Volvo A25C S/N 5350V11726 1999 yr. model The tranny has developed a shifting issue. 2 days ago was shifting fine then would not shift out of 1st gear. Reverse and 1st gear was fine. Went through diagnostics and replaced the output shaft speed sensor. Checked all wiring from ecu to and through trans. Solenoids all ohmed within spec and output sensor and turbine speed sensor all ohmed within spec. Wires to the cannon plug all had good continuity. Dropped oil and filters, oil looked great with no metals present, filter was clean. Put everything back together and shifted fine for a little while then when the shifter placed in D would attempt to start in a high gear and would not go into reverse. Reset the key and it would shift into reverse or forward but then will not go into reverse again and starts trying to pull out forward in a high gear. Volvo guys seems to think it is an ecu which I'm hoping it isn't because they are scarce as hens teeth. Appreciate any advice or troubleshooting tips.
Shift solenoid for second gear, or the K2 iirc band in the back of the trans has failed. Id check pressures next.
I may not have explained the situation adequately. When the trans is reset by cycling the key on and off the trans will shift into forward and shift through all the gears but when you stop and shift into neutral when you shift back into the D position about 90% of the time it will not go into gear but if you move the shifter just a tad forward toward the 3 notch it will engage a gear but that is when it tries to pull out in a higher gear and then will not go into reverse. I took the shifter out and the little plastic cam wheel that engages the micro switches is a little worn on the D and R notch which is understandable since that is what is used the majority of the time. I took the shifter to Volvo and talked to their mechanic and service manager and asked if the shifter could cause that condition but both of them said that it would not since the micro switches are only sending a signal to the computer for electronic shifting purposes and furthermore the computer should always send a signal to the trans for 1st gear start no matter what the shifter indicates in the forward gear. I did check the solenoids with a multimeter and ohms and all of them read correctly.
Depending on software, some start in first, some start in second. So yours obviously starts in first. The shift strategy says 1st gear shift to 2nd at speed X and 3rd at speed Y. If the speed sensor doesn't pick up the needed signal, it holds 2nd and prevent the shift to 3rd. If 2nd doesn't pull faster than first, machine holds in first. So in first , a clutch pack is energized by a solenoid. To shift into second a brake band applies to stop a planetary set in addition to the clutch pack effectively changing the gear reduction. If the band is not applying, the speed can't increase, and you can't reach 3rd. This is why i suggested checking pressure. The service manual will tell you what ports have what pressure in a particular gear. If you need this information, one of us should be able to post it for you.
I attached a couple files. One is the trans schematic, the expanded view is of the solenoid pack with a legend of what solenoids are energized in what gear. Additionally I included the pressure check points and the selector switch check. There's not a lot of info as they are ZF transmissions
Thank you for these. I'll trace these down. Someone told me that you could shift these manually to determine if it was your ecu or not. They weren't sure the procedure to do that though.
I think, the automatic functioning rules overall. I think the only exception is if you have a gear hold switch. It would look like a gear with a padlock inside. Then, I think you could select and hold the individual gear.
Problem resolution results. Kept tracing this thing down and I was stumped at the erratic nature and continued to check solenoids, wiring and changed out the shifter just to eliminate that as a possibility. After a shutdown and startup shifted fine forward and would not go into reverse, then started in a higher gear before downshifting when started rolling. This has been the norm. Took the plug out of the ecu just to read ohms on everything one more time. Everything checked fine so snapped the plug back into ecu and started up again and lo and behold it wouldn't shift out of 1st gear but would shift into reverse normally. Big clue there. Took plug loose and reinstalled and wouldn't shift into any gear. Took plug out and inspected it closely with a pair of reading glasses and a flashlight and could see several damaged pins with the worst ones on the ground plugs. Took a knife and a pick and straightened them as best I could and carefully reinstalled the ecu connector, started the truck and it ran and shifted into all gears perfectly. Ran it around to check and hit some rough roads and bounced around a bit and started not wanting to shift into reverse again. Looks like I am going to be replacing a few pins but at least it was nothing mechanical or the ecu which i discovered are scarce as hens teeth. I'll update again when I get the pins changed but I am 99% that is my problem. Thanks for all the suggestions and responses. I feel kinda stupid that I didn't inspect those more closely when I was checking all the circuits.
Awesome! Thanks for posting the resolution. It is so annoying when threads are left unfinished. Glad your A25C trans issues was elec related as mine was not....
Had some female pins damaged in the 55 pin Bosch connector to the trans ECU. Having a hard time finding them. 4 prongs in the female pin. I think that they are single locking bosch crimp connector pins. Having a devil of a time locating them. Have to rebuild some of the connector especially the ground connectors. Any help sourcing them would be appreciated.
https://www.digikey.ca/en/products/result?s=N4IgTCBcDaIAoEsB2ACAxgeyUgpmgLhgE4gC6AvkA
Hope this helps a little, maybe for cross reference of pn
Thanks for the help locating these. I was able to use the PDF of the prosis info and they were still good part numbers with Volvo and ordered enough to hopefully get me going. Thank you Fun.
That site referenced by SlatcoAu, they're everywhere that has mail service and they get you any terminals or housing you can imagine. We use them a lot.