault.

I think the previous owner knew the transmission was flakey and got rid of it. You need to get it scanned because you'll spend more time and money throwing parts at it and never knowing if the transmission is any good or not.
 
#18 ·
If this is the original A 604 from 1991 and it is determined that the trans needs work, most trans shops will switch it out for a reman 41TE and not want to spend the money putting in all of the upgrades into this transmission.
 
#25 ·
Land vehicle Vehicle Car Full-size car Sedan
So an update:

I replaced the TCM with an updated version, and the transmission performed worse. As in, it would go into all gears, but when going to upshift from 1st to 2nd, it would go into neutral and rev...which obviously alarmed me. But with a cycle of the key, I could reset it and it finally upshifted, but poorly. I also noticed the fluid level reading way above the full mark, when I know for a fact I did not overfill it during the last filter change...

So, as a last resort, I drained the fluid from the pan, then added a bottle of Lucas Transmission Fix, per the advice from a former Chrysler mechanic who worked during the late 80s/early 90s, and topped it off properly with ATF+4, while running in neutral to be sure I didn't overfill it. Took it for a drive, and the first upshift was rough...and then...

Fine! It's been shifting fine since. The TCM did its few cycles to learn and the shifting smoothed out and now it upshifts and downshifts as it should. I have decided that this transmission is probably worn, as it does have high miles, and the internal seals are probably leaking, causing a lack of fluid pressure, which gave me a lack of upshifting sometimes, but now can still perform with the Lucas additive, since its thicker and is made to seal internal leaks and eliminate slipping. I don't expect it to go forever, but it appears to be a fix for now, and is doing its job again.

Thanks again for the all the help and advice! In a couple days I'll let you guys know how its doing!
 
#26 ·
Nice car! Beautiful for its age.

Yep, the fact that the rough shift disappeared after the introduction of Lucas indicates the seals had hardened and there were internal leaks. Just keep in mind that you've meerly bought time. Lucas didn't really fix the problem.

Glad it works for you now.
 
#28 ·
I don't expect it to go forever, but it appears to be a fix for now, and is doing its job again.
Lucas helped put several years on my 1992 Imperial's original transmission before I ended up replacing it. The only reason I replaced it was because first I got a great deal on a 1991 Fifth with a rebuilt tranny with a known history and I had damaged the torque converter from accidentally overfilling. Between you and me, the original transmission probably could have lasted up until I sold the car had I not overfilled that day.

It was getting lazy with reverse once in a while, and would bump shift at stop signs, but Lucas and an updated TCM improved that transmission quite a bit.
 
#30 ·
So I've gotta find a '90-'91 with the updated TCM...

Sidenote: Today I drove to work and the car had reset itself it seems, as I did not disconnect the battery this time, just parked it last night..and today it shifted just fine other than occasional kinda rough 1-2 upshift, but not consistent, other times it shifted 1-2 smoothly..and drove to work just fine, not going into limp mode at all. So I'm very confused...perhaps an updated TCM will solve this new issue?
My 92 Grand Caravan LE (long gone after 399, 992 miles and 6 yrs ago) had that same problem, locking up in 2nd gear at 147K. I had the dealer change the fluid and filter, had it checked at two other reliable trans shops, of which no one could find anything wrong, and drove it to NC from Michigan and back. Ran flawlessly. The problem resurfaced when the ambient temperature dipped down to about 25 degrees in November. Had a Chrysler re-man installed, and the dealer urged me to get a new controller, which I did, (along with the rear main seal), and it lasted to 265K. The new controller made the shifts noticeably smoother. You may be on borrowed time, but the controller couldn't hurt, assuming you have the original on the car. (My minivan's controller was mounted on the firewall.)
 
#31 ·
I did replace the controller with the updated finned type from a '91 Caravan. It has indeed made a difference. It's still a little wonky from 1-2 upshift, but not nearly like before. Obviously the trans is living its final days, but it's at least doing its job for now! Gentle accelerating and not driving it hard, combined with watching the fluid level, seems to be working for now.
 
#34 ·
'90-'93 Imperials are one of my dream cars, and I have been desperately hunting for a good one near me. They are definitely a rare bird. I'll find one eventually!
 
#35 ·
Don't give up - it took me three years to get mine! I eventually had to go to Vancouver because there were never any up for sale around here.

Reluctantly sold it in November after 8+ years due to medical issues that forced me to buy something easier to get into.
 
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#36 ·
Imperials seem to have almost disappeared, probably because there are so many high priced failures: air suspension, ABS, transmission, etc. Things those of us who've had these cars can take care of but would scare off most owners so the get rid of the car.
I really liked the 1990 Imperial I had, fully loaded, dark blue in and out. It felt odd in that it was narrow, but otherwise it was a good car.
 
#37 ·
It felt odd in that it was narrow, but otherwise it was a good car.
I had a bit of a tough time adjusting to the Pacifica because of the width after the Imperial... those first couple months, I kept parallel parking a good foot away from the curb because I was worried about getting onto the sidewalk.

Did a quick search this morning - there's one 1992 Imperial listed in Alberta, none in BC or Saskatchewan. That's about the same as when I bought mine - usually around spring and summer is when they start popping up for sale, and then it'll be 2-4 up for sale for the whole year over the entirety of western Canada. Rarely in Sask.

My advice is go for 1992-1993. Then you get the auto headlamps and the 3.8 engine was standard.
 
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#39 ·
Wanted to update on the Dynasty. The Lucas really has made a difference, and the transmission does its job fairly well now about 95% of the time. It still has a hiccup here and there that will send it into limp mode, but a quick pull over and cycle of the key resets it and then it goes back to shifting perfectly fine. These A604's are so odd. Lol in the meantime I'm saving up for when the trans finally gives up the ghost, which I'm sure isn't too far in the future.
 
#40 ·
The early Ultra Drive / A604 4 speed transmissions had their share of problems. But over the years there have been updates and service bulletins to the point that the current 41TE is almost a different transmission and if you want your A604 rebuilt, most transmission shops will just replace your A604 with a reman 41TE because it would be expensive to put all of the updates into your A604.

The original A604 trans in my 92 Plymouth Grand Voyager made it until about 90,000 miles til it developed problems with the internal seals leaking and it would not shift into 4th gear. At one point this transmission was averaging 30,000 to 40,000 miles between rebuilds. The A604 made it about 30,000 miles until something in the planetary gear set let loose and it did a hard downshift into 1st gear on me. No limp in mode it was locked into 1st gear and no reverse. The second time it did the same thing again around 30,000 to 40,000 miles. A hard downshift into 1st gear.
The first time a Chrysler dealer did the rebuild. The second and third rebuilds, I should say reman trans, were done at Kerry Brothers Trans shop in Waukegan Illinois. They have been in business since 1951 or 1953. The third reman trans was the charm. I must have well over 150,000 miles on it and it is still going strong.

This Plymouth Voyager is my only vehicle and it is my daily driver.
 
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