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Throttle Position Sensor issues


D-VO
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Hi guys,

 

I had posted in my restoration thread that I suspected my TPS was bad, and I still believe that unit was, when I would check resistance across the sensor, moving it back to the "idle" position resulted in an inconsistent reading when attempted multiple times. I bought a Mazda TPS kit from Boostinmini (quality hardware, highly recommend), and installed it using this graphic:

 

http://i.imgur.com/aJ4AoFN.jpg

 

So I get it all together, and I'm still getting somewhat erratic voltage readings (+/- 0.2 V) when the throttle plate snaps closed... could this be shaft seals? I bought a rebuild kit for the throttle body, but I really wanted to just start the car and drive it around the block before I do that.

 

If the TPS is sending a signal indicating non idle position (some throttle applied), will that create a no-start condition?

 

Thanks in advance.

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Unless the TPS voltage is waaaayyy off it won't affect engine startup. The ECU has a different sensor on the throttle body (opposite end of the throttle shaft from the TPS) to detect if the throttle is at idle. However, the ECU doesn't really look at any of that stuff for startup.

 

Remove the throttle cable and try your "snap the throttle" test again. If the TPS output is more consistent you likely have a bad throttle cable. Typically the cable starts jamming/sticking because the main engine ground strap is missing or not making good contact... thus when you run the starter motor the large amperage flow doesn't have the proper path back to the battery and it looks for the next suitable grounds. Those turns out to be the throttle cable and a skinny wire attached to the intake manifold by the EGR valve. The throttle cable heats up and burns like a fuse slowly buring out... this frying "welds" it to the cable cover and makes it sticky. Another potential issue is the cruise control cable which connects to the pedal underneath the dash with a sliding joint (so it has no effect when the cruise is OFF and you're pushing the pedal with your foot)... verify that isn't binding.

 

Folks that take the throttle body apart often have issues with the throttle return spring and don't get it re-installed correctly. If I remember correctly it is supposed to have three turns worth of pre-load... two is pretty hard to do by hand so that third turn is really tough.

 

There is a small electric motor at the rear of the throttle body assembly (covered by the black plastic/rubber piece sticking out) that drives a screw shaft in/out; the end of this shaft has the "idle switch" (also called "nose switch") and in turn presses on the throttle linkage to adjust the idle throttle opening. That assembly is the "Idle Speed Controller" (ISC) unit. If you stand on the passenger side of the car and look over the top of the engine, just to the firewall side of the throttle cable you'll see a 6-sided slotted screw head. That's what adjusts/calibrates the actual throttle position to your car's ISC. Turn it clockwise 1 or 2 full turns to crank the throttle open and try starting as a test. If it makes a difference you'll need to follow - to the letter - the idle speed ISC/MPS/TPS adjustment procedure in the FAQ corresponding to your model year. You'll also see what looks like a carb's idle speed adjust screw at the bottom/center of the throttle body (to the right of the throttle cable when you're viewing from the passenger fender)... this is the "fixed SAS" screw. It's a backup minimum idle adjustment that SHOULD NOT ACTUALLY BE TOUCHING THE THROTTLE LINKAGE in normal conditions. You should be able to turn that screw 1 complete turn before it "touches" the throttle linkage and starts moving stuff. It's purpose is to serve as a backup in case the ISC assembly fails or goes whacko and tries to totally close the throttle plate.

 

mike c.

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Mike,

 

Great post, extremely informative!!!

 

Unfortunately there was nothing wrong with the TPS, it turned out that the wire from the coil to the distributor wasn't seated properly for some reason. I charged up the battery today and my dad was futzing around with the coil wires, so I cranked it and it fired up! Got the idle set around 1000rpm for now, and I drove it around the block. I'm glad it was something silly. I will refer to this post for when I rebuild the TB, good tips there.

 

Again, thank you so much for the information. This community is part of the reason I got back into these cars.

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Good to hear it was a simple fix, if you're having doubts by using the aftermarket Mazda TPS there is something you should know about the shared ECU pin for the MPS TSP and MAS

Sometime when using the trouble codes a faulty TPS is a false positive for faulty MAS

http://imageshack.com/a/img903/5786/yhtLVO.png

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