Throttle Position Sensor Voltage Climbs!

I have several issues going on with my '88 Mustang. The issue I'm working on right now is a random high idle. I've reset the base idle many times. I've cleaned the salt & pepper shakers. I've replaced the IAC valve with a new genuine Motorcraft part. After doing further research, I decided to try the throttle position sensor. The throttle body is BBK so I thought I would put a Motorcraft TP sensor on the car. I adjusted it to .99 V with KOEO, tighten the screw, recheck and it's now at 1.00 V. I loosen the screws and re-adjust it and now it's reading 1.01 V. Every time I check it, the voltage is HIGHER! I did a COMPLETE base idle reset with the car again, got it back down to .99 V, tighten the screw and then watched the voltmeter climb AGAIN. I've gotten readings as high as 1.68 V.

Why is the voltage climbing? Is my PCM bad?

Could this cause the car to idle high at random times? Last week I drove the car and got a red light after 1/4 mile and it was fine. I got a red light a mile later and it was at 1500 RPM. Another red light a mile later and it's still at 1500 RPM. After about 5 miles of driving off idle it finally settle down.

I can't get the car to go into reverse without shutting it off and waiting for the input shaft to stop spinning. When the car is off, I can hear the shaft howling. Is the high idle causing the transmission to keep spinning or is this a completely separate issue?
 
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Read and follow directions.
Welcome to :SN:
 
Read and follow directions.
Welcome to :SN:
Yep, I've done all of that, most of it I've done at least twice. :(

Last voltage check before closing the hood 15 min ago was 1.86 V.
 
Haven’t checked for codes this year. Last year all the smog codes came up because the thermactor stuff has been removed. There is a high pitch squeal when the IAC is unplugged that may be a vacuum leak. The car idles at 600 RPM all day with the IAC unplugged, which leads me to believe something is telling the IAC to open up when it shouldn’t.

I put on a 93 Cobra intake, 1.7 rockers, MSD ignition & distributor, Derale fan and controller, 3G alternator. No MAF.

I have not run a wire from the engine block to the PCM ground. Could that be causing the TP sensor voltage to gradually rise?

The TP sensor was high even when I unbolted it. It kept rising for at least 30 min before I walked away. It was not resetting when I shut the ignition off.

At this point, I’m scratching my head why the voltage steadily rises. This should have been a 5 minute install but turned into half a day of head scratching and I’m considering selling the car to get rid of the frustration. :( the service history all the way back to 1988 shows idle issues.
 
The first step is to check for codes.
I bet there is a vacuum leak. Do a smoke test.
Why would you need to add a ground? You bolted on the grounds that are supposed to be there?
I am suspecting a vacuum leak as well. According to the trouble tree mentioned above, when you do a 3G alternator swap you need to put a heavy gauge wire from the engine block to the PCM ground bolt (even though it seems like overkill to me and if it's necessary why don't any of the 3G swap kits include it).

I'm trying to fix issues one at a time. Even if there is a vacuum leak, why would the voltage be climbing on the TP sensor? I'm testing it with KOEO so a vacuum leak SHOULD be irrelevant, right?

I'll run another code scan tonight and see what comes up.
 
I did spray some CRC Electronics Cleaner into the harness before install and swabbed it out as best as I could. The new Motorcraft TP sensor was packed with dielectric grease from the factory.
Then testing the sweep of tps with a meter and making sure it returns to the same resistance every time would be my next step.
 
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Have you tried to clean and tighten up the connections in your salt and pepper shakers?
Have you checked the output voltage of the alternator where it connect to the starter solenoid when this is going on?
 
Have you tried to clean and tighten up the connections in your salt and pepper shakers?
Have you checked the output voltage of the alternator where it connect to the starter solenoid when this is going on?
I cleaned the S&P about a year ago and thought I had solved my problems. Unfortunately my first drive with the car this year about two weeks ago made it apparent that it was not the issue. I re-cleaned them and applied fresh dielectric grease this evening.

The tests I've been performing are Key On Engine Off so the alternator should be irrelevant.
 
This evening, I re-cleaned the S&P shakers, and then used a wire brush to make sure the TP sensor connector is clean. I then pulled out my scan tool decoder and discovered that the Duracell batteries I had put in there last year had corroded and apparently made the scan tool worthless.

I talked to one of my Ford mechanic buddies and he suggested I check the orange wire to see what the reference voltage is. I grounded my voltmeter to chassis ground and the initial reading came out to 5.55 volts. Then it started climbing to 5.56, then 5.57... This seems to line up with the steady increase in the voltage coming out of the green wire.

So what does it mean when the reference voltage is gradually climbing, and for that matter, why is it higher than 5 Volts?
 
Well, I feel REALLY dumb and embarrassed.

I used my meter and checked the battery voltage. It came up to 14.85 Volts and was climbing... I checked the battery voltage in my pickup truck and it was 14.35 and climbing... I pulled out an AA battery and the voltage was 1.8 and climbing. I started questioning the accuracy of my voltmeter. I dug out a cheap dollar store voltmeter that my wife purchased long before she met me and started testing things. The needle didn't move and the numbers made more sense. I started shopping for a new voltmeter but then I noticed that her old voltmeter has a battery test function. I tested the battery in my meter and it read REPLACE.

I replaced the battery. Reference voltage reads 5.05 volts and the TP sensor was reading .82 volts.

I want to apologize for wasting everyone's time for my own stupidity and I really appreciate the input provided.
 
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I’m glad you found one big problem,
I’d still want to run the surging idle checklist and the smoke test with the hanging idle.

As far as reverse, most T-5 s do not have a reverse syncros, so they can be picky about reverse. I suspect your clutch needs adjusted to disengage farther to make it happy. I have better luck getting it in reverse quietly and consistently on first start up by letting the clutch out in neutral, the pushing the clutch ALL the way down to shift.
 
I’m glad you found one big problem,
I’d still want to run the surging idle checklist and the smoke test with the hanging idle.

As far as reverse, most T-5 s do not have a reverse syncros, so they can be picky about reverse. I suspect your clutch needs adjusted to disengage farther to make it happy. I have better luck getting it in reverse quietly and consistently on first start up by letting the clutch out in neutral, the pushing the clutch ALL the way down to shift.
I spoke with a transmission guy and he also said it sounds like the clutch cable needs adjustment. I installed the steeda adjustable cable last summer hoping that it would solve my problem. I did not see an improvement but I may have to make some adjustments.

When I started this project back in 2011, there were no such issues. I had used the car as a daily driver in the summer for several years. The car was on blocks until 2020 when I got it repainted and my wife gave her blessing for me to put it back on the road. That was when all of the high idle and reverse engagement issues started.

I'm not in a position to spend money on it this year. I'm hoping to at least limp it to a car show this weekend and then I can worry about solving problems next spring.
 
For the record, anything between 0.5 and 1.25 volts at idle is acceptable for a tps throttle setting.

Upon startup, the ECU takes that measurement and assigns that as an idle value and adds a certain voltage over that (I believe 3v) to signify WOT. Each time you start the car, it takes this baseline voltage.

There is no need to chase 0.99 volts at all.
 
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I spoke with a transmission guy and he also said it sounds like the clutch cable needs adjustment. I installed the steeda adjustable cable last summer hoping that it would solve my problem. I did not see an improvement but I may have to make some adjustments.

When I started this project back in 2011, there were no such issues. I had used the car as a daily driver in the summer for several years. The car was on blocks until 2020 when I got it repainted and my wife gave her blessing for me to put it back on the road. That was when all of the high idle and reverse engagement issues started.

I'm not in a position to spend money on it this year. I'm hoping to at least limp it to a car show this weekend and then I can worry about solving problems next spring.


Troubleshooting loss of signal ground

Revised 21-Aug-2013 to rewrite the MAP/Baro testing of the signal ground and moving the testing the computer’s internal signal ground test procedure to the last place.

1.) The computer pin 46 signal ground is a critical component: it provides ground for the
Map/Baro, TPS, ECT, EGR position sensor and ACT. Signal ground is used in many circuits that have analog inputs to isolate the electrical noise. It is always separate from power ground, although both may have a common connection origination point. Signal ground usually has some conditioning that reduces the electrical noise to prevent false readings. The black/white wire (pin 46) is signal ground for the computer. It provides a dedicated ground for the EGR, Baro, ACT, ECT, & TPS sensors as well as the ground to put the computer into self test mode. If this ground is bad, none of the sensors mentioned will work properly. That will severely affect the car's performance. You will have hard starting, low power and drivability problems. What sometimes happens is that the test connector black/white wire gets jumpered to power which either burns up the wiring or burns the trace off the pc board inside the computer. That trace connects pins 46 to pins 40 & 60.

2.) Troubleshooting signal ground problems:

Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. That means disconnecting the battery positive cable since there is always some current draw due to the computer and radio circuits.[/b]

A.) With the power off, measure the resistance between the computer test ground
(black/white wire) on the self test connector and battery ground. You should see less than 1.5 ohms.
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B.) MAP/Baro circuit: check the resistance between the black/white wire on the MAP/Baro sensor and the negative battery cable. It should be less than 1.5 ohms. If it isn’t you can figure that you have serious problems with the wiring for the computer or the computer’s internal signal ground. Check the resistance between the black/white wire on the MAP/Baro sensor and then the black/white wire on the EGR and the same wire on the TPS. Repeat the process with the black/white wire on the ACT & ECT sensors and the MAP/Baro sensor black/white wire It should be less than 1 ohms. Higher resistance than 1 ohms indicates a problem with the 10 pin connector or the splice inside the main harness where the wire from the 10 pin connectors joins the rest of the black/white wire

C.) Engine mounted sensor circuit: Check the resistance between the black/white wire on the TPS and battery ground. It should be less than 1.5 ohms. Higher resistance than 1.5 ohms indicates a problem with the 10 pin connector or the splice inside the main harness where the wire from the 10 pin connectors joins the rest of the black/white wire.

Repeat the process for the ACT and ECT sensors. Your results should have the same specifications.

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See the graphic for the location of the 10 pin connectors:

Diagram courtesy of Tmoss & Stang&2birds

TPS_IAB_Pic.jpg


See the graphic for the 10 pin connector circuit layout.
salt-pepper-10-pin-connectors-65-jpg.68512

The injector power pin is the VPWR pin in the black 10 pin connector.



3.) Testing the computer's internal signal ground:
Remove the passenger side kick panel and disconnect the computer connector. There is a 10 MM bolt that holds it in place. Measure the resistance between the black/white wire and pin 46 on the computer wiring connector: it should be less than 1.5 ohms. More that 1.5 ohms is a wiring problem. If it reads 1.5 ohms or less, then the computer is suspect. On the computer, measure the resistance between pin 46 and pins 40 & 60: it should be less than 1.5 ohms. More than that and the computer’s internal ground has failed, and the computer needs to be repaired or replaced.

See the pictures below for help finding and fixing the burnt computer trace.


hash-146a243133771eeba54f17b17d721b1f-jpg.646628


The fix is some careful soldering of a small jumper wire across the burnt section of copper trace.

20160325_234845-jpg.553347


If the ground for the TPS goes bad, the TPS output voltage increases and the idle speed may fluctuate..
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Clutch Adjustment for stock and aftermarket setups
Revised 28-Apr-2016 to include need for longer cable with single hook quadrant

Clutch adjustment with stock cable and quadrant
There is a self-adjusting mechanism for the clutch cable. As the clutch wears the cable tension changes, that is why there is a self-adjusting mechanism built into the clutch pedal. On the clutch pedal there is a toothed plastic quadrant with a ratchet pawl that engages the quadrant. As you pull up on the clutch pedal, it ratchets and takes any slack out of the clutch cable. Mess around with the pedal or even bump it while the end that fits in the clutch fork is loose, and it tries to take up the slack in the cable. That has the effect of shortening the cable.

Do the clutch adjustment first before considering any other problems. With the stock plastic quadrant and cable, pull up on the clutch pedal until it comes upward toward you. It will make a ratcheting sound as the self-adjuster works. To release to tension of the stock quadrant, use a screwdriver to lift the ratchet paw up and out of engagement with the quadrant teeth.

You can use a screwdriver to lift the ratchet pawl off the quadrant teeth. That will allow you to move the quadrant to allow more slack in the cable. If the cable is too loose, pull up on the pedal. The ratchet pawl will allow the quadrant to take up the slack in the cable and lock the adjustment.

The following applies to aftermarket clutch cables and quadrants.
Adjustable cables are used when the plastic quadrant is replaced with an aftermarket metal quadrant. The downside to this setup is that you have to get under the car to adjust the clutch. Add a firewall adjuster to an adjustable cable setup and you can spare yourself from getting back under the car to make any fine adjustment that you desire.

The fancy 2 and 3 hook quadrants are for use with stock length cable and a firewall adjuster. Use the firewall adjuster and screw in and out to take the slack out of the cable and get the clutch engagement point just where you want it.

Binding clutch cable
A binding clutch cable will make the clutch very stiff. If the cable is misrouted or has gotten too close to the exhaust, it will definitely bind. The binding common to adjustable cables is often due to misplacement of the adjusting nuts on the fork end of the cable. This will also cause the cable to wear and fray. Both nuts should be on the back side of the fork so that the domed nut faces the fork and the other nut serves as jam or locknut to the domed nut.

Clutch pedal adjustment
Clutch pedal adjustment with aftermarket quadrant and cable: I like to have the clutch completely disengaged and still have about 1.5” travel left before the pedal hits the floor. This means that I have only about 1” of free play at the top before the pedal starts to disengage the clutch. Keep in mind that these figures are all approximate. When properly adjusted, there will not be any slack in the clutch cable. You will have 4-15 lbs. preload on the clutch cable. With a non-adjustable cable and a firewall adjuster, the cable needs to go in the second hook of the quadrant. Single hook quadrants used with a firewall adjuster may make the clutch too tight, causing it to slip. The possible exception to this is the Maximum Motorsports cable which is a bit longer to make it work with a single hook quadrant.

Adjustable clutch cable tips:
Loosening the cable adjustment nut (throwout bearing arm moves to the rear of the car) moves the disengagement point towards the floor.

Tightening the cable adjustment nut (throwout bearing arm moves to the front of the car) moves the disengagement point towards the top of the pedal.

Firewall adjuster tips
Turning the firewall adjuster IN makes the engagement point closer to the floor since it loosens the cable. You have to push the pedal to the floor to disengage the clutch. Too loose a cable and the clutch won't completely disengage and shifting will be difficult. Gears will grind and you may have difficulty getting the transmission in first gear when stopped.

Turning the firewall adjuster OUT makes the engagement point farther from the floor since it tightens the cable. You push a short distance to disengage the clutch. Too tight a cable will cause clutch slippage.

Aftermarket solutions to the problem:
The quadrant needs to be replaced if you use any type of aftermarket cable or adjuster. My preference is a Ford Racing quadrant, adjustable cable and Steeda firewall adjuster. The adjustable Ford Racing cable is just as good as the stock OEM cable. It allows a greater range of adjustment than a stock cable with a aftermarket quadrant and firewall adjuster. Combined with the Steeda adjuster, it lets you set the initial cable preload and then fine tune the clutch engagement point to your liking without getting under the car.

Using a stock OEM cable, firewall adjuster and a single hook quadrant may result in not having any free pedal travel before the clutch starts to disengage. I found this out the hard way.

See Summit Racing - High Performance Car and Truck Parts l 800-230-3030 for the following parts.

Ford Racing M-7553-B302 - Ford Racing V-8 Mustang Adjustable Clutch Linkage Kits - Overview - SummitRacing.com Cable and quadrant assembly $90
fms-m-7553-b302_w.jpg


The Ford Racing Adjustable cable is available as a separate part:
Clutch Cable, Adjustable, Ford, Mercury, 5.0L, Kit FMS-M-7553-C302_HE_xl.jpg

[url=http://www.summitracing.com/parts/SDA-555-7021/]Steeda Autosports 555-7021 - Steeda Autosports Firewall Cable Adjusters - Overview - SummitRacing.com
Steeda firewall adjuster. $40

575166


Fix for the quadrant end of the cable popping out of the quadrant when installing a replacement cable courtesy of Grabbin' Asphalt
attachments\575167




Help for those who have replaced the clutch assembly and are still having problems with adjustment:
The next step doesn't make much sense it you already have the transmission installed, but just for sake of discussion, here it is:
The throwout bearing sits in the clutch fork arm with the wave springs pressing on the rear flange of the throwout bearing.
throwout-bearing-placement-in-clutch-fork-arm-gif.86276

Major differences between the distance between the flywheel surface and the clutch fingers may require tinkering with the clutch fork pivot ball. Stack your old pressure plate, clutch disc and flywheel up like they were when installed in the car. Tighten down all the pressure plate bolts and measure the distance between the clutch fingertips and the flywheel face.
Too much thickness will cause the clutch fork arm to sit too far back to get the clutch cable on the quadrant. It may even sit against the rear or the bell housing hole for the clutch fork arm. In that case, reduce the pivot ball height.
Too little thickness will cause the clutch fork arm to sit too far forward and bottom out against the front side of the bell housing hole for the clutch fork arm.. This will prevent the clutch from fully disengaging.
In other words, the clutch fork arm should sit positioned midway or a little towards the rear in the bell housing hole for the clutch fork arm when the cable is properly tensioned.
 
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