Throttle Position Sensor Testing SVO

JCullen

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Jan 2, 2020
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I am building a pickup with an '85 2.3 turbo fuel injected SVO engine. It is running but pops and bucks when I push down the accelerator pedal much at all. It can be revved without too much popping when under no load. I have been told the timing should be advanced for a turbo beyond factory 10 degrees.

I am in the process of changing the Throttle Position Sensor since the test voltages did not come out as directed. When testing the voltages I got just 3 volts across the red wire and the ground. When I moved the + probe to the middle terminal that goes to the ECU, no voltage ever shows. ( Tutorial said 5 Volts across red to ground and then .9 Volts at the middle terminal at closed throttle.)

Now that I have replaced the TPS I get basically the same. I traced the wiring to the MAP sensor and I have 5 Volts there on the middle terminal which comes from the ECU but I get nothing coming out of the MAP sensor to the TPS. I bypassed the MAP sensor and got the 5 volts at the TPS but again, no reading at all from the middle terminal.

No matter what, if I place a digital OHM meter across the two outside terminals, or one outside and the middle terminal, I get no reading of resistance at all. And that is with the old TPS and the new one. Knowing OHM's Law, you can not have a voltage drop across any device unless there is a resistance. Does anyone know how these variable resistors are made and why there are no resistance readings from them?

I can not imagine that more than one new or old TPS are bad and will not read resistance. I read where the TPS is a variable resistor or potentiometer. I am assuming the MAP is also a potentiometer that is varied by atmospheric pressure changes.
I am using a rebuilt ECU from an 85 2.3 turbo FI. I also have an ECU from an 87 Turbo coupe but all the voltages are the same. So I don't think it is the ECU.

I feel that I must get this TPS matter understood and fixed before I will get the vehicle operating as a 2/3 turbo should. Any help will be taken seriously.
 
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junkyardwarrior

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Jan 10, 2011
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You'll want to see 4.5-5.2v reference. You're seeing 3. That doesn't mean that the ECU is bad, or it could, but that could mean a wiring issue. One strand of wire might carry a little current through it but not enough to supply the current required to run the TPS with it's variable resistance. With that said, it would be wise to check the integrity of the harness from TPS VREF to the pin on the ECU, with a load on it, via backprobing.

Forgot to mention that there was a limited timeframe where Ford used inferior wiring in/around the engine. I had an '84 SVO that had this wiring and it was trash. I ended up buying another harness from an 86 I think it was.
 

JCullen

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Jan 2, 2020
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Thanks. I was going to retrace all of my wiring today. This is a Frankenstein project and I made my own wiring harness so that will be on me.
You'll want to see 4.5-5.2v reference. You're seeing 3. That doesn't mean that the ECU is bad, or it could, but that could mean a wiring issue. One strand of wire might carry a little current through it but not enough to supply the current required to run the TPS with it's variable resistance. With that said, it would be wise to check the integrity of the harness from TPS VREF to the pin on the ECU, with a load on it, via backprobing.

Forgot to mention that there was a limited timeframe where Ford used inferior wiring in/around the engine. I had an '84 SVO that had this wiring and it was trash. I ended up buying another harness from an 86 I think it was.
 

JCullen

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Jan 2, 2020
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Indiana
Thanks. I was going to retrace all of my wiring today. This is a Frankenstein project and I made my own wiring harness so that will be on me.
I did check my wiring and it seemed to all be good. So, even though I am not sure the throttle position sensor is set in the correct position, I started the engine and it ran really well compared to before. I took the pickup out for a test run and wow, the feel of the turbo kicking in was jaw dropping. It still needs tuned a bit but I did put some miles on it this afternoon. The power, and especially the torque was great. When I had a 1.6 Miata and 5 speed in it, it would not pass 55 in the hot summer.....no aero at all.
But this SVO turbo plows right through to 75 mph in no time, without even a hint of a strain.
I do have some local help coming later to help tune the timing, TPS and then a turbo tuner is coming north to his home town and will help tune.
 

JCullen

Member
Jan 2, 2020
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Indiana
I did check my wiring and it seemed to all be good. So, even though I am not sure the throttle position sensor is set in the correct position, I started the engine and it ran really well compared to before. I took the pickup out for a test run and wow, the feel of the turbo kicking in was jaw dropping. It still needs tuned a bit but I did put some miles on it this afternoon. The power, and especially the torque was great. When I had a 1.6 Miata and 5 speed in it, it would not pass 55 in the hot summer.....no aero at all.
But this SVO turbo plows right through to 75 mph in no time, without even a hint of a strain.
I do have some local help coming later to help tune the timing, TPS and then a turbo tuner is coming north to his home town and will help tune.
Thank you for giving me some direction. One thing I found is when I put in the 87 Turbocoupe ECU, it ran well. When I switched to the rebuilt 85 ECU it did not run well.
Being a Frankenstein I am not surprised and this is what my problem has been all along....trying to get parts to work together. But it seems I am nearly at the end of the tunnel and I thank everyone for their help.
 
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65ShelbyClone

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Sep 9, 2000
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There is something not right. The '85 ECU is expecting 30lb/hr injectors and the '87 is for 35lb injectors.They don't run well with a mismatch. Are your injector tops brown or light green?

I have been told the timing should be advanced for a turbo beyond factory 10 degrees.
If you want a blown head gasket, yes. It should be set at 10º with the SPOUT out.

Does anyone know how these variable resistors are made and why there are no resistance readings from them?
They are a conventional three-terminal variable resistor. The only reason to get a short circuit or open circuit

I read where the TPS is a variable resistor or potentiometer. I am assuming the MAP is also a potentiometer that is varied by atmospheric pressure changes.
No, the MAP sensor is a transducer with some integrated electronics.

I took the pickup out for a test run and wow, the feel of the turbo kicking in was jaw dropping. It still needs tuned a bit but I did put some miles on it this afternoon. The power, and especially the torque was great. When I had a 1.6 Miata and 5 speed in it, it would not pass 55 in the hot summer.....no aero at all.
But this SVO turbo plows right through to 75 mph in no time, without even a hint of a strain.
With 3" exhaust they make about 170hp to the wheels and over 200ft-lbs, so yeah, more than a 1.6 Miata(and the 2.3's crankshaft bolt doesn't fall out on its own).
 
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