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TPS undervoltage

corkster

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Hey all,

My Mom has a 1993 ford explorer that has been running a bit rough of late. I jumpered the edc and got three codes; two saying the engine is running rich (one for left side and one code for right side), and one code saying the TPS was running at a low voltage.

I checked the wiring harness to the TPS with the KOEO and found that it is indeed running low with a voltage of 4.87 when it should be 5.0-5.1 +/- .1 volts.

If the TPS is getting low volts from the harness, 1: would that cause the engine to run rich? and 2: how do I fix the low voltage issue?

If it was the sensor itself I cold fix it easy enough with a replacement, but I have no clue when it comes to figuring out why the voltage coming into the sensor would be too low.

Any tid bits of info would be great! Thanks all.
 


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MrShorty

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1st question: Which TPS code did you get, and was it a KOEO code (before separator pulst) or a CM code (after separator pulse)?

2nd question: Which two leads were you testing to get the 4.87 V? The TPS is a basic potentiometer with three leads, which means there are three possible voltages that can be measured. I don't have a wiring diagram right in front of me, but the voltage across the "fixed resistor" should be 5 V, which is the 5 V "reference voltage" generated by the PCM. The other two voltages should add up to 5.

If the reference voltage is low, that suggests to me some problem in either the Vref wiring (a loose or corroded connection) that is causing a voltage drop in the Vref circuit, or a bad ground that is throwing off the measurement of Vref.
 




manaen

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when your getting 4.87 volts where are you grounding your multimeter?

BTW MrShorty, great analysus!
 




corkster

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Hey MrShorty, Thanks for taking time to respond to my question :)

The Code I got was 122; which according to my haynes manual is TPS below minimum voltage. It was a KOEO test with a jumper wire and count the happy flashes of the check engine light.

and the leads I checked on the wiring harness were the reference voltage terminal and the ground terminal.

Being mechanically retarded (as I am so prone to being) I'm not sure what the Vref wiring that you mentioned is. Is that just the wiring leading to the plug that plugs into the TPS?

Again, thanks for your reply!!
 




corkster

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Manaen,

I am grounding to the ground terminal in the wiring harness that plugs into the TPS.

Thanks for taking time to reply!! :)
 




MrShorty

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Hey MrShorty, Thanks for taking time to respond to my question :)

The Code I got was 122; which according to my haynes manual is TPS below minimum voltage. It was a KOEO test with a jumper wire and count the happy flashes of the check engine light.
There are two types of codes output during the KOEO test: KOEO codes and CM codes. Refer to my "notes on pulling EEC-IV codes thread in the EEC-IV forum if you need help distinguishing them. KOEO codes represent hard faults (here and now faults) where CM codes (without an accompanying KOEO code) could indicate an intermittent fault. It could be good to know if your fault is intermittent or not.

and the leads I checked on the wiring harness were the reference voltage terminal and the ground terminal.

Being mechanically retarded (as I am so prone to being) I'm not sure what the Vref wiring that you mentioned is. Is that just the wiring leading to the plug that plugs into the TPS?

Again, thanks for your reply!!
Vref is a constant 5V reference voltage supplied by the computer to various sensors, especially those like the TPS that are actually resistors and can't generate their own voltage. On my '92, it's a grey/red wire originating at pins 46 and 49 of the PCM and going to several sensors. If there is a bad connection somewhere in this circuit, it can introduce a voltage drop that can throw off these sensors, including the TPS. If you move your voltmeter to the appropriate pin at the PCM and check Vref, do you still get 4.87 or do you get something closer to 5.0? If you don't have one, you'll need a good wiring diagram to identify the correct pin at the PCM.
 




CDW6212R

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The other voltage wire at the TPS should be just under 1.0 volt(.990), that's the one usually causing issues. It changes between that 1.0 volt and 5.0 volts, the other two should always stay the same. Good luck,
 




manaen

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looks like your 93 has the same reference setup on the PCM it is a gray/red wire and it is on pins 49 and 46 of the PCM connector.

49 and 46 are wired together in the harness so you should be able to test either wire, and they are both gray/red
 




corkster

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Thanks for the reply guys.

So do I just disconnect the pcm plug and shove my positive tester terminal into the hole on the plug with the gray red wire and then ground the negative tester terminal to the body somewhere to check the reference voltage at the pcm?
 




MrShorty

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If you do it "by the book," you'll use a breakout box. Glacier posted a primer on using a breakout box somewhere, it seems. However, unless you get lucky, a breakout box is usually more expensive than a typical DIY'er can justify. I backprobe connectors carefully and do just fine.
 




manaen

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yeah, a straight pin inserted into the rear of the connector usually works fine, just be extra careful not to accidentally ground the connector. This, Bad Mojo!!
 




CDW6212R

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Be very careful with direct PCM terminal testing, they are easily blown I hear.
 




X-North

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I know that im not reaslly in the good post but can anyone give me a link with picvtures of to extract obd-1 codes for 1992 explorer. Im not good in english so maybe with pictures i will understand better ...thank you
 




cjones101812

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have you fixed it yet?? if not, have you checked for proper ground??

to check for ground:

Voltage drop test - set meter on 2v scale, place RED lead on TPS ground and BLACK lead on NEG battery terminal. Turn key to RUN. max reading should be no more than 0.05v. if higher you have resistance in ground circuit. check and clean ground connections.
 




corkster

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Thanks Cjones,
I'll try that later today.

I have opted not to mess with the PCM for fear of making a mistake that will have very nasty consequences. (I'm not a very good mechanic you know).

And thanks to everybody else who has taken the time to respond as well!
 




cjones101812

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I know that im not reaslly in the good post but can anyone give me a link with picvtures of to extract obd-1 codes for 1992 explorer. Im not good in english so maybe with pictures i will understand better ...thank you

here is a copy of a post i put in a diff thread:

to check codes, refer to this link. (note - you can use either method, your 94 works basicly the the same as 89-95 5.0s)


http://forums.stangnet.com/showthread.php?t=698148
 




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