Bad shifting automatic

d252 · Post by **d252** » 27 Jun 2021, 13:46

Dear readers

Im working on a volvo 850 (1994) with a 2.4L and a automatic gearbox, lately its has been shifting very strange.
The first few minutes the car always shifts normal but after that it will sometimes (like 1 out of 3 shifts) not shift until 5000-6000rpm then stay there for about 5 seconds and after that will shift like 2 gears in a row or do a realy hard shift. All the shifts feel kinda hard when its acting up.

the strange part is that if i disconnect the battery the car will shift normal for about 2 days to a week and after that the problem comes back.

what i did since the problem occured.
Checked the input from the sport/eco switch
Changed the automatic oil (old one didnt smell burned and as for the color it was kinda normal to me)
Checked the gear position sensor for blocking from metal shavings (its also a bit magnetic so i guess that one is fine)
The car does not throw any fault code so is assume that its not a bad sensor

does anyone have a possible solution for this problem? My current suspect is the TCM because of the battery part.

thanks in advance
PS excuse me for the bad english

Post by **misha** » 27 Jun 2021, 15:12

TCM will not reset itself or reset DTC when battery is disconnected...like ECU.

Since you have a '94 you allready have diagnostic ports (OBDI) under the hood attached at ECU(s) box.

All you need is a pencil and a piece of paper to write down the flash codes from ALL modules in the car.

https://www.google.com/amp/s/www.matthe ... volvo-850/

d252 · Post by **d252** » 28 Jun 2021, 11:10

Car only throws fault code 444 on a3 which means "No power supply to valves in hydraulic unit" the rest of the ports all throw code 111, i have reset the code on a3 and after a big test drive the fault stays clear but the bad shifting continues.

Post by **misha** » 28 Jun 2021, 13:34

If there are not codes stored or pending... in A2...yes in A2....because TCM sometimes requests from ECU to turn on CEL....try with re-learning procedure after you reset ecu by removing negative battery cable for 10-15min.

Re-learn procedure:
http://www.volvotips.com/index.php/850- ... rocedures/

I would also check A6 & A5 for stored or pending codes.

Post by **Clemens** » 28 Jun 2021, 14:21

Did you drain the ATF once or twice? If never done before, you might need 2 or three drain and refills with at least 100 miles of driving in between.

Post by **Roger_850T** » 28 Jun 2021, 19:56

There's a potentiometer on the back of the throttle that is an input to the transmission computer (TCM); it uses that to help decide when to shift. When mine went bad it caused all sorts of weird shifting. If you take it off and turn it by hand, mine felt "crunchy". In reality, the coils on the winding had just worn out. I'd pull it off, see how it feels, test with an ohm-meter, and see if it's smooth or not.

Good luck!

Roger