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Discussion Starter #1
I'm trying to sort out a car right now. SBF with a procharger and a FAST XFI on it. We inherited this problem so we've been chasing gremlins and think we have it close just have one lingering issue. The TPS singnal is very un stable. We checked the car without it running anf turn the throttle blade and it reads 91% wot. Then we start the car and put it on the transbrake and when its on the floor the tps isgnal is all over the place from 60-90%, it looks like a heart monitor. The most troubling part is that its intermittent. The owner emailed me 9 datalogs from his house and it worked fun, then we get to the track and its fuckign up again. We'll work on it in the pits get it good then go up to the line and it happens again. Its getting irritating. So far we have checked grounds, replaced the tps,moved the msd and all other ept to avoid intereferance, and checked the harness and pin as much as we could. Anyone have any thoughts?

Bill
 

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You say you checked the grounds, where are they located?
 

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Discussion Starter #3
one is on the firewall, one the block and the others to the battery. I wasn't a huge fan of the on of the firewall so when we were testing it i ran a wire directly to the neg batt terminal to rule that out and the problem continued.

Bill
 

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When it is screwing up check the ground signal at the TPS and the 5V reference.
 

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Do you have any additional sensors hook-up? like fuel pressure, oil pressure,......
If so unplug them one at a time and see if you have another bad sensor somewhere.

Second thing is the sensor itself. What voltage are you getting?

If you have any question you can also call
(252) 452-4998
Paul Powell
 

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Discussion Starter #7
No nothing to extra ordinary, only things hooked up are the injectors, crank trigger, dist, tp, and all the normal fast stuff. It did have a bunch of bracket gear in it, delay box, and a bunch of rpm activated switch we unhooked all that and it continued. As far as the voltage i think we were getting 4.8 or so to the sensor and with the car off get full voltage back to the box but its hard to stay next to the car with it running on the transbrake to check the output voltage at the box itself.

Bill
 

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Discussion Starter #9
thats another good question and i'm not sure that would tell us if the feed is breaking up because another sensor would be doing the same thing, although as far as i've seen all the other readings on the log look good.

Bill
 

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Do you have a schematic we can see?
 

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you could be getting electrical interfernce on the return circuit from another component. such as ignition coil, alternator, ignition box.
 

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Discussion Starter #13
we moved all the elec components far away, even had the coil wire run out the window lol. Plus usually interference will affect alot of sensors not just one.

I'm pretty sure fact has a wiring diagram on there website i know i have it in the help section of the xfi software but don't know how to post it.

I think thats the next step is re run all the wires to make sure theres not a short or bad wire something.

Its getting annoying

Bill
 

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I want to add, I chased a glitch with my Accel dfi for 3 days not too long ago. Checked all wiring, swapped boxes and sensors. Never did go away. I put a different alternator in that I had on the bench on a whim. Issue never came back. My code was a ignition driver overcurrent. It would shut the engine down. Re-set code and it would restart. Electrical stuff can be a pain in the ass.
 

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The 5v reference is shared by the TPS and the MAP sensors. The sensors grounds all go to the same pin on the ECU.
 

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i say look at the alternator. not a fast system but on the ford mustang they run the large wire from the alternator right beside the cam sensor wiring. a blown diode in the rectifier will introduce noise into the circuit and cause a lot of strange problems. could be doing the same thing on your car with the tps circuit.
if you have access to an old analog charging system tester you can check that pretty easily.
 

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Find someone or try to use a two channel labscope if possible. Hook in at the tps connector 5v rev on one channel and the tps signal on the other channel. If the 5volt reference varies only a little this will throw the signal voltage way off. With the two channel scope on there you can see every little blip in the traces so that should tell you what is happening. If everything looks ok but you still have the problem move the 2nd channel to the computer end of the signal wire and recheck it. Good luck hope this makes sense. The Snap-on Vantage or Modis are the shit, you can even record the info then go back and look at it frame by frame for further analysis
 
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