Throttle Position Sensor

[sideshowbob]

My car is an 88-89 Highline 635 CSi, have posted already that I am having issues with the TPS, original unit seems to have developed a fault resulting in rough idle and engine momentarily cutting out.
Replaced with a second hand unit of unknown origin that seemed OK but has now developed similar faults to original, albeit not as frequent cutting out.

My car has a TPS with a 6 pin round plug and is no longer available from BMW, Bosch it would seem still make them but thy are out of stock everywhere and on back order and are expensive at around E300 (Euros).

I have noticed that there are similar Bosch units available that look the same but have a rectangular 3 pin plug, am I right to think that the 3 additional pins are something to do with the automatic transmission ?

As I am about to have the car converted from auto to manual, would this have any bearing on which TPS I can use ? Or Is there a possible way to modify car in such away as to use the more modern and more freely available 3 pin type of TPS ?

[wattsmonkey]

Here's an extract from the excellent motronic no-start guide from Hi Performance store - you'll find the whole thing with pictures with digging.

I converted my burgundy car to manual and kept the auto trans TPS with no ill effects.

Hope this helps to identify the problem before going down the parts replacement bingo route, to quote the much missed Brucey on this forum!

1986-1988 535i Automatic

Ok, lets suppose, that like me, you have a daughter that insisted on having an E28 535i with an automatic transmission, and you caved in. Don't fear, it's still a much cooler car than those her friends drive, and the throttle position switch isn't too bad. The switch looks totally different from the type in the other E28s. It's under a big round rubber covering located on the throttle body towards the front of the car. This switch is actually two separate switches, one for the idle position, and one for full throttle. If neither is engaged, the ECU assumes you are in cruise. This switch does essentially the same thing as the other type, but it communicates in a different way. It sends a full throttle signal to the automatic transmission control unit (located in the dash near the headlight switch). That unit then sends a signal to the ECU saying "full throttle". I don't want to turn this into a transmission article, but know that if your BMW automatic is shifting hard, won't upshift when it should or has various other issues, it could be this switch!

With the cover off look at the switch assembly and you will see that it's actually two switches. As you face the switches from the front of the car, the switch on your right (driver's side of the car) is the full throttle switch, and the other is the idle switch. Caution, this is correct, various automotive manuals have this backwards!

To test the idle switch unplug the connectors (each terminal has its own connector) and measure the resistance between the two terminals on the SWITCH side of the connector. With the throttle at idle, you should have continuity (essentially zero ohms) between the two terminals. Advance the throttle and once it's off idle by any significant amount you should have NO continuity, or infinite ohms. This is actually a tough test to do. I had to use a very long probe to reach that back connector. It's also tough to get those connectors on and off. I did it, and took a picture of it for this article and accidentally erased it. So the picture I have isn't the actual test.

The position of the sensor can be adjusted, so if it doesn't test correctly play with the adjustment before you buy a new switch. To adjust it, loosen the mounting screws and rotate the switch. With the throttle at idle rotate the switch until it clicks. It should click as the throttle reaches idle or just barely before. This switch almost never fails or gets out of adjustment so it's not likely to be a problem.

The picture below gives a good idea of what a good test of the idle switch should look like with the throttle at idle. Be sure to disconnect the switch before you do this test to avoid the risk of damage to the ECU.



Testing the full throttle switch is very different from testing the other throttle positions switches on Motronic cars. First of all you don't disconnect the switch, and you will be testing for voltage, not ohms. With the ignition ON, use a volt meter with pointy terminals and back probe the wires going into the switch. That means shove the pointy terminal into the back of the connector so it makes contact with the wire.

When back probing you should get the following results. With the throttle at idle, you should see about 5 volts between the black wire (terminal 3) and ground. You should also have about 5 volts between the black wire and the brown wire (terminal 1).

Now measure voltage between the yellow wire (terminal 2) and the brown wire. Voltage should be about .7 volts with the throttle closed and increase as the throttle opens to about 4.78 when it's fully ope