TPS electrical testing isn't always conclusive

mallard ducks · 07-13-2009, 09:53 AM

The TPS test procedure in the WSM or Clark's isn't always conclusive.

Ever since getting my 944S 3-4 years ago it's suffered from a part-throttle hesitation - my TPS always tested to spec at the DME plug and at the switch itself(and it's always been properly adjusted) so I'd always discounted it as the source of the problem.

I finally decided what the heck, I"ll replace the TPS anyway.

Nice. The part-throttle hesitation is gone. What a huge difference. Took me a couple of minutes to adjust to driving without it!

Anyway, the old TPS wasn't operating entirely correctly despite testing good.

If you're suffering from a part-throttle hesitation, don't assume your TPS is good just because it tests good.

bazar01 · bazar01's Garage

It's just two sets of normally open contacts. It's not too hard to test those unlike the variable resistor type.

pauloregon · 07-14-2009, 08:12 AM

Interesting comments, Mallard. I am still chasing a part-throttle acceleration hesitation problem. Since as bazar01 points out, the TPS is just two SPST switches, it shouldn't have anything to do with the throttle position except at the low end and high end. But, as I experiment with my hesitation problem, after changing a number of parts and checking grounds, I wonder if my TPS is the cause of my problems. Most recently, I can easily reproduce the hesitation on the freeway, but then I can slow down, push the throttle to Wide Open and accelerate thru the hesitation zone on the tach with no hesitation noticed.

In my problem, the hesitation is at the same rev/gear combination where I spent most of the time driving on the highway at 65mph. Even though it doesn't make sense, could 20 years of holding the throttle in this position for most of my driving each day be due to a "worn TPS"?

I'm replacing my fuel filter and cleaning my fuel screen this weekend, but if this doesn't fix the problem, a new TPS is next.

pauloregon · 07-14-2009, 08:36 AM

One more comment: Looking at Bruce Arnn's website and his TPS schematic, you can see that the TPS is not two switches. It's one SPST switch and one potentiometer. I really think it's the pot resistance, just like we see with the ARM wiper/track issues, that's failing in some of these cases.

Mask of Destiny · 07-14-2009, 09:19 AM

Quote:

Originally Posted by pauloregon

One more comment: Looking at Bruce Arnn's website and his TPS schematic, you can see that the TPS is not two switches. It's one SPST switch and one potentiometer. I really think it's the pot resistance, just like we see with the ARM wiper/track issues, that's failing in some of these cases.

There are two versions of the TPS. The one used in the 944NA is just two switches, the one used in the 944 Turbo has a potentiometer like you describe.

Basically, the DME has 3 fuel maps, an idle map, a part-throttle map and a WOT map. When the throttle is closed and the idle switch in the TPS flips, the idle map is used. When the throttle is all the way open and the WOT switch flips, the WOT map is used. If neither switch is triggered, hte part-throttle map is used. Both the idle and WOT fuel maps are 1-dimensional. The amount of fuel delivered is just a function of engine RPM. The part-throttle map is 2-dimensional so fuel delivered is a function of engine RPM and the output of the AFM.

If everything works fine at WOT, but not at partial-throttle I would suspect a problem with the AFM or a vacuum leak.

mallard ducks · 07-14-2009, 12:37 PM

Could be the switch not coming off the idle map as quickly as it should, even though it shows it's doing so when testing it?

My part-throttle hesitation didn't manifest itself unless the engine was fully, fully warm, i.e. switch tests good when switch is cold and no hesitation when first starting to drive, but as switch gets hotter from engine heat the problem starts.

After 20 years, probably all the wiring, switches and sensors in our cars can be viewed with "suspicion"

bearone2 · 07-14-2009, 01:26 PM

Quote:

Originally Posted by mallard ducks

The TPS test procedure in the WSM or Clark's isn't always conclusive.

Ever since getting my 944S 3-4 years ago it's suffered from a part-throttle hesitation - my TPS always tested to spec at the DME plug and at the switch itself(and it's always been properly adjusted) so I'd always discounted it as the source of the problem.

I finally decided what the heck, I"ll replace the TPS anyway.

Nice. The part-throttle hesitation is gone. What a huge difference. Took me a couple of minutes to adjust to driving without it!

Anyway, the old TPS wasn't operating entirely correctly despite testing good.

If you're suffering from a part-throttle hesitation, don't assume your TPS is good just because it tests good.

the switch needed to be burnished, cleaned with an eraser(where the fingers make contact), to eliminate carbon tracking, which is the hesitation spot.

i've been doing this for years on many bosch tps's in older volvo's with fi.