BMW 4HP22, 4HP24 Automatic Transmission No Upshift, Stuck in 1st Gear, Limp Mode
This answer is applicable for many BMWs in addition to the one listed below.
I have an E32 ‘88 735i BMW with an intermittent transmission problem. It occasionally decides not to upshift out of first gear. I thought this was a “limp home” mode problem, but don’t know how to check. Also, most of the time it works fine, as long as it doesn’t miss the first shift. When it acts up, it sometimes does it from the start, or sometimes does it at a light, after I’ve already been driving. Is it electrical, switch, harness, related, or perhaps related to the manual/sport/economy switch by the shift lever?
Your 735i has the 4HP22 automatic transmission. These transmissions were used on most BMW 5, 6 and 7 series 6-cylinder models from the early ’80s through the early ’90s. The 4HP24 is basically the same transmission, but is stronger and was used in the 750i/il and the 850i/ci V12 models. Early models have standard hydraulic valve bodies and later “EH” models have electrical solenoids to manipulate the hydraulic valves, via a control unit.
These transmissions do have a history of common faults. On EH models, when a fault is registered in the transmission control unit, a warning will be displayed in the instrument cluster and the transmission may go into the infamous LIMP MODE. Note that not all faults or problems will be registered as a program fault. When the transmission goes into limp mode, you will see a TRANS PROGRAM warning or a gear image with an exclamation point in the middle. At this time, the transmission will also be locked in 3rd gear, for forward movement …. starts in 3rd … stays in 3rd (reverse, park and neutral will also be available ….. depending on the fault). Non-EH models will not register a fault on the instrument cluster and do not go into limp mode when they have hydraulic or mechanical faults.
Common faults for these transmissions include;
* A-clutch wear-out (typically no forward movement, but often reverse does work)
* Sticking hydraulic valves in the valvebody (shifting issues, possible limp mode)
* Failed electrical valvebody control solenoids (shifting issues, possible limp mode)
* Overdrive solenoid shorting (touching) against the transmission case (limp mode)
* Oil leakage through electrical harness plug on transmission (shifting issues and/or limp mode)
* Any issues with the electrical control system (EH models), such as: Control unit (moderately rare), EH control switch, downshift switch, shifter switches, etc. (usually limp mode and functional faults)
When the transmission will not upshift out of 1st gear (very often, exactly as you have described your symptoms), this is typically due to a sticking hydraulic valve in the transmission valvebody. In this case, we recommend:
* Fluid flush and some driving time.
* Check for fluid in the harness plug on the driver’s side of the transmission. If there is fluid, clean it out with brake parts cleaner spray, Also pull back the boot on the vehicle-side harness plug and clean out any oil in the back-side of the plug housing. Reassemble and test. Obviously, the plug will become oily again, but if this temporarily cures the faults, we can address the oil leakage issue by replacing the in-trans harness and plug.
If the shifting issue has not cleared up, the valve body may need to be replaced (assuming that the fault is due to a sticking valve). The valve bodies are no longer available from BMW. A used valve body (from a known good transmission) can be installed without removing the transmission.