I thought I would share what seems so far to be a successful repair to my ’97 throttle body APP. There tends to be a lot of traffic on this forum regarding problems with the TB and my car was not immune to them. This turned into quite an epistle so I’ve broken it up into entries of symptoms, removal and testing, alleged repair and results.

The APP or Accelerator Pedal Position sensor is part of the throttle body. Other cars it is called a TPS or throttle position sensor. On these it is much more complex.

Let me repeat that this is for a ’97 and I have no idea if this is useful for other years with the 4.0 engine. The TBs on the 4.2 and supercharged cars are surely different and may not be a problem to begin with. My VIN number ends in 016878 putting it late in the model year. As far as I know this is the original throttle body for the car.

Mike

 

Start the car cold and it would operate almost fine. Occasional engine fault light when the accelerator pedal was released to idle. Cruise control would work normally.

Typically warming up and hitting the freeway it would also be normal but after 50 miles the engine fault light could be brought on by easing it back to idle. Cruise control would drop out over bumps in the road.

Once fully warmed, stopping the car and letting it heat soak for a half hour then starting off again it would be at its worse. Engine fault light most of the time that transitioned into the poor performance message and finally the check engine light would latch on. The car would still run and was manageable on the freeway. The TB would cool down after about 40 miles and the engine fault light would go away, the cruise control would mostly work again, and the car would drive almost normal.

The car was acting like there was a loose wire or bad connection on the APP (accelerator pedal position) sensor. All the trouble was associated with idle position or near idle. The Check Engine code was P1227 when it showed up. A code listing I found indicated the APP was at fault.

Next testing and exploring...

 

I removed the TB and ran a set of measurements using an ohm meter on all combinations of the five pins on the driver’s side (LHD) of the unit. Setting the throttle plate in four different positions including idle I got a set of measurements that had a good pattern to them. As expected, cold, it didn't look bad.

To simulate the heat soak condition I waited until Diana left for the barn and put the TB in the oven. 200 degrees and 1.5 hours the TB casting was up to 200 using the temp probe on my multi meter. I took the TB back on the bench (using gloves) and took another set of measurements. Ah-ha! The circuit on the top and bottom pins of the five pin plug was completely open from the bottom end of the throttle’s range and was barely readable at wide open. As the TB cooled down the measurements improved.

Next, digging in...

 

Removed the APP sensor case from the throttle body and opened it up. More complex than I expected but staring at the circuits and seeing where the top and bottom pins lead you can see where the problem would be located. Using a magnifying glass to stare at the pieces I could see that the six tiny brushes were showing their 15 years and 145k miles of wear. It was a fair guess that one of the brushes was not making contact when the TB was hot and barely making contact when cold. Using my finger and lightly touching the brushes revealed one was particularly weak.
My “fix” was simply to bend the weak brush up a bit taller so it would have more contact force on the resistor tracks that make up the position sensors. Not seeing anything else that might be a problem I cleaned the assembly with tuner cleaner and reassembled.

 

Started the car and it scared me with an engine fault right off the bat. I had actually forgotten to plug the five-pin connector back into the TB. That’s what I get for eating dinner after reassembly.

Plugged it in and gave the engine bay another thorough check then started the car. Ran great, let it warm up and still seemed fine.
Took it out for a lap around the block, then two then a few, then a few miles. Twenty miles later it was still acting like a normal car so I parked it for some heat soak. Started it a half hour later and no faults or hiccups. Another few miles and it still ran fine. Ran it around the next day then put it on the road for the week.

700 miles later I’ve almost forgotten how bad it was acting. Even on the roughest parts of the freeway where it would ALWAYS drop the cruise control it is rock solid. I haven’t seen an engine fault light since I started it fully assembled.

Normally I won't drive this car unless I can see it thoroughly warmed up; at least 15 miles round trip. Had a grocery run to do today and just couldn't see using anything else in the driveway. The car so far is running great. Knocking on wood, we'll see how long my fix lasts. I'll try to remember to post reports every few thousand miles. It gets about 600 miles a week.

Blue skies,
Mike

 

Nice fix...thanks for sharing !

 

That's awesome! I often wonder what they do to a part when they send it off to have it overhauled. I mean seriously, what about overhauling a throttle body makes it cost $500? I guess it's a supply and demand thing. I'm sure there's a source somewhere where you can buy those little brushes. Maybe you should go into business rebuilding throttle bodies. Think about it, if you did one per day and made $350 in profit after paying for parts, that would be $7000 per month.

 

I had assumed it would be as such. You've done exactly what I intended if/when mine ever tossed a code. I recall reading a post somewhere of someone's visit to the rebuilder in Tennessee. He stated they pulled the car in, repaired it and had it outside in an hour. That alone told me there wasn't much going on as most of that time would be spent on r&r.

Kudos to you, good job!

 

p.s. just don't get too crazy with the tension and cause it to wear the tracks out.

 

That's almost what I'm expecting to find the next time I have to open it up. You can see the angle of the brushes has to increase as they wear. That will make the wear accelerate. I imagine the originals were at least 1/8" to 1/4" longer making for low wear on the resistor tracks.

The brush that I adjusted was extra soft. As if it had been annealed and lost its spring character. That makes sense since it is closest to the throttle shaft and is subject to the most heat transfer. (yes, that is a stretch of the imagination but the best guess I've got so far)

Anyone got a low mileage throttle body (off of a 4.0 XK8) on the shelf that they THOUGHT was a problem? Open it up and take some closeup pictures for us....

 

From the first test I find the throttle has a small dead zone from idle position. It seems the throttle plate has to move a small amount from idle before any response in the engine, then it’s a small jump. This is noticeable both in the accelerator pedal and when the cruise control picks up the throttle after coming off a downhill stretch.

I suspect the worn brushes in the APP sensor have actually changed their “calibration”. The contact point on the resistor tracks has advanced towards the idle position as the brush gets shorter due to wear. See the attached sketch. This change over time may have put the brush out of range on the resistor track. It would be nice to know what the original length of the brushes was.

 

This needs to be a sticky!

The quality rebuilders give a pretty good warranty. I suspect they are replacing the brushes and maybe the corresponding plate, too.

Great work!

 

Just over 1000 miles today and still no faults.
11 cold starts, 8 hot starts, longest non stop 260 miles.
I'll report back if any faults pop or at about 5k miles.
Damn this car is running good!

 

That was quite revealing...thanks. I'm facing a similar problem with an old windshield wiper motor on a 67 S-Type I'm restoring. Guess I have to open it up. I can't get it to the park position. Might also have something to do with a Positive vs Negative Ground and running it backwards. The spring leads/points on the internal cam I bet.

 

OregonJag,

Is it possible to give the spring an additional wind while still on the car? The purpose is to increase the pedal pressure.

 

Not worth the trouble. I think by the time you fabricated the surgical tools to do that you could have R&R the TB, R&R the throttle arm, and wind the spring tighter. However, I don't think I could get another turn out of the spring on my TB. It didn't unwind much when I removed it so it's pretty stiff to start with. Maybe later years had a lighter spring.

 

This is the 5,000 mile report. It actually made it 4,000 miles before it started sporting symptoms. The first 3500 miles it ran like it was new and I was loving it. Then I noticed the ENGINE FAULT light flicker on me just like it used to do. Then not for another week, then more frequently. Last weekend I dove into the throttle body again to see what might be the cause.

From what I could tell from the original pictures the resistor tracks on the APP were almost worn through. The one that I diagnosed as the trouble had a soft brush spring so I bent the spring to make a stronger contact. I didn't have any dielectric grease when I reassembled it so I just used tuner cleaner on it and put it back together.

When I opened it up last weekend the resistor tracks looked about the same but putting an ohm meter to it showed it obviously had completely worn away the carbon resistor track in one spot. In hind sight some dielectric grease might have made it last longer. How much longer is anyone's guess. I don'[t have an idea of how to fix this so my current diagnosis is an unrecoverable APP. It's really too bad you have to buy the whole throttle body for this one wear-item part.

Got on the web last week and found a NOS throttle body in someone's EBay auction and now it is supposed to be on its way to me. The picture showed it still in the plastic bag from Jaguar so if it works on my car I'm guessing it's good for another 150,000 miles. I'll report back after I get it mounted.

So, on a rather poetic note, I ended the day with the odo sporting 150,064 miles. The poetic part being the orange ENGINE FAULT lamp and red POOR ENGINE PERFORMANCE lamp both lit. Reminds me of the guy that came in second at Daytona a few years back. Crossed the finish line upside down and on fire, but still got second.
Mike

 

Oregonjag,

I think you did a superb bit of engineering diagnosis. And I certainly agree that it is a pity that the APP cannot be bought seperately. I wonder is the people that overhaul these things have a source for this part only. I bet they do. But would probably never tell us.

Thanks again for a great presentation revealing the results of a great bit of work.

EZDriver 2000 XK8 1995 XJ6

 

I had mine rebuilt at Auto and Truck Electronics, the Tennessee place and am very happy. Runs perfectly. They opened repaired and resealed the stepper motor input and opened and presumably R/R the APP side. They did not clean the unit. Fast turn around, $375. IMO it is the best solution. Definitely DIY!

 

2000 miles on my replacement TB and it hasn't missed a beat (knock on wood).
As usual, time spent shopping/searching will save you a little bit of dough. It's just that old balance of time vs. money. Got a good price on that NOS part still in the plastic bag at about 12% of the cost of a new one from the dealer. You can find replacements almost every day in the $500 and up range. The rebuilds are a great deal if they replace the wear items like the electronics inside the APP. I was told by one place they can't replace the APP and I would have to buy the whole unit. I couldn't tell you if other places have found a source or if another application will work on the AJ26 throttle body. These are the same units that fit on a span of XJ8s with AJ26 engines so there is a bigger TB population out there than just XK8s.

Just past 152k yesterday, time to change the oil again.

Mike