Ideling around 1100 to 1200 and won't shift into overdrive

Tiny
88FORDVANCLOUGH
  • MEMBER
  • 1988 FORD E-SERIES VAN
  • 141,200 MILES
I have a 1988 ford econline 150 club wagon with the 5.8L engine and 4 speed auto transmision. I am having a problem with it idling high and shiftinf into over drive. I have a after market temp gauge installed would tht not let the computer know the van is warm and cause it to just keep dumping gas in it? And it will not shift into over drive untill 63mph could these be linked?
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Wednesday, June 20th, 2012 AT 7:20 PM

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Tiny
CARADIODOC
  • EXPERT
How is your temperature gauge wired in? Does your instrument cluster have an original temperature gauge that wasn't working? There are two different coolant temperature sensors. One is for the computer. You can't tap into that to do anything that will affects its resistance and the voltage it sends to the Engine Computer. The other sensor is for the dash gauge or warning light. If you aren't sure, disconnect your aftermarket gauge, then see if it shifts right.

Ford used to have a lot of trouble with their coolant temperature sensors for the computer. One of the common symptoms was a high and erratic idle speed. That will also cause excessive fuel consumption because liquid fuel doesn't burn. It has to be a vapor. That's why we needed a choke; to dump in extra fuel in hopes a large enough percentage would vaporize and burn smoothly. If anything changes the reading from the computer's coolant temperature sensor, it will adjust fuel delivery accordingly, so it could be related.
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Wednesday, June 20th, 2012 AT 7:34 PM
Tiny
88FORDVANCLOUGH
  • MEMBER
Yes my dash temp guage stopped working. I got a scanner and the koer codes that come up are a code 12-cannot control RPM during self check or idle speed control problem. And code 23 throttle position sensor is out of range. Is that supposed to mean is bad? I just replaced the IAC valve thinking that that's what code 12 was and tht didn't help at all
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Wednesday, June 20th, 2012 AT 8:40 PM
Tiny
CARADIODOC
  • EXPERT
You're smarter than most people to ask about the throttle position sensor. WAY too many people would just throw a new one on and expect it to be fixed. In reality, codes never say to replace parts; they only indicate the circuit or system that needs further diagnosis. That said, it's true that the sensor is the problem about half of the time.

You can test the TPS with a digital voltmeter but the glitch you're looking for can occur much quicker than what a meter can detect. Since you already have a scanner, you can read the same TPS voltage on the sensor menu, but here again any dropouts in signal voltage can occur too quickly for you to see. The Engine Computer WILL detect those though and set the code.

You CAN read a hard fault with a voltmeter. The TPS is fed with 5.0 volts on one wire. The ground return wire will have about 0.2 volts. You're interested in the signal wire. There are mechanical stops that only allow it to go from 0.5 to 4.5 volts from idle to wide-open-throttle. If you find 0.7 to 3.8 volts, that's okay too. The point is it sets a code when it goes to 0.0 volts or 5.0 volts. That can only happen with a broken wire or a broken contact inside the sensor.

If you find the signal voltage sweeping smoothly from low to high, the 5.0 volt feed and ground wires have to be okay.

The advantage to reading sensor voltages with a scanner is that you'll see what the computer sees. If you use a voltmeter you might find a nice 0.5 to 4.5 volts at the sensor's signal terminal, but if that wire is broken before it gets to the computer, or if there's a poor connection in the computer's plug, there is a very weak circuit called a "pull-up" circuit to apply 5.0 volts internally to force the computer to set a code. Without that pull-up circuit and with a broken signal wire, the voltage could "float" to some random value due to other circuitry in the computer and that random voltage could be an acceptable value that the computer would try to run on. Because of that pull-up circuit, very often you will see 5.0 volts for the TPS voltage when you unplug the sensor. Some car models do the same thing with a "pull-down" circuit to insure the voltage goes to 0.0 volts when there's a defect.
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Wednesday, June 20th, 2012 AT 9:48 PM

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