AZ_JC

I have my son's 1987 944 Turbo in my garage at home... It suddenly started totally cutting out between 1600 and 3000 RPM. Has ignition signal but no fuel, my background is BMW's so what I'd expect is the system has a fuel shut off for deceleration between those RPM. Is the Throttle Position Sensor sticking in the closed throttle position causing the system to think it's in deceleration mode when it should reading an open throttle? Pulled the cover of the TPS, checking the white micro switch that senses throttle closed - with an ohm meter it is open at all times, button on top that contacts the throttle arm moves just a small amount and will not stay in the up position. Unplug the TPS and the engine runs normal between those RPM. Before I buy a replacement... my question is am I on the right track?

Tom M'Guinn

Sounds exactly right to me. There is an overrun cut off above 1600 or so. The TPS has two signals -- a microswitch that tells the DME if the throttle is closed or not, and a pot that tells the KLR (knock/boost computer) when to trigger the full throttle maps. If the microswitch is telling the DME that the throttle is closed, it will cut off the fuel above about 1600. So sounds like your BMW experience translates over just fine. Only thing that doesn't necessarily sync up in your post is when you say the microswitch stays "open." Not sure what you mean by that, as the switch is designed to ground (which I'd call 'closed') when the throttle is at idle, but maybe we're just using different terms and/or the states aren't clear with the TPS off the car. You might want to run through all TPS diagnostic listed in the link below before buying a new one and see the link regarding fixing your own. There is an adjustment for when the microswitch opens and closes, and the issue could be in the harness or DME/KLR (e.g., cracked solder joints), but odds are the TPS is just old and bad.

http://www.clarks-garage.com/shop-manual/fuel-06.htm

http://www.arnnworx.com/catalog/inde...id=2&chapter=2

AZ_JC

Thanks for the reply - By open I meant an open circuit, no continuity, infinite resistance or OL (Open Loop). From what I read on Clarks doesn't cover the turbo TPS testing (6 pin connector) specifically - he indicates a closed circuit at throttle closed, open circuit when throttle is open. Opposite of what I'd expect the way this car is acting. Mine being an open circuit at all times would lead me to expect a high idle because it never sees closed throttle. Contacts #4 & #6 are what the micro switch are connected to on the TPS and are always open regardless of throttle position. Same for the switch it's self - always an open circuit. I never get a zero ohms reading indicating closed throttle, always open circuit indicating an open throttle. This covers the turbo pretty well: http://www.arnnworx.com/catalog/inde...id=2&chapter=1

Tom M'Guinn

That clarks link does cover the turbo -- it's near the bottom after the n/a and 16 valve motors. The other link gives more theory of operation though, so is more than enough to figure out if yours is good or bad. Not sure what to think of the open v closed thing, but sometimes it is counterintuitive with the tps off the throttle body as to which direction represent a closed throttle vs open throttle?

AZ_JC

Couple of things I've discovered: Reinstalled the TPS and the car runs like it always has.... no problem between 1600 ~ 3000 RPM. But a firm hit with a small hammer can cause it to cut out between that range. My testing has been done with the TPS off the car, so the contact inside the micro switch must be lose and moving around. Should have tested it in place to get accurate results, just the action of removing it caused it to shift from a closed circuit to open confusing my testing. Now I can get it to test either open or closed.... just by hitting it with a hammer! So now my test results match the symptoms - when stuck in open circuit it runs, when stuck in closed circuit it wont run between 1600~3000 RPM. I am guessing it will run even better when I replace the TPS.

Tom M'Guinn

Good news is you seem to have found the problem. Congrats! Bad news is that part, with a $1 microswitch and a $1 potentiometer inside, retails for nearly $300 these days. If that gives you heartburn, some people have been successful replacing the microswitch from a (cheaper) Bosch TPS used in a BMW, such as:

http://www.944online.com/cgi-bin/yab...num=1443147694