Strange TPS? STRANGE NO MORE!!! - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 01-18-2007, 11:04 AM Thread Starter
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Strange TPS? STRANGE NO MORE!!!

Mine - '03 Rubicon, 40,000 miles
Occasional sputtering, popping, backfiring, when accelerating.
Sometimes part throttle backfire.
If it was a carb I'd suspect accelerator pump or maybe a hunk of flotsam in the bowl.

As FI it's exhibiting typical TPS problems.
Throws code "TPS High" - verifies suspicion.
Code returns after clearing almost immediatly.

In fact, when driving it felt like the computer did not know I changed foot position, it would falter, then you could feel the MAP sensor compensate for the lower vacuum with more fuel. Then once it "caught" you could feel the feedback system working to get it right. It was interesting to play with like that.)


Voltage check: 5 volts reference - OK, ground - OK, 5 volts on the wiper throughout sweep range - no change! Bingo!

Disconnected - Ohmmeter check on TPS shows no change in resistance through out the range of movement. (Tried both Digital and Analog Ohmmeters.) Bingo - TPS gone south -- Right? Certainly not rocket science.


New one from the dealer - Ohmmeter check shows exactly the same thing! Wiper does not change in resistance.
Must be bad from the factory.

Swapped it for another out of the dealer's stock - we were going to send the first new one back as defective.
Resistance check shows the same - the wiper does not change in resistance! 2 bad new ones?

Now the real puzzlement - installed that one - voltage changes as it's supposed to -- but how? I must have done the resistance check wrong. Disconnected - tried the Ohmmeter again - does not show a resistance change! Reconnected, voltage sweeps as it should!
Drivability problem gone, runs fine again - but it makes no sense!???

The service manual shows the TPS as just a simple variable resistor. We found another one under a work bench at the dealer - but the older version - it sweeps with the Ohmmeter like expected.

All the other TPS's I've dealt with over the years have been simple resistors - potentiometers - and can easily be checked with an Ohmmeter. But this one stymies me!

I even tried using the Ohmmeter with opposite polarity - a difference in the Ohmmeter reading could indicate it's an active device rather than passive - semiconductors involved inside. No difference.

Sure, the symptoms and code indicated the TPS as being defective, but I'm a believer in verifying before throwing parts at it.

Why won't it show with the Ohmmeter?

Anybody?
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post #2 of (permalink) Old 01-18-2007, 11:08 AM
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Re: Strange TPS?

RRich, I think you're being lead down the garden path. My guess is that something other than the TPS is causing the problem such as: <ul type="square">[*]The CPS is flaky and causing the bucking and backfiring.[*]The MAP sensor can't follow the pressure fast enough.[/list]How are you monitoring the voltage?
From the FSM:[ QUOTE ]
OPERATION

The TPS is a 3–wire variable resistor that provides the Powertrain Control Module (PCM) with an input signal (voltage) that represents the throttle blade position of the throttle body. The sensor is connected to the throttle blade shaft. As the position of the throttle blade changes, the resistance (output voltage) of the TPS changes.

The PCM supplies approximately 5 volts to the TPS. The TPS output voltage (input signal to the PCM) represents the throttle blade position. The PCM receives an input signal voltage from the TPS.

This will vary in an approximate range of from .26 volts at minimum throttle opening (idle), to 4.49 volts at wide open throttle. Along with inputs from other sensors, the PCM uses the TPS input to determine current engine operating conditions. In response to engine operating conditions, the PCM will adjust fuel injector pulse width and ignition timing.

The PCM needs to identify the actions and position of the throttle blade at all times. This information is needed to assist in performing the following calculations:
˛ Ignition timing advance
˛ Fuel injection pulse-width
˛ Idle (learned value or minimum TPS)
˛ Off-idle (0.06 volt)
˛ Wide Open Throttle (WOT) open loop (2.608
volts above learned idle voltage)
˛ Deceleration fuel lean out
˛ Fuel cutoff during cranking at WOT (2.608 volts
above learned idle voltage)
˛ A/C WOT cutoff (certain automatic transmissions
only)


[/ QUOTE ]
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post #3 of (permalink) Old 01-18-2007, 01:50 PM Thread Starter
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Re: Strange TPS?

Could be - but?
1. Drove like a bad TPS (in my experience with many defective or scratchy TPS's.)
2. Code thrown was TPS High and almost instantly reset after clearing.
3. Voltage check before disconnecting - back probing showed the wiper was not sweeping - stuck at 5 volts always.
4. Resistance check on the bench showed wiper was not sweeping.

5. Replacing with the new one cured the symptoms. (But it is possible that's a fluke - the wrapped wiring harness to the TPS is shared with the MAP. Measuring while wiggling showed nothing - but?)

The new one failed the resistance check - acted just like the old one. Checked it several times. Yet installed it passed the voltage check. Wiper sweeps from .29 (.28 is spec) to 4.7 or so.

So - why doesn't it show as a variable resistor when an Ohmmeter is used, but functions?

Has Ohm's Law been repealed by the Democrats?

I'm going to take apart the one I took out - maybe it's more than just a resistor.


He He - as someone once said "I don't understand all I know."
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post #4 of (permalink) Old 01-18-2007, 02:27 PM
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Re: Strange TPS?

If you can't measure it's resistance sweep out of the circuit, then it's bad. If a pin were bad in the molded part of the connector then installing the connector to the harness could put enough pressure on the pin to heal the connection. Then if you pull out the TPS and measure it and it's healed, it can drive you nuts.

I'd sure be wary of the fix for a few miles down the road. If that pin in the TPS is bad, but unseen, then it will fail again.

Something doesn't add up.
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post #5 of (permalink) Old 01-19-2007, 02:59 AM
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Re: Strange TPS?

I only know you need to probe the TPS when connected. No idea why, but disconnecting it is probably the cause of the funny readings.
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post #6 of (permalink) Old 01-19-2007, 04:55 AM
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Re: Strange TPS?

That makes no sense. Being a Pot it should have a constant resistance from power in to ground. And a varying resistance Power to output and varying output to ground as the throttle moves. The only thing I can think is that your ohm meter was set wrong or is on the fritz. Have you tried putting 5v or so to the power in and grounding the other side and seeing if voltage on the output varies as the throttle moves? The only thing I can thing of is that the pot was replaced with some kind of solid state devise. [img]/ubbthreads/images/graemlins/confused.gif[/img]
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post #7 of (permalink) Old 01-19-2007, 10:52 AM Thread Starter
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Re: Strange TPS?

No sense at all!

The Ohmmeter(s) I used was first a digital (not the best to check for glitches/scratchy) then an analog - when I was standing by the Jeep. A few minutes later at my bench I used another digital and another analog Ohmmeter. Then at the dealer I used theirs - a digital Fluke. All were the same.

The loose pin idea - possible - they appear tight, but even a tiny movement breaking the connection inside could do it.

If there was a solid state device in it it could explain it - but I would think switching polarity when measuring betweenm pins it would give different readings - like a diode forward or backwards. But it was always constant like a passive device.

The idea that voltage has to be applied in order to read it - an Ohmmeter does exactly that.

Sorry, I didn't get a chance to take the old one apart yesterday - I had broken/frozen pipes to deal with.

There has to be a simple explanation - unless the space aliens are playing tricks.

I'll post what I find.
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post #8 of (permalink) Old 01-19-2007, 12:08 PM
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Re: Strange TPS?

It being a solid state device might be the only answer.

What was the impedance (ohms) measurement from +v to gnd and from signal to gnd?

I can see where a solid state optical device might work better than a physical potentiometer. Resistive pots have a limited resolution and are prone to getting dirty....even if sealed.

But...I would expect to see a difference in resistance measurements, especially if you reverse the polarity.

Ok...ya gotta take the bad one apart and let us know...
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post #9 of (permalink) Old 01-19-2007, 01:24 PM Thread Starter
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Re: Strange TPS?

My wife reminded me of what may have caused it - I'd forgotten all about it.
A couple weeks before this started, I blew a hose on my on-board air system (not the air conditioner) - it blew green PAG oil all over.

I sprayed 409 cleaner around, and probably some brake cleaner, then used a garden hose to wash it down. I was careful not to flood the TPS or MAP, but they got wet anyway. That probably killed it.

It looks like it's sealed pretty well, but?

I wonder - it may be a good thing to coat the TPS with the rubber tool handle stuff or even liquid electrical tape. And make a gasket to seal around where the TB shaft pokes into the TPS.
It's certainly not a device that generates heat.

Maybe it's just better to carry a spare TPS off road - someone else may need one on the trail. They aren't all that expensive, and the extra weight won't be noticed.

I'll get those resistance measurements.
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post #10 of (permalink) Old 01-19-2007, 05:39 PM Thread Starter
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Re: Strange TPS?

Bad TPS:
737 Ohms total - end to end
Wiper to one end: 59
Wiper to other end: 678
Sweeping the shaft does nothing to change resistance.

Then I took it apart - not near what I expected. No indication of any type of solid state device, not even a diode.

The round thing has the resistance material on the inside - it's a continuous carbon strip. It's NOT like regular potentiometers that the ends are not tied together.

The carbon ring fits over 2 contacts kinda like shoes - they touch the inside of the resistance ring. There is/was a small ring that made contact with the metal part of the ring - but when I was cutting it open it went flying - probably by now it's with all those socks that disappear in the dryer.

The shoes are NOT spring loaded, and they are NOT a tight fit into the resistance material. Contact seems to depend on the assembly twisting slightly - caused by a clock spring - not shown here, and the TB shaft itself.

So - ANY bit of moisture or dust getting in would get pressed into the resistance/carbon ring, causing malfunction.
Sealing around the shaft - no O ring, no gasket, nothing but a crooked path the moisture would have to get past. My old one was dusty inside the carbon ring - the dust was pressed /ground into the resistance material - I looked at it with a high power lens - couldn't believe how much junk was embedded in the carbon ring.
It sure seems like a much better design would be in order, especially for a price tag of $50+.

That may explain why the "new one" read open when it was bench tested, but worked after it was mounted on the TB - the TB shaft wasn't putting "twist" on the assembly causing contact with the shoes.

It looks like it should be a frequent failure device, yet simple to fix. Steam cleaning, pressure washing, river fording - anything that allows moisture - or dust - to get in the TPS is asking for trouble. When possible, keep it dry and clean, and carry a spare.
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