Symptom: poor throttle response, check engine light on. OBD2 Codes P0622 and P0123. On the center of three wires for the throttle position sensor and ground I get 1.44 volts at idle and 1.85 volts at 2000 RPM. Idle should be in the 0.3v range.

Alternator is as 13.5 when running. Reacts nicely when all accessories turned on. Battery is newish, I replaced the cable connections at the battery after a no start condition a few weeks ago (corrosion). Battery rest volts about 12.5V.

Ground between first wire (nearest passenger side) and battery negative is perfect 0.000 Ohms. Wire looms look good all around.

Tried the following; no change.

• Replaced idle sensor.
• Cleaned connections at TPS and PCM.
• Reset by disconnecting battery cables and connecting to each other for a minute.
• Reset by disconnecting negative battery cable for 5 minutes then cycling the accelerator pedal slowly three times.

Yes, I'm a newbie (its a friends car). Any advice will be appreciated.

Engine idles OK, not great. With or without TPS plugged in.

 

Are you getting about 5-volts between ground and the other wire? If the ground wire is near zero volts measured back to the battery, and the supply to the sensor is around 5-volts, then I suspect your new TPS is also bad. The aftermarket TPS sensors are notoriously bad quality.

It's never a bad idea to remove and clean up the grounds on the engine block bu the dipstick.

 

I'll double check those voltages, thanks.

Regarding the TPS itself, do you have a recommendation for a good one? The one that he purchased was by Standard Motor Products.

 

OEM if you can. If you go aftermarket, test before installing. Honestly, I'm not overly worried about oem since it's easy to test and replace.

 

More testing. The new TPS off the engine with a clean 5vdc reference:.

• Between reference (driver's side) and ground (passenger side) 5.01v at rest 5.01v at full throttle
• Between return (middle wire) and ground 0.26v at rest and 4.8v at fill throttle.

Reinstalled on the engine, ignition on:

• Between reference (driver's side) and ground (passenger side) 5.16v at rest 5.16v at full throttle
• Between return (middle wire) and ground 1.42v at rest and 4.3v at fill throttle.

So something in the car is giving this TPS a little over one volt. There is no cruise control on this car (wasn't a chosen option).

 

Nice test setup! The TPS signal gets split off to both the engine computer and the trans computer. I'd try unplugging the trans computer to rule that out as the culprit. I'd unplug the engine computer next and see if you're getting voltage on the wiring harness from a short somewhere. Then I'd probably snip or de-pin the signal wire to the engine computer to see if that's pulling it up. I believe the engine computer has a relatively high value pull-up resistor as people have noted the voltage seems high when they unplug the TPS and measure the wiring harness side of the connector.

 

Thanks for the compliment. That voltage source was my son's idea. Also does constant amps so I can electroplate Nickle onto steel (much safer than chrome). Those used to be high dollar- now they are inexpensive.

This is a manual transmission car, so probably no computer for it?

Also, possible to put a resistor in the line instead of replacing the engine module?

 

I was assuming automatic, so correct, there is no automatic trans computer. If it's somehow the engine computer, adding a resistor inline would make it worse.

 

Perhaps the throttle plate isn't closing completely.

 

It closes completely and the idle motor works as well.

 

I measure the voltage with the key on between the ground terminal on the connector with the negative battery post: 18 millivolts. Then I cleaned the two engine grounds and got 16 millivolts. Same 1.4V between the center terminal and the ground terminal.

Looks like my only option now is to replace the powertrain control module. O's has them special order, programmed for the VIN and mileage, for just over $200.

 

Update: Both local parts stored could not sell the PCM. So I bought one from car computer exchange dot com. Cost was just under $400 including shipping. Fixed the issue.

 

Your TPS voltage is way too high (should be ~0.3V at idle, max ~4.5V at WOT). P0123 (TPS high voltage) and P0622 (alternator field control circuit) suggest either a bad TPS, wiring issue, or PCM problem. Since you’ve checked grounds and connections, try swapping the TPS it’s likely faulty. If that doesn’t fix it, inspect the wiring between the TPS and PCM for damage or corrosion. If all else fails, the PCM might be the culprit, but that’s rare. Double-check alternator wiring too P0622 could be related.

 

You should read the thread prior to responding.