Failed Throttle Position Switch and fix
01-14-2011, 02:19 AM
#1
Three Wheelin'
Thread Starter
Failed Throttle Position Switch and fix
I was searching through the 928 forums and came across some posts on TPS failures. It appears that these are quite troublesome. The TPS includes a microswitch for idle (you can hear it click when manually opening the throttle), as well as a soft contact for WOT (it opens at something like 80% throttle but doesn't click like the idle switch). You can find pics of the TPS insides on the 928 forums.
I checked the 2 switches using a DMM on the TPS, and found that neither of them were functioning (ie infinite resistance between centre pin and each outside pin for all throttle settings). To double check I disconnected the TPS and drove the car - absolutely no difference. Time to pull this baby apart (they are glued shut). The trick to opening these is to use a utility knife to score all around the joint and then use a small screwdriver to wedge between the top cover and the base (main unit).
Upon opening the TPS, a whole bunch of coolant escaped - wow. I figured this happened when the coolant to throttle body elbow pulled out about 18mths ago. The coolant enters along the spindle of the throttle body (I can't believe how much ended up inside). Coolant is a great insulator!
After cleaning the TPS out with contact cleaner and brake cleaner, I checked all the soldered connections and found 2 dry joints with intermittent contacts which I then re-soldered. The original soldered joints weren't to substantial so I can see how these could fail over time. I also used some emery boards to clean the contacts on the WOT contacts. Apparently from what I have read on the forums, the contacts/soldered joints on these units are prone to failure over time.
It is interesting that the car actually ran "OK" with the TPS faulty or disconnected. The main fault was a stumbling idle when restarting from hot.
However, with the TPS fixed, the idle has dropped from around 1000rpm to around 900rpm, and there feels to be a bit more top end power now (the ECU can now use WOT maps). I ran some power runs on my performance box, and the power increase appears to be around 8-14KW (10-18bhp). There is certainly an increase in performance above 5000rpm, as previously the car felt a little breathless in the upper rev range.
Although the TPS now works perfectly, I have ordered a new one regardless as the case gets slightly damaged on disassembly.
I suggest that those that have an old TPS should test it. You never know what you may find.
Good Luck
I checked the 2 switches using a DMM on the TPS, and found that neither of them were functioning (ie infinite resistance between centre pin and each outside pin for all throttle settings). To double check I disconnected the TPS and drove the car - absolutely no difference. Time to pull this baby apart (they are glued shut). The trick to opening these is to use a utility knife to score all around the joint and then use a small screwdriver to wedge between the top cover and the base (main unit).
Upon opening the TPS, a whole bunch of coolant escaped - wow. I figured this happened when the coolant to throttle body elbow pulled out about 18mths ago. The coolant enters along the spindle of the throttle body (I can't believe how much ended up inside). Coolant is a great insulator!
After cleaning the TPS out with contact cleaner and brake cleaner, I checked all the soldered connections and found 2 dry joints with intermittent contacts which I then re-soldered. The original soldered joints weren't to substantial so I can see how these could fail over time. I also used some emery boards to clean the contacts on the WOT contacts. Apparently from what I have read on the forums, the contacts/soldered joints on these units are prone to failure over time.
It is interesting that the car actually ran "OK" with the TPS faulty or disconnected. The main fault was a stumbling idle when restarting from hot.
However, with the TPS fixed, the idle has dropped from around 1000rpm to around 900rpm, and there feels to be a bit more top end power now (the ECU can now use WOT maps). I ran some power runs on my performance box, and the power increase appears to be around 8-14KW (10-18bhp). There is certainly an increase in performance above 5000rpm, as previously the car felt a little breathless in the upper rev range.
Although the TPS now works perfectly, I have ordered a new one regardless as the case gets slightly damaged on disassembly.
I suggest that those that have an old TPS should test it. You never know what you may find.
Good Luck
