Throttle Position Sensor Repair

The 951's TPS is not normally considered a repairable item. If you're trying to keep up with maintenance on a 25 year old car, some of what is "normal" may have to go out the window. This is pretty much the situation on my TPS.

In the process of looking into why I'm seeing a high idle (1100 rpm), I went through the usual checks of the TPS. And sure enough the micro switch wasn't closing (testing for continuity between pins 4 & 6 with throttle at idle). I could hear the clicking but testing the contact confirmed a problem. This can be checked at the switch or at the DME, I was suspect of the switch simply because it seemed sort of covered in oil film. This might have been a simple adjustment but adjusting the TPS while the mounted on the engine is a real trick. I found it easier to just pull the throttle body and test/tweak the TPS on the workbench. Side note: installing cap screws for mounting the TPS will allow on-engine adjustment with a small hex wrench.

With the throttle body removed, I verified the problem again and attempted to adjust the switch position. That was no help, as I couldn't get continuity between the two switch pins of the TPS. So, I'm about to throw this piece of expensive plastic into the trash and just was considering how my split finger fast ball has really been zinging lately, when I thinks ... wait a minute. Why not cut this piece of crap open and see what makes it tic. 

So with a little pressure from the old woodworking vice and a machinist's scribe, I poke at a suspect corner and am rewarded with a little plastic pop. A similar thump to the opposite corner and I starring into the mysterious TPS.

Heck, this is nothing special. Just a small switch and a potentiometer. But, it was all covered in a film of dirty oil, so with a little brake cleaner, I get a good look at the internals. Electrically speaking, above is my interpretation of what is happening. Caution: Some TPSs have fibers attached to the potentiometer's wiper-arm. These help to make contact with points across the resister. Be careful not do damage these fibers.

Notes: The throttle position switch consists of a switch and a potentiometer. The switch is used to signal the DME to enable the idle stabilization system. 

TPS Test Procedure (from Turbo manual):

1. TPS Idle Contact:

1.1 On cold DME control unit plug connect Terminal 2 and ground with ohmmeter.
Indication: Throttle closed = 0 - 10 ohm
Throttle open (~1 degree) = infinity ohms

1.2 If there is no indication, carry out testing directly on TPS, Terminals 4 and 6.

1.3 If TPS is okay, check power flow to DME plug terminal 2.


2. Throttle Potentiometer

2.1 Remove TPS plug. Connect voltmeter with Terminals 1 and 2 on plug and switch on ignition.
Indication approx. 5 V

2.2 Switch off ignition and pull plug connector on K/CP (KLR) control unit. Connect ohmmeter with terminals 22 and 23.

Move throttle slowly to full load position. Instrument must swing continuously to the final value without breaks.
Indication: Idle position 320 - 670 ohms
Full load position 2.7 - 4.7 K ohms


3. Checking Throttle Valve Cut-Off

3.1 Pull plug off TPS and bridge Terminals 4 and 6 with auxiliary wire.

3.2 Start engine and increase speed to 1600 rpm. The motor should begin to surge.

After the clean-up I recheck the switch and holy-moly, its working now. 

So, maybe I can use this puppy after-all. Checking the potentiometer, it seems fine too. So with a little RTV to reseal the case, I remount it to the throttle body. Since I have the meter right here, now is the perfect time to set the position of the TPS and verify the fix.

So ends a successful hour of tinkering.

Throttle Body Resealing

Actually this is not the end... upon further study into why and how the oil film making its way into the TPS, I find that I likely have worn seals in the throttle body (there is a great post on this in RennList). My buddies on RennList patiently explain to me that  when under boost, air pressure is placed all along the intake system and finds a way out via the worn seals. Once past the seals on the passenger side of the throttle body, the oily air goes directly into the TPS. Sometimes the TPS cap will even be blown off. 

So, I need to pull the throttle body again and replace the shaft seals.     

When I get the hood open and get a look at the TPS, sure enough the cap is popped open. Hmmm, note to self...maybe RTV is not the right stuff for closing this thing up. ;]

From here, I started through a long process of locating the parts needed to reseal my throttle body. I incurred quite a bit of expense in finding the correctly sized o-rings - several of them are  un-common sizes. Too large and they will promptly leak when typical boost pressures are applied.  Too small and they are easily cut (damaged) during installation. Naturally, the o-rings can only be acquired in quantities of 100.  This is how I come to offer throttle body reseal kits on my website.  If you're interested in doing this procedure on your throttle body, the kit provides everything you need. More on it here.

Throttle Body Comparison Pics

Here is a pic of the 951 vs. 944 throttle bodies.

Intake Comparison Pics

Here is a side-by-side peek at the 944 vs. 951 intake manifolds. The finish is not OEM.