Jumping Gears at Speed

[TomMan7531]

Hi,

So last night it would seem my 87 635 with the 4 speed automatic with switchable function decided that at speed such as 70 mph with my foot static it kept bouncing from 2000 rpm to 3000 rpm, I assume changing back and forward from third and fourth. When I put the car in sport mode it stopped, again just because it stayed in the lower gear I assume

I have no idea if its connected, but the instant fuel gauge also was being temperamental. Hope someone can help and please say my gearbox isn't dying.
Thanks.

[dwcains]

Just guessing, but from your 2 clues, I'd say look for a vacuum leak.

[pit]

hi, too.
i just read about this in the german forum with same description and that´s the possible reason and what to to:
corrosion on throttle-potentiometer(-connector), fixed with contact-spray / cleaning,
under-voltage at EH-ECU (10V) caused by relais, located in fuse-box, fixed same way or replace it,
3rd, the round connector at the gearbox may have corrosion, dirt or what ever causes bad contact.
...i think, no need to explain how to fix it.

hope that helps.

greets from germany

pete

[Brucey]

I agree with the above post; the most common cause of this malfunction is indeed the throttle potentiometer; if this is intermittent, it will signal a kickdown to the gearbox ECU.

Depending on the age of the car it will either have a black TPS in a small plastic housing or an arrangement under a rubber boot. Either sort can come good with some contact cleaner (or WD40 come to that) . The rubber boot type can (usually) also be salvaged even if the potentiometer track itself is worn; it is usually possible to remount the unit slightly off-centre so that the wiper arm is working over an unworn part of the track.

My own '87 635 suffered the exact same fault and I was able to fix it as described.

Before starting to take everything apart, it is a good idea to monitor the voltage from the TPS so that you are sure that there is indeed a fault of this sort; it is the most likely thing to cause the symptoms you describe but it isn't the only possibility.

cheers

[TomMan7531]

Thanks Guys, I will take a look tonight as I found yesterday it didn't do it when the cruise control was active. I will report back my findings.

[sansouci]

Uh, standard WD-40 isn't a good contact cleaner. Try deoxit or equivalent....

[Brucey]

Uh, standard WD-40 isn't a good contact cleaner. Try deoxit or equivalent....

carbon track potentiometers are not standard contacts, and nor do they go bad in the same way as other switches, either. I have used WD40 in many carbon track potentiometers (against the advice of folk who thought they knew better) and it has worked fine. List includes hi-fi, camera, car stereo, engine management, etc etc

cheers

[TomMan7531]

So update.

I gave it basic clean due to weather and took it out for a spin and no good, worth knowing it does do when the cruise control is on as well, I have attached what mine looks like, so the 'boot' version as Brucy called it. I also checked it with a multimeter and seemed ok with no resistance.

IMG_20170516_192910.jpg (1.03 MiB) Viewed 6290 times

Any more ideas?

[Brucey]

er, yeah... 'no resistance' either indicates that your potentiometer is faulty (fused into a solid lump, maybe...), or that you maybe didn't use your multimeter properly when you tested it....?

cheers

[TomMan7531]

I have been playing around with this and still no luck, It has started holding on to revs for ages before changing gear but if I lift my foot slightly it changes up so I'm guessing the TPS is requiring replacement.

I am struggling to find the part though, all I can find is 13631273265 which is from what I can see the common one from the period, as you can see from the picture above mine does not look like this and looks more like the one below, does anyone know what this is and where I can find it? Or can you convert it to the more common TPS? As looking at my picture it looks aftermarket due to exposed wire. Hope I'm being clear :)

Capture.PNG (217.01 KiB) Viewed 6006 times

Thanks,

[Brucey]

that looks like the usual arrangement that you will find under the rubber boot. On the far side are the switches that give the usual TPS function (idle, WOT) and facing the camera (behind the harness connections is the black potentiometer housing. The housing is held to the backing plate with several small screws so that it can be adjusted for position. Because this mounting also allows a little lateral movement, and the centre (moving part) isn't located precisely, this allows the potentiometer to be reset such that a worn area in the track is avoided. The white bit in the middle is the part that attaches to the throttle spindle. If you squirt contact cleaner and/or WD40 at the white bit, it will also get to where it needs to for cleaning/lubrication purposes. [FWIW mine went bad exactly like yours 12 years ago; I gave it a dose of WD40 and moved it slightly and it hasn't missed a beat since then.]

With the ignition on, monitor the potentiometer wiper voltage as the throttle is moved. You should see a nice smooth variation in voltage between zero and five volts; sudden changes usually mean that there is a bad part (dirt or wear) in the wiper track.

FWIW I think you can convert to the later combined TPS/potentiometer unit (as used on the post 7/87 models) but you will either need to splice the wiring loom or fit the matching plug connector from the later wiring loom. Unlike the gubbins under the rubber boot, you can't adjust the elements in this unit independently of one another, and when it goes intermittent, it is usually headed for the bin.

cheers