help with odd running issue

[mikek340403]

So long story short I bought a merkur again (had a few both xr4ti ' and scoprios when I was younger). Low miles 86. (114k) started with fixing the leaking valve cover and replacing the cap, rotor plugs and wires. Fixed the turn signal (hazard button failure) and got the dash working. Bought one on ebay. So after all this. It still doesn't run correct. Slight miss at idle. So I dug a little deeper. Started to reset base idle. (Car would die when idle air control valve was disconnected) got the base idle correct (about 750 rpms) went to reset the throttle position sensor and found the voltage at the signal wire to be .15volts. Loosened the screws and Tryed to ajust it to the .9 to .95 volts. The max I could get it to was .40 volts. So I wanted to see what the sweep was. At wide open throttle the voltage at the signal wire was only 2.1 volts. Just to make sure the supplied voltage to the sensor wasn't throwing things off I checked and found I had 5.1 Volts at the supply voltage. So I ordered a replacement one. New standard parts (the brand standard) went to check just to make sure I was correct on my daig. Plugged it in and now the sweep is to 3.25 volts max. No break in sweep and again 5.1 volts on sensor supply. If I am correct it should be a range to 0 volts to 4.5 volts. So my question is what should the signal to ecu be for the throttle position sensor be? Am I correct with the 4.5 volts max (or in that range) any help would be great. Thanks

[Grayson]

That 5V Vref that is fed to the TPS is also fed to the VAM's potentiometer and the barometric pressure sensor. I would expect the TPS to sweep from approximately 0-5V if you're using a screwdriver to sweep the sensor in your hand. Installed on the throttle body and set properly, it should sweep from ~0.95V closed to ~4.0V at WOT.

I wonder about your wiring, or the ECU's ability to provide 5V. It's not supposed to be a loaded circuit, so it might be sensitive to resistance in the wire. If you measure the resistance between ECU pin 47 and the Vref pin at the TPS connector, it should read less than 5 ohms. The resistance between the Vref pin and ECU pin 46 and/or ground should be less than 10,000 ohms. If either of these are out of spec the wiring might have too much resistance or the ECU may have an issue.

There is also a chance that some weird combination of settings causes you to have the throttle plate too far open in order to maintain a 750rpm base idle, so when you set the TPS to read ~0.95V at that position, you only get ~3.25V when the plate is all the way open. Perhaps a little too much throttle plate opening is required to overcome a bad ignition setting or something.

[mikek340403]

Sorry. Forgot to clarify that when I tested the new sensor it was with a screw driver. Never mounted. With this being said if I am back probing at the throttle position sensor connection even if there was high resistance I should still see a set voltage. As this is a signal back from the sensor. I my be wrong here but isn't it the 5 volt reference is supplied and the sensor sends back a voltage based on what the throttle is (aka idle would be say .95 volts and the rest would be shed to the ground, or wide open throttle is 4.5 volts and the .5 volts left over would be on the ground) I also have a 5.1 volt reference to the sensor (also checked other sensors and had them) so I don't think the problem is in the resistance on the 5 volt reference. Also is there any other cars that we can use a throttle position sensor from. Like a 5.0 mustang as they are readily available. Idk input on this weird problem would be great ad I'm baffled at the moment and I'm a professional mechanic by trade...

[timandsam]

What wires are you checking return voltage at?

[Ed Lijewski]

Re the odd idle, check compression first.




YMMV

[mikek340403]

Thanks for the pic. Out of curiosity I wanted to check if I had a bad new sensor. I went a got one for a 86 mustang 5.0 (due to the 5 volt reference, signal wire ,and ground wire being the same with same connection) checked that one and same deal. Unable to get 4 volts at max movement (with screwdriver). This is being done with key on engine off due to having to remove the idle air control valve to reach it. I may check compression but I'm ting to get this correct before I move futher. Even if I find low compression on a cylinder I still have to get this fixed before I could have it running correctly.

[Ed Lijewski]

Are you back-probing only the red and green wires (as Tim asked above)?

YMMV

[mikek340403]

Green/white wire is the wire I am getting signal from sensor on. Tried to back probe from that wire to the black white wire and the battery ground with no change. The orange/white wire I have the 5 volt reference at.

[mikek340403]

Also forgot to note that the max sweep I got on the original sensor was with the sensor off and using a screwdriver to see what the max the sensor could read.

[mikek340403]

Ok I figured it out. It was my meter at home. I grabbed the one I have a work and now I have correct signal. Sorry should have verified the signal with a nother meter.

[eaton53]

Ok I figured it out. It was my meter at home. I grabbed the one I have a work and now I have correct signal. Sorry should have verified the signal with a nother meter.

Sucks when your equipment makes think you have a problem and you really don't. But it's good that you got it sorted.

[Ed Lijewski]

True this: I went back and forth in my mind about suggesting you might check the DVM or it's battery--I've seen a lower than expected DVM reading caused by a faulty unit or low battery more than once--but hesitated to suggest it because you noted you're a professionally trained tech and might find that insulting.



YMMV

[mikek340403]

Haha. Just guess it's time to get a good meter for home. Lol.

[RichBaker]

That 5V Vref that is fed to the TPS is also fed to the VAM's potentiometer and the barometric pressure sensor. I would expect the TPS to sweep from approximately 0-5V if you're using a screwdriver to sweep the sensor in your hand. Installed on the throttle body and set properly, it should sweep from ~0.95V closed to ~4.0V at WOT.

I wonder about your wiring, or the ECU's ability to provide 5V. It's not supposed to be a loaded circuit, so it might be sensitive to resistance in the wire. If you measure the resistance between ECU pin 47 and the Vref pin at the TPS connector, it should read less than 5 ohms. The resistance between the Vref pin and ECU pin 46 and/or ground should be less than 10,000 ohms. If either of these are out of spec the wiring might have too much resistance or the ECU may have an issue.

There is also a chance that some weird combination of settings causes you to have the throttle plate too far open in order to maintain a 750rpm base idle, so when you set the TPS to read ~0.95V at that position, you only get ~3.25V when the plate is all the way open. Perhaps a little too much throttle plate opening is required to overcome a bad ignition setting or something.

Sounds to me like either something in the circuit is drawing too much current, or the regulator internal to the ECU is going south....