Annoying Transmission Issue.

[ozbmwnut]

Help! My "86 E24 auto has an issue with gear changing. Although i have owned the car for almost 2 years, it has hardly been driven, and this issue was evident only after my first drive at speeds of around 100 kmh. The issue is, at that speed the car continued to jump from 4th gear back to 3rd, then to 4th again. This occurred continuously as i was on the freeway, for a journey of around 25 kms. As you can imagine, it made the trip far from enjoyable. My mechanic has the car with him atm, and he has just rang me saying he THINKS the issue is with the "sport/economy"(?) switch on the centre console next to the gear selector.. He has had the car on a Dyno, and says he has managed to stop the car from doing it if he "fiddles" with that sector switch on the centre console.. Has anyone had this issue before? Or any other suggestions? if so can someone describe the part that may be at fault, and how easy they are to replace(?) I am based in Melbourne, Australia. Any advice would be greatly appreciated, and thank you in advance, regards, Steve.

[Brucey]

IME the most common cause of this fault is the TPS, not the mode select switch. On E24s fitted with the EH gearbox, the TPS includes a throttle angle sensor (which reports only to the gearbox ECU) as well as the idle/WOT switches. It isn't uncommon for the TPS to go bad (esp in storage) and this causes an erratic signal to the gearbox ECU.

It turns out that the signal from a erratic TPS is interpreted by the gearbox ECU as being that of you wanting a kickdown PDQ, so when you are in 'E' and 4th gear (cruising), the car will drop into 3rd gear as if you were wanting to get a wriggle on.

You can diagnose that it is the TPS and not the switch when driving the car. If the switch is bad the box will shift into 3 at any speed. However if the TPS is bad the gearbox will become more reluctant to shift into 3 the faster you go (i.e. as normal). The gearbox is programmed to drop into 3 if (when in 4th) any significant throttle is used below ~100kph and the 'lockup top gear' will hold at increasing throttle settings the faster you go. By the time you are doing about 150kph or so it practically takes WOT to provoke a shift to 3rd gear. If the TPS is faulty the shift behaviour at high speed will be 'normal' pretty much, but if the mode switch is bad it will still drop into 3rd gear regardless of speed or throttle setting.

Note that the TPS will often improve by itself in a car that has been stored; simply though use the wiper can get cleaned up a bit.

There are two versions of the EH TPS; one with a large rubber boot over it and another (fitted to the later models I think) which is a smaller unit with a single harness connector to it. The former type can be fiddled with; the potentiometer housing can be moved very slightly off-centre and this allows the wiper arm to access a previously unworn section of the carbon track inside. This means that even a worn unit can often be persuaded to work again.

cheers

[plip1953]

From memory (but it's a bit hazy) isn't the speed at which this problem is said to occur about the same at the speed at which it goes into gearbox "lock up"? Could this be a factor in the equation?

Does the car operate perfectly normally if Sport or Manual modes are selected? In particular, if a gear change occurs spontaneously when in Manual mode that, to me, would tend to support the idea that the selector module is faulty.

[baders]

This could be as simple as a faulty kick down switch. If you have the appropriate ETM, you can work your way through many test procedures on the electronic transmission control.