TPS check

[pete c]

I suspect a bad TPS. I did a search and couldn't find a procedure. I seem to recall sdpatt or somebody saying that you should get a varying resistance as you move it through it's travel. I get an open with a momentary resistance at some spots. I checked a new one out of the box at AZ. It measured the same. I decided to hold off on getting it.

An explanation of the procedure or link to it would be appreciated.

BTW, I have a chronic idle problem, no matter how many times I reset it, some pinging, even with 93, some hesitation, most notable at higher speed in 5th gear going up hill and lean codes, both sides.

This leads me to suspect a fuel starvation problem, since higher revs under load such as 70 mph uphill is the time you are requiring a greater fuel flow rate.

I don't have a fuel pressure gauge and would prefer not to have to buy one. The fuel pump is original, I suspect. I changed the fuel filter about 20-25K ago. When I turn the ignition to on after sitting all day, the pump runs, very quietly for about 2-3 seconds. When my slo had FP issues, it would stay on forever.

A vac leak is another thought. Does the 'spray carb cleaner under the hood till the idle picks up' method work well on shos? Any common trouble spots to look first?

I gotta do something quick. The annoying idle was bad enough. Now I got powerloss and having to pay another 20 cents a gallon to keep the pistons from melting (so far).

 

[bertha66]

When I did my 2nd 60k matinance the machanic checked the fuel pressure and said that it was getting weak, he also said that when tha car got to the point where it did not idle and felt like it was starving undel a load to change out the fuel pump. In a bout a year that is what it started doing and I put in new pump and no more problem. pump replacment was about $60. And took less than a hour to do. It may pay you to have the pressure checked.

 

[Rockledge]

I suspect a bad TPS. I did a search and couldn't find a procedure. I seem to recall sdpatt or somebody saying that you should get a varying resistance as you move it through it's travel. I get an open with a momentary resistance at some spots.

http://fordfuelinjection.com/index.php?p=30

 

[KC2SHO]

I don't know why you would get an open at any point on a TPS. Maybe you don't have the multimeter set on the proper scale? You should see decreasing resistance as throttle is opened. You can do it on the car as well, tap the ground and the return signal wire, I forget the colors right now, you should see increasing voltage as you open the throttle. This is not an acurate test though. I believe somewhere near 5 volts is wide open? Sorrry I don't have my reference material here at work. Oh, and you must use the ground wire at the sensor! Any other ground will not do.

I've always been told by my brother, an accomplished ford mechanic, that the only way to test for a bad TPS is to use an oscilloscope. Or, maybe I should say the only really accurate way. A multimeter does not respond fast enough to give you real picture of the entire resistive surface, maybe an alalog multimeter would be better if you can get your hands on one. A slight dip or jump would be easier to notice on something that is NOT limited to samples per second. Despite working with oscilliscopes on a regular basis I have never tested one this way.

 

[Charlie Hammond]

I suspected mine as well....I used a "true RMS" voltmeter. It's just a potentiometer, 3 terminals, middle is reference, (or ground), top terminal increses resistance as it is turned, bottom decreases resistance as turned. Resistance level should be linear, smooth through entrie range of motion. Goes from .489 ohms at rest, to 1.489 ohms at other end of motion.

 

[KC2SHO]

A digital meter has a sampling rate. You may pas over a bad area and never see a dip on the meter. Now I'm not saying don't test your TPS with a DMM just becuase of this. I would probably make a preliminary test with the DMM myself, but I would not trust that the TPS is 100% just becuase of a successfull test with a DMM.

 

[pete c]

I tried different scales. Been using one of these damn things for over 20 years now! I seem to remember the guy at AZ saying you needed a battery as well as a meter to check it. Apparently it is more than a simple potentiometer. Probably has a transistor or diode or something else in there that needs to be biased properly to work.

Thanks for all the responses. Either I'll get this damn thing fixed soon or the used parts market in the northeast is about to get a little more supply. Just what can you get for chopping up a stock old 120K mtx gen 2 anyway? The slicers are decent. That oughta be worth something.

 

[KC2SHO]

Apparently it is more than a simple potentiometer. Probably has a transistor or diode or something else in there that needs to be biased properly to work.

That's very possible. I've never tested one that wasn't on the car already nor have I looked for a shematic of it. Now you've got me curious.

 

[Rockledge]

Do all DMMs use a sampling rate? And what exactly is the difference between "True RMS" and a "Regular RMS"? It's kind of a hard concept for me to grasp right now.

I've done a web search, and from what I can gather, Regular RMS = Average Signal Level, in other words, the meter averages out the signal(s) it is getting and displays that number. This is sometimes referred to as "Calibrated RMS".

True RMS, on the other hand, does not do typical averaging; instead it uses the actual signal readings (manifested as sine waves) over a certain period of time and then computes the "true" signal based on that. There is more math involved in these calculations, obviously. (Maybe this is why an oscilloscope works best?) Supposedly, True RMS is better for testing signals that are capable of being distorted.

Does this describe it accurately?

Also, it is notable that Ford recommends aginst using a True RMS meter, this is from my '94 Factory Manual CD:

True RMS DVOM's should not be used with the Pinpoint Tests because they may display different voltage readings depending upon if the DVOM is turned on first and then the test leads are connected, or if the leads are connected first and then the DVOM is turned on. Also they may not auto range to the same range every time and some display significantly different values depending on the range selected.

 

[KC2SHO]

Rockledge, Yes. Since it's a digital instrument, it must convert analog information to digital by sampling that analog data at a preset interval.

RMS doesn't even come into play here. We are dealing with DC signals so there is no sine wave to sample.

 

[SHOZ123]

You can also check it by voltage. Back probe the plug and observer the voltage readings with the key on as you run the throttle through its full range of motion. It should be a smooth curve.

 

[KC2SHO]

I just checked the resistance of a spare TPS between pins A and B. It measured .4 kohm at min and 3.6 kohm at max.

 

[SHOZ123]

What you really want to see is a nice smooth transition in voltage or resistance through out the entire range. Any jerky readings will give poor driveability.

 

[KC2SHO]

Yes, as it has been stated a few times already in this thread. I didn't think it needed stating again. I was just giving the range for reference.