Hey guys, I've got a couple of questions on testing / verifying voltage for the TPS. Specifically, I'm asking because I want to know a bit more how the TPS values affect the ECM, and what "ranges" are acceptable.

I've read that the IDEAL idle voltage is about 0.525 volts. This is the "peak" (as the kids say) voltage for closed throttle / idle TPS value.
Also ideal is 5 volts at wide open throttle.

And... as close as you can get to either is ideal.

I have tested two working AC Delco sensors, and the ranges that I get are:

0.68v to 4.31v
and
0.86v to 4.41v

Are either of these acceptable? I can get the sensor to actually get down as low as 0.525 and as high as 5, but only when the sensor is disconnected from the throttle body. When installed in the throttle body, it's physically incapable of doing either the suggested starting and suggested high.

Anyone have any thoughts on this? Again... not asking about it because my car is running good or bad, just tryint to understand this for documentation purposes.

Also, is there any benefit to highing a slightly higher low-end (like 0.575)? One video I saw suggested this.


Appreciate any thoughts! Thanks!!!

 

Transducers work off a 5v ref voltage. They all operate between 0.5 and 4.5 volts. This is to allow room for above or below operating voltages. The calibration between those numbers can change how a sensor works. If the low end was zero instead of 0.5v, the system wouldn't know if voltage was missing due to a cut wire etc.

Lets say you have a pressure sensor. You want to measure oil pressure and know pressure will not exceed 100 PSI. You buy a 100 psi sensor. Since the sensor is calibrated to 100 psi, your oil pressure on the gauge will show accurate pressure. Now you replace that with a 200 psi sensor. The sensor still operates in the 0.5-4.5v range however now that the sensor is calibrated for a higher pressure, the pressure it detects is the same but the voltage returned to the gauge will be lower due to the sensor calibration.

Might not seem like a lot but when you put a 5 bar map sensor into a vehicle that's calibrated for a 1 bar sensor, boost pressure will not work properly until the system is told to operate with a 5 bar sensor. Then the return voltages will be correct.

Sounds like both TPS you checked are working fine. There's a slight discrepancy but close enough to still work within limits. That's also why there may be a calibration sequence so that the computer can detect the low and high voltages to know what the limits are and will adjust accordingly.

 

Thank you! I appreciate that!

So, the only thing then that I'm curious about... does it only use it as a reference? Meaning... the ECM doesn't have pre-prescribed values in the fuel map table (for example) that it references... it merely associates the lower value (whatever that is) with idle, and the higher value (whatever that is) with WOT? As long as it's within the range of .5 to 4.5?

So... if my idle TPS value is .85, that doesn't make any difference... meaning that there's no real benefit to it being 0.525volts, for example... ? As in, it's not producing more fuel or "suggested" throttle indicator to the ECM at idle because it's .03v higher?


Thanks!!!

 

ECM sees the return voltage from the transducer and it needs to be between 0.5 and 4.5v. All it sees is the voltage but has no idea what it means. The ecm will then use whatever part of the fuel map that voltage is programmed for based on other inputs such as temperatures MAF etc.

OEM tuning only gives programming for typical daily driving events so tolerances don't need to be too exact. That's why aftermarket programming has become so popular.

 

The ECM contains the values that the transducer is expected to produce in its tables, then as the O2 sensor's signal indicates during ACTUAL operation, it adjusts its fueling accordingly. As long as the engine's ACTUAL behavior matches the tables closely "enough", it can handle variations, within some range or other.

The values you have posted are all well within what the ECM is capable of accepting gracefully. These are just CARS after all; not any kind of precision machinery or anything remotely like it.

You can physically adjust the sensor's position to get it closer to "ideal" if you want. No harm in doing that. We always all used to look for idle values around .55 - .60 as "in range", and anything above 4.0 at WOT as ... WOT. Whether doing so will actually make any difference to how the car runs, I have my doubts. It's true however, that getting it within an "acceptable" range at idle, is MUCH more important than at WOT. It's been my experience over the last 45 years or so however that it isn't all that critical. There's enough long-term averaging built into the ECM to null out ALOT of that sort of thing, as long as it's consistent and repeatable.

Don't forget, you're working with 1970s technology, stretched to its absolute limit. Just think back to the cell phone you were using in 1976 (about the time that microprocessor was introduced to the market) to watch your TikTok videos with, as I'm sure you were doing at the time. Temper your expectations accordingly.

Slightly higher values at idle will make the car "feel" more responsive temporarily until the ECM learns what its "low" value is. Takes maybe 10 - 25 run-stop cycles before the ECM fully averages it out and returns to equilibrium.

 

Another consideration is what year vehicle, and what system is involved. The TPS installed in a carb like the E4ME has a lower curb idle tolerance range. The TBI systems (including crossfire) had a different range for the closed throttle point. The early TPI (adjustable) systems had a tighter range, and the later non-adjustable TPI systems had a bit wider range for the closed throttle point. These also all varied as to what was assumed to the the WOT voltage.

 

Ok, so... it may seem like I'm obsessing a bit, but I see a lot of videos of configuring the TPS, but it's all really the V6 model, and none really on the TPS for the Iron Duke style, except for the Cross-Fire Injection cars (82, 84 Corvette, 82-83 Firebird and Camaro), and the older Cadillacs that used these same TBI units on the V6. For reference, the TPS sensors seem identical to the Iron Duke Firebirds and Camaros, and Cross Fire Injection (as they are on the TBI 300 for the 4 cyl Fieros).

Here's what I see in the various service manuals:


1986 Pontiac Fiero Factory Service Manual (for the 2.5 TPS):

"The throttle position sensor (TPS) provides a voltage signal that changes relative to the throttle valve. Signal voltage will vary from less than 1.25 volts at idle to 4.5 volts at wide open throttle and is non-adjustable"

"The TPS signal is one of the most important inputs used by the ECM for fuel control and for most of the ECM controlled outputs."

... and then a few pages later:

"If the MAP sensor fails, the ECM will substitute a fixed MAP value and use the Throttle Position Sensor (TPS) to control fuel delivery."

1.25 volts at idle to 4.5 volts


1984 Corvette Factory Shop Manual:

My 84 Corvette uses the same TPS... they seem basically interchangeable. The connectors are the same, and the TBI units are basically the same. The part numbers are different, but I notice that the TPS for the Cross-Fire Injection (original part numbers) are for a TPS that, while it's identical to the Fiero's... has elongated holes for the screws that allow for adjustments. Here's what the 1984 Corvette "Shop Manual" says about the TPS:

"The TPS can either be misadjusted, open, or shorted. If it is out of adjustment, idle quality may be affected, or Wide Open Throttle (WOPT) performance may be poor."

Here's what the 84 Corvette shop manual says about TPS voltage:

"Signal voltage will vary from abouyt .5 volts at idle to 4.5 volts at Wide Open Throttle."

0.5 volts at idle to 4.5 volts


1985 Pontiac Fiero Factory Service Manual (for the 2.5 TPS):

For the sake of completeness, I also checked my 1985 Serivce manual, and it says:

"At a closed throttle position, the output of the TPS is low (approximately .5 volts). As the throttle valve opens, the output increases so that, at wide-open throttle, the output voltage should be approximately 5 volts."

~0.5 volts at idle to 4.5 volts


I think it's as some of you have said, it likely takes a snapshot when the ECM is first powered and learns the idle values. So, I decided to use my TPS harness that I got from XFire Performance...




...and we measured the voltage (min and max) with the harness plugged into the 1985 Fiero 2m4 SE, but with each TPS properly installed into our spare TBI unit (just for convenience). We got the following voltages:





Hard to see the boxes because I wrote the information down on the blank sides. I wasn't JUST concerned with the starting voltage, but also the maximum range available between each of the sensors. So, here's the final outcome with the sensors by make...

0.86 - 4.42 = 3.56 (Delphi)
0.81 - 4.36 = 3.55 (GM)
1.00 - 4.35 = 3.35 (Standard Motor Product)
0.82 - 4.32 = 3.50 (Car Quest)


So, you'll see that the Delphi one has the highest range, barely... with the GM one (original made in USA sensor from New Old Stock), gave the second highest, almost the same as the Delphi. SMP and Car Quest were a bit lower.

Now, when I installed the original GM one into my modified TBI. This was a TBI 300 that I sent to XFire Performance to bore out to 46mm... quite literally, to the stock bore of a TBI 400, and even using one of their left-over 46mm Rochester TBI lates. I got a range of 0.68 - 4.31 with my multimeter. This actually ended up giving me a higher total range of 3.63 volts, with a starting voltage of 0.68. The only reason I can think of is that this TBI was modified and configured with the idle screw set on a bench (idle is fantastic in this car), which gives me a little bit more range at stock.


I think in closing... there's a few things I've noticed...

- By 1986, TPS is non-adjustable? Maybe adjustment wasn't necessary... unless 1986 ECM's had different programming.
- Universally agreed, most important sensor for determining fuel delivery in this system
- Universally agreed, takes priority over MAP sensor if MAP fails
- .525 seems to be ideal baseline, but that isn't the whole story...
- Some sensors have a higher range than others, and range is probably a more important factor


So ... not like I've just unearthed the meaning of life... but just wanted to make some sense of this... would love to hear anyone's thoughts other than, "Todd, you have too much time on your hands..." which I don't, haha... just like to solve problems.




Seems the best I can get is 0.68 with a maximum of 4.31, which seems pretty decent and fairly close.


Haha, well, I was born in 1978. The first cell phone I got was in 1997 and it was analog. My first computer was an 8088 KayPro PC with an NEC V20 processor running at 13mhz that was intentionally declocked at 8.77 (with a switch to 4.77) to be compatible with the speed of dos programs at the time. It had a Hercules monochrome monitor (green phosphorous), running DOS 3.21a. First game was Starflight on dual 5.25" floppy disks, and I installed it on my 20mb hard drive, which was a Seagate ST-225 RLL/MFM hard drive. My parents got it for us in 1984 for Christmas. We had the most powerful computer on the block! Haha...

But yeah, the car runs fine, I'm just trying to really dial in this system, because for some silly reason, my daughter wants to make this the most efficient and well-running Iron Duke Fiero ever... she's already installed a harmonic balancer from the newer cars, converter it to a serptentine belt system, even has a decoupler pulley she installed on the alternator so it reduces drag on the belts when you down shift or are engine braking.

She's way smarter than I am ... when I was her age, I was knocking over port-O-potties, my friends and I were getting drunk and shooting houses with paint ball guns, and basically being retarded. She's already written a book, and has a YouTube channel where she does engineering stuff ...

 

I think that this is being WAY over thunk.

Back in otto skool, my prof's made a big deal out of TPS's, TPS settings/adjustments and how influential the TPS is. Then, non-adjustable TPS's came out on cars. Huh? But I thought the adjustment was super critical?

So one day, I'm dicking around w/MY Crossfire car, it's running with the hood open, air cleaner off, and IDK why but I get the urge to start messing w/the TPS. So, I hook up my scan tool and bring up Datastream (live data) and look at the TPS while it's idling and the voltage is not perfect. So I get my torx tools out, start adjusting it to "perfection" and as I'm doing this, w/the engine running I start noticing some things. Since it's CFI, I can see the injectors firing. When I rotate the TPS toward lower voltage, the fuel output decreases -while I'm moving it. The engine stumbles accordingly and when I stop moving/adjusting it, it resumes normal operation/idle. When I rotate the TPS toward higher voltage the opposite happens, where the injectors pulse more, while I'm moving the TPS. Again, the engine stumbles accordingly and when I stop moving the TPS, it recovers and resumes normal idle. Huh. It idles the same whether I stop moving it at .40v or at .60v. So, I set it to spec, then run a "test"....With the engine idling i use a small screw driver to manipulate the TPS arm by itself, and what I see is? More of the same! I can push the TPS arm from .50v all the way up to over 4v and while I'm moving the arm "up", the injectors shoot more fuel -just like an accelerator pump would do, and the IAC opens some. Any point at which I stop moving the TPS arm, everything settles back into a normal idle. All the way to "WOT" on the TPS arm, same thing. I could hold the arm steady as far as it would travel and the engine idled normally.

I don't think that the TPS does much other than tell the ECM what the throttle plate "velocity" is, and in which direction. And I definitely don't think it matters where the base idle voltage is set. I see people post about setting it at 0.575v. Give me a phucking break. 0.575??? Does anyone think that the ECM GAF about 5 thousandths of a volt?! 0.005v? -or any thousandths's?? It does not. I think you could set the TPS to 1.0v at idle, and it'd work just fine. I'd like to reproduce that same experiment on the '89 and see what happens with it, compared to the '83 CFI.

 

Yeah... that's kind of what I've gotten from this too... I see a lot of videos about making sure the TPS is properly adjusted, but it doesn't even seem to make a difference. Biggest thing for me, what I got out of this, is that different sensors have a larger range, and operate either more smoothly, or less smoothly. And it seems the least choppy and biggest range the pentameter can get, the more ideal the performance (not meaning horsepower, but efficiency / drivability) will be.


Thanks!

 

I appreciate all the information everyone provided. If it helps, my daughter took all of this and made a video for it, to support Iron Duke and Cross-Fire Injection, since these are the only ones she knows properly applies to the TBIs she had access too (300/400 TBI). Thanks again!