TPS sensor

africansnowowl · May 14, 2020

I've been trying to get the drivability back to normal in the 87' GT I bought. It would start fine and idle at a normal 800ish rpm, then after driving for a little bit, the idle would climb to 1400 or so. I read through the high idle check lists. Long story short, I replaced all the sensors, (O2, IAC, ECT, ACT, EGR delete)all motorcraft where applicable, replaced the dry rotted vacuum lines, double checked timing(13 degrees). Still had the idle issue. I had originally set the TPS to .95. Checked it again the other day and the voltage was 1.8. Figured the sensor was bad so put a new motorcraft TPS sensor and the voltage was still about 1.8. After drilling out the holes, the best I could get it down to was 1.1. The reference voltage was 8.1 volts. What would be the most likely cause of this high voltage? I traced the reference wire back, and where it meets the EGR wire, some of the sheathing has came off but the wire still looked good. I wouldn't be upset if I had to replace the computer, bc a MAF conversion with an A9P was going to be the next thing I do to the car.

Mustang5L5 · May 14, 2020

8.1 volts for reference voltage it too high. It's going to throw off all your sensor readings.

Speed density ECU's are cheap compared to mass air ecus. I'd try and pick up a new ECU to try if you trace that 8.1 volts back to the ECU

africansnowowl · May 14, 2020

It appears as though the only stop for the reference wire before going to the ECU, is the salt & pepper connection. I would think a jacked up connection there would cause low voltage?

Mustang5L5 · May 14, 2020

High resistance connections would lower voltage.

Still should verify your 5vref. Is that is really registering 8v then that’s going to throw off sensors

africansnowowl · May 15, 2020

So I did "the fix" to the salt & pepper connectors. I was going to just delete them, but settled on this for now. After that, my TPS voltage is now at a normal range. I set it to .90. However, the reference voltage is still high at 7.2 volts. I traced that back to where it enters the connector and the voltage stayed the same. I'm assuming the ECU is on its way out?

90sickfox · May 15, 2020

The ECU may be done already....but you need to check the reference voltage wire at the ECU. The reason you need to do that is because if you are back feeding voltage from a short it could damage the new computer or cause you to have the same issue. It's under 300 bucks to have the computer refurbished. If you buy a used computer you should have it sent off anyway to replace the caps and stuff that are known to go bad.

Mustang5L5 · May 16, 2020

Check the wire exiting the ECU. The internal voltage regulator should’ve regulate to 5v but sometimes they fail. It can be replaced if you are good with electronics. There are places that will refurb, but check eBay it classifieds for used speed density ecus. They tend to be cheap. Under $100 and sometimes $50.

africansnowowl · May 16, 2020

Well I was planning on a MAF swap anyways, so a new computer isn’t a big deal. I’ll check the wire going into the computer firs though.
Slightly off topic question... my car is an original 302/5spd, so would an A9P work? All the threads I see are about cars that swapped 5spd to auto or other way around. A9Ps are about $100 cheaper than A9Ls right now.

africansnowowl · May 17, 2020

I followed the reference wire around the engine compartment, aside from where it splits at the TPS/EGR, it looked good as far as sheathing. The TPS, EGR, and MAP all had 8.2 volts on the reference wire. Pulled the ECU and reseated the harness connector just in case it was messed up. Still had 8.2 volts on that reference wire. Probed the reference wire coming out of the ECU and it has 8.2 volts. So its safe to say the ECU is bad correct?

jrichker · May 18, 2020

africansnowowl said:
I followed the reference wire around the engine compartment, aside from where it splits at the TPS/EGR, it looked good as far as sheathing. The TPS, EGR, and MAP all had 8.2 volts on the reference wire. Pulled the ECU and reseated the harness connector just in case it was messed up. Still had 8.2 volts on that reference wire. Probed the reference wire coming out of the ECU and it has 8.2 volts. So its safe to say the ECU is bad correct?

Yes, the ECU is bad.
That high voltage will do damage to the logic circuitry inside the computer. All the computer's internal electronics run at 5 volts DC +/-. 25 volts. Once you get outside that range, all sorts of weird things can happen.

africansnowowl · May 21, 2020

So I replaced the ECU with a new one... still have the 8.x volts on the reference wire. The same voltage is on the wire no matter where I check it... wire going into the ECU, at the EGR, TPS, MAP, and salt connector. What am I missing? Where could the extra voltage be coming from?
The car actually idles and drives really good currently, but that high voltage concerns me.

jrichker · May 21, 2020

africansnowowl said:
So I replaced the ECU with a new one... still have the 8.x volts on the reference wire. The same voltage is on the wire no matter where I check it... wire going into the ECU, at the EGR, TPS, MAP, and salt connector. What am I missing? Where could the extra voltage be coming from?
The car actually idles and drives really good currently, but that high voltage concerns me.

You either have a bad voltmeter or a miswired sensor that is picking up voltage from someplace other than the computer's Vref.

TPS sensor

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