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Need to reduce TPS voltage?

Old 01-15-2009, 08:37 PM
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Default Need to reduce TPS voltage?

I have a turbo Jeep where I need to fool the ECM into open loop in boost. JD suggested using a hobb switch to bypass the TPS on the onset of boost. My buddy Pete tried this in his turbo wrangler and it worked. The only issue is the CEL for voltage too high. I assume that the ecu doesnt want 5 volts back to it. I think it wants nothing high than 4.875.

I need to know what resistor to use to drop 5volts to 4.5 volts. That will allow me to eliminate the CEL.

Thanks for the tip JD. It will work as you instructed I think if I can get the voltage down a bit.
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Old 01-15-2009, 09:02 PM
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Default Re: Need to reduce TPS voltage?

fet designed to limit voltate to the specified limit, or a resistor perhaps. 10ohm should be close?
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Old 01-15-2009, 09:19 PM
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Default Re: Need to reduce TPS voltage?

Need to find out how much current is on that path...

V=I*R

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Old 01-15-2009, 11:27 PM
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Default Re: Need to reduce TPS voltage?

---- it. He got one in there that drops it to 3.7 which puts it back into open loop but soon after it throws a tps cel. He says it absolutely works but then throws the CEL..... this sucks.
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Old 01-16-2009, 02:13 AM
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Default Re: Need to reduce TPS voltage?

How exactly are you dropping the voltage? You are replacing the signal, right?

Call me tomorrow if you need to.
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Old 01-19-2009, 02:05 PM
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Default Re: Need to reduce TPS voltage?

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Old 01-19-2009, 10:30 PM
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Default Re: Need to reduce TPS voltage?

I wired it up yesterday. The Hobb switch is only a N/O switch.

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Old 01-19-2009, 10:51 PM
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Default Re: Need to reduce TPS voltage?

That's going to up the voltage that the ECU sees from the TPS, but it doesn't remove the TPS sensor from the equation. If you set the potentiometer to simulate WOT at a 30% throttle angle there's no way to keep it from driving the signal too high at WOT, and there's your code.

You need to swap one signal for the other. You can't do it according to my schematic as your Hobbs switch is apparently the wrong style (I have a 5 terminal 3 psi Hobbs somewhere, they come in rainbow flavors). Use the existing Hobbs switch to actuate a "reed" switch, which is like a super fast relay in fact some are just digital chips, which exchanges one signal for the other.
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Old 01-20-2009, 08:31 AM
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Default Re: Need to reduce TPS voltage?

Originally Posted by Joseph Davis
That's going to up the voltage that the ECU sees from the TPS, but it doesn't remove the TPS sensor from the equation. If you set the potentiometer to simulate WOT at a 30% throttle angle there's no way to keep it from driving the signal too high at WOT, and there's your code.

You need to swap one signal for the other. You can't do it according to my schematic as your Hobbs switch is apparently the wrong style (I have a 5 terminal 3 psi Hobbs somewhere, they come in rainbow flavors). Use the existing Hobbs switch to actuate a "reed" switch, which is like a super fast relay in fact some are just digital chips, which exchanges one signal for the other.
If the ECU is sending 5+ and I jump it through the POT and reduce it down to 3.5 volts like I am doing how is it going to increase the voltage? I checked it with a volt meter and it works the same no matter what the throttle angle is when the Hobb switch is closed.

And there is no TPS CEL as of yet and I drove it all over the place. My buddy wired it the same and he has no CEL either. There is a hiccup when the switch engages but I believe it because of the rapid fuel change. I need to play with the boost setting on the Hobb switch I think.
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Old 01-20-2009, 02:07 PM
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Default Re: Need to reduce TPS voltage?

Well, the upper branch's resistance is derived from the TPS and the pot. You change one value, you change the voltage going back via TPS.

Meh, who knows. Glad it's working, though. All that hard work and then having to ---- around to be able to drive on even a basic level is super frustrating.
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