Hello,

I have a 1990 740 that has about 227,000 miles on it. Recently replaced the fuel pump and a week later, though it functions normally in the city, when I take the car onto the freeway, it will run in the overdrive gear instead of Drive.

I checked the relay behind the ashtray, it looks fine.

The mechanic where I usually take my car told me over the phone that it could be a kickback cable; however, when I looked in a Chilton service manual, there was no mention of a kickback cable anywhere.

If I change the transmission oil will that be a good start to eliminating my troubleshooting options? Can anyone explain to me the procedure for changing transmission oil?

Any other troubleshooting advise would help alot. Thanks.

 

I don't have this particular car or transmission, but I'll make some general comments.

It's not clear what you mean by this. At highway speeds, the transmission should be in overdrive gear, if it has overdrive. "Overdrive" is like a higher top gear, technically it really means that the output shaft of the transmission turns faster than the input shaft (which is connected to the engine crankshaft). This allows the car to run at highway speeds while the engine rotates at moderate RPM.

Perhaps what you mean is that the engine is running too fast (high RPM), because the transmission is in a lower gear.

The term you heard is not "kickback" (a usually shady business or bribery tactic), but "Kick-down". On the highway, if you are in top gear as normal, and you need a burst of power (usually for passing), you can punch the accelerator pedal and cause the AT to "kick down" into a lower gear - sometimes referred to as "passing gear". The higher engine RPMs of the lower gear give you higher torque and horsepower for the temporary acceleration you need. So I think you are saying that the car is running in "passing gear" all the time, as if it has "kicked down" but not up-shifted again into high gear.

Older automatic transmissions had a linkage, usually connected through a cable or a rod, to activate a kick-down valve that creates the downshift to passing gear. However, many newer transmissions use an electrical connection to a kick-down solenoid instead. Someone else might know right away what the 1990 740 AT uses - but I'll see if I can find the info for you. If it's a solenoid, then it could be that you (or your shop) disturbed the wiring to while working on the fuel pump. Old wires, insulation and connectors under the car are often brittle and/or corroded due to age, heat and oil exposure.

Rather than changing your own tranny fluid, you should start by checking the level. First, identify the dipstick for the transmission. It is functionally similar to the engine oil dipstick, but the handle is often yellow insead of red, and the stick itself is usually about 1/4" wide and flat - better to hold and see the level of the thinner transmission fluid, vs. the engine oil dpstick that is usually like a rod or a thick piece of wire. Your owner's manual will probably have a diagram showing where the AT dipstick is found. Next, unlike the engine oil, the tranny fluid level is checked with the engine running. Warm up the car and put it in park with the parking brake set and with the engine idling normally.

BE CAREFUL not to touch rotating fans, belts, ignition wires or hot manifolds with either hand, or your hair, clothing, neckties, jewelry etc!!!! You do this at your own risk, and think twice about it if you are not familiar with working around a running engine! Wear eye protection (safety goggles can be found at the 99¢ store).

Pull the AT dipstick out, wipe off the end with a paper towel, re-insert fully and draw it out again. Then hold it horizontally while you look at the end, to see if the wet fluid residue is between the marks for a "Warm" or "Hot" transmission. There are also marks for "Cold" or "Kalt", but I recommend you do the warmed-up check. If the fluid is low, then you add it (now with the car turned off) through the same tube that the dipstick goies in - buy a long and narrow-tipped funnel at the auto parts store, and make sure you buy the correct kind of AT fluid as listed in the owner's manual.

If the fluid level is correct, but the problem remains, then I'd recommend you take it to a mechanic you trust - it is probably a simple connection as mentioned above, especially since the problem occurred soon after the fuel pump service. If the fluid is very low when you check it, and adding it corrects the problem, then you probably have a leak. In any case, actually changing the fluid is probably not going to fix the problem - though in a general sense it's good to flush the transmission and change the fluid at the intervals recommended in your manual.

As mentioned, I suspect the problem is likely electrical, but I'm not very familiar with that particular setup. Good luck.

 

Thank you, Joel, for your help and advise. This is very instructive and will go along way in helping me to resolve this issue.