TPS voltage won't go lower than 4.95v..

[TC93]

Have an idle surge on a 91 LX, and while going through jricker's check list, the TPS voltage is at 4.95 when checking. Put new TPS on and still same 4.95. I know that should be WOT voltage, but any ideas why now?


Gonna take intake off tomorrow and check wire harness for bad wire, but anyone had this problem before?? Any pointers, for things to check just to help point me in the right direction, will be greatly appreciated..

 

[Daggar]

What's the voltage going into the TPS?

 

[wicked84gt350]

Sounds to me that you are reading the reference voltage from the computer and you need to be reading the modified voltage leaving the tps because the tps is a passive sensor which means it modifies the voltage given to it from the computer before it returns to the computer. With the key on engine off you can set it up to .99 volts 1 volt + is off idle to the computer but I am sure you all ready know this.

 

[HISSIN50]

If the TPS return was reading 4.95, the car would not start (during cranking, the injectors would not fire).

The above thoughts were mine too.
However, if that's not the case:

If you disconnect the sensor and both wires still read the same, look for a short between them. If the sig return falls to zero volts once the sensor is disconnected, I'd be thinking the TPS has an internal short. A continuity check could confirm this.

Good luck.

 

[TC93]

That voltage is on the green and blk wires. Didn't check coming in.....


Back to how things are going...

The idle surge was very bad when I started, it would jump from about 850 to about 2100 after the car idled for a few minutes. Got car to idle decent when just idleing in neutral, but when test drivng it has the dreaded surge and occasional shut off when stopping. Thing is, check engine light will be off when the car is 1st started, then come on about 15-20 seconds after car starts. So before checking anything else want to get this TPS voltage straighten out.. And pretty sure this will cure the surge... (hopefully).......

 

[vristang]

Have you pulled the codes???


jason

 

[TC93]

Have you pulled the codes???


jason

AAww man thats another thing, can't get any codes even though CE light is on. Tried the jumper pin thing but nothing happens, no blinking CR light. And I don't have an analog voltmeter. Plus don't know anywhere, who can scan OBD-1's. Well scan them for free like Advance does for OBD-2's....

 

[stang&2Birds]

http://www.amazon.com/Equus-3145-Digital-Ford-Reader/dp/B000EW0KHW

If the TPS voltage was always ~4.9v, then disconnecting would make NO DIFFERENCE. The EEC checks the TPS voltage at start-up. If it's not within range, it gets flagged as bad and it's not used.

A bad TPS will *not* cause the car to surge at start-up if you don't touch the throttle.

 

[TC93]

http://www.amazon.com/Equus-3145-Digital-Ford-Reader/dp/B000EW0KHW

If the TPS voltage was always ~4.9v, then disconnecting would make NO DIFFERENCE. The EEC checks the TPS voltage at start-up. If it's not within range, it gets flagged as bad and it's not used.

A bad TPS will *not* cause the car to surge at start-up if you don't touch the throttle.

Doesn't surge at start up, only after it has idled for a minute or 2......

 

[jrichker]

TPS adjustment:
Wire colors & functions:
Orange/white = 5 volt VREF from the computer
Dark Green/lt green = TPS output to computer
Black/white = Signal ground from computer

Always use the Dark green/lt green & Black/white wires to set the TPS base voltage.

Do the test with the ignition switch in the Run position without the engine running.

Use the Orange/white & Black white wires to verify the TPS has the correct 5 volts
source from the computer.

Adjusting the TPS fails to resolve the problem:
If the adjustment does not work to get the voltage below 1 volt, you probably have a
bad signal ground. The black/white wire is pin 46 and is signal ground for many things. If it
burns up inside the computer you get multiple faults and cannot pull codes..

Check the black/white wire resistance. Connect one ohmmeter lead to the black/white
wire on the TPS and one lead to the negative post on the battery. You should see less
than 1.5 ohm, more than that indicates a problem. Always take resistance measurements
with the circuit powered off.

Clean the 10 pin salt & pepper shaker connectors.

.


Check out the black/white wire on pin 46 and you will see that it serves as signal ground for
several things.

Diagram courtesy of Tmoss & Stang&2birds

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
(website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

 

[TC93]

Thanks, J-man!!!

Have your site saved with all diagrams and was going to run through it later. While looking yesterday, I could see the EGR wires were showing, and was wondering if the TPS and EGR shared anything. And from the looks of it they do. Will check out OHM's today,, and give the harness a once over...


Thanks again.!!

 

[noslow1986]

mine does the same thing 4.86

 

[lutzgt86-91]

They share sig return #46 i had a problem last year with my 91 gt I thought the fuel pump was bad changed it ran for about 2min then died ran the circuit from relay back no problems started car was looking under the hood saw some sparking behind lower intake and noticed that the sig return was grouned out on the lower intake thiswas killing the fuel pump I now that this is not your same problem just some fuel for thought

 

[TC93]

Checked the ohms on the black wire at TPS to NEG side of battery, showed none. And Went to the EEC connector to check pin 46 and mine is grey/red not blk/white. But checked the ohms between pin 46 and ground right below the EEC. Showed about 2 ohms.... Also checked the same grey/red wire at tjhe EEC tester and ground had the same 2 ohms.

Sounds like a short somewhere,... Because just for heck of it,, I checked the voltage at the black wire on the TPS to engine gorund. It showed 4.94 volts also.. Swapped engine harnesses, just to rule it out. Still got same..
Swapped the MAF, EGR, ECT, TPS with known good ones. Only 2 I haven't swapped yet are the ACT, and MAP. Is it possible one of those causing this problem.

MAN!!!!,,,,Wish I had access to ALL-Data again

 

[superhuaman]

voltage from the tps black wire to the engine block should not have read 4.94...that would mean that the black wire is open.

 

[TC93]

voltage from the tps black wire to the engine block should not have read 4.94...that would mean that the black wire is open.

Thats why I was thinking a short also, the black wire shouldn't have any volts where grounded...

 

[superhuaman]

maybe the black wire ISNT grounded? if it was ground yet shorted to power, then it would bring the power down to 0volts...

 

[TC93]

maybe the black wire ISNT grounded? if it was ground yet shorted to power, then it would bring the power down to 0volts...

Aww.... I get what you are saying. I will check continuity from pin 46 up to the TPS and other corresponding sensors...

 

[TC93]

Got continuity between pin 46 and BLK wire on TPS...


Check engine light won't come on right away. When the car starts, it will come on about 15-20 after car is running. And now when I go to set TPS voltage, I have none between, GRN & BLK wire. But with either wire to engine ground I have the 4.95. And on the ORG wire I have the 5 volts...

Just to rule out the EEC tired another one just to be sure it wasn't internal. But got same results...

So it looks like I have a short if I am getting 0 volts between the BLK & GRN wire?

 

[superhuaman]

the fact that your meter reads 0 volts between the green and black wires means they are the same voltage, or rather, there is no difference in voltage between the two. the fact that there is 4.95 volts difference between the black wire and engine ground, suggests that there is a problem with the ground wire in the fuel injector harness. the black wire should be grounded. and its not. i dont know why you keep saying you "have a short somewhere". it is open.