Using a potentiometer instead of IAT

tmoss · Aug 11, 2006

MLC Stang said:
Guys,

The ACT and ECT sensors have a much, much greater effect on A/F ratios than on spark. In fact, the two sensors combined perform the job that used to be done by the choke on a carburetor.

I'm glad you posted this - it made me look back and realize I posted the wrong table...........

tmoss · Aug 11, 2006

Synned said:
I would just check voltage there, right, since I can't check resistance?

Thanks
Joe

See the link MLC gave you - the circuit and measurements are described in there.

95Vert · Aug 11, 2006

TMoss, since I'm doing an a9l swap, will I see/feel a difference from replacing the act with a resistor? I know the timing pulled comes in at higher temperatures in an a9l(thanks DON). And wouldn't 2.80 or 3.84 ohm resistor work better since the best temperature for our engine is right around 196*F?

reddy351 · Aug 11, 2006

MLC Stang wrote: "You can see by the chart on page 275 that if you put a 50 K-Ohm resistor in place of the ACT, the computer will think that the incoming air temperature is always about 58 or 59 degrees F. So, when it’s really hot outside, your engine will be running pig rich."

It sounds to me that you could use resistors to replace the ACT to do the same thing that replacing jets in a carb used to do. You could make it intentionally rich or lean (more than the ACT would do) for drag nights(or whatever). Is that true? If so, a guy could keep a myriad of resistors to suit the track/air. A poor man's twEECer!

Synned · Aug 11, 2006

reddy351 said:
If so, a guy could keep a myriad of resistors to suit the track/air. A poor man's twEECer!

Or just a potentiometer instead of those resistors

Okay, so to figuring out my problem....with cold starts

It kind of makes sense that if the car thinks it is hotter than it really is before when it would cause my hesistation, than I guess it would think its hotter than it is in the morning, which could cause my hard cold starts. I'm thinking there is something maybe wrong with the wiring back to the pcm. The resistor made a difference with the performance area of my problem but did not help with the cold start issues.
If there is an open in the wire, it would be more resistant, so it would tell the computer the car is cooler than it is, not hotter. So what could cause the act to be off with a lower resistance instead of a higher resistance?
Wow that was a confusing paragraph, sorry!

HISSIN50 · Aug 11, 2006

Cool discussion.

Do we know that comparing A9L to T4/U4 processor-ing works?
I thtough there are different thresholds and means of modifying spark and fuel tables in each case................ I guess not.

I'll add my two cents. Justin, Joe (Synned) and I have chatted about this a bit. As a means to an end - pinging when the underhood temps get rediculous - this resisitor thing seems to help a bit for some folks.

My one comment (I'm trying a resistor out myself - have been for a couple weeks. But ambient temps are only in the mid 90's, which is too cool for when my issues arise):

My chrome CAI heat soaks the IAT. I've checked calibration at various temps on the bench and it's about as accurate as I care to interpolate. But when the IAT is in the CAI, it seems to heat soak BAD. If I can narrow down how much heat soak there is over a given range of effective temps, I might simply use a resistor inline with the stock thermistor (installed in the CAI) so as to reflect more accurate intake air temps. I think if I had the IAT in a stock CAI, it wouldn't get heat soaked like it is, ergo I wouldn't need to do any of this crap. My engine temps stay at 190* (even in 110*F temps) but my IAT is the sensor that skyrockets as a function of time at said operating temps (SN95 underhood temps are rediculous. Add chrome to that [CAI] and all bets are off).

For the hard start folks: I have noticed that it takes an addition 1-2 'whirs' for the motor to catch. I'm not concerned as this is the worst temp delta we will see (as we head for Fall, the resistor value will correlate closer to actual temps). I think I'll go back to the stock plumbing/wiring once temps get below triple digits for most of the days (I live in AZ).

Just some random thoughts to toss in the pot. Good discussion. I applaud folks for not turning this into a pissing match, as most folks who bash such an idea haven't even tried it in its true dynamic to accurately comment. I have to say that I was a bit surprised by what I saw. Previously I had wondered if the puter would richen things up a bit once it saw IAT temps skyrocket, and doing the resistor have the opposite effect of what I desired. I'm still lookin for definate info on that but this seems to work for some folks in any event.

Synned · Aug 11, 2006

HISSIN50 said:
Cool discussion.

Do we know that comparing A9L to T4/U4 processor-ing works?
I thtough there are different thresholds and means of modifying spark and fuel tables in each case................ I guess not.

I'll add my two cents. Justin, Joe (Synned) and I have chatted about this a bit. As a means to an end - pinging when the underhood temps get rediculous - this resisitor thing seems to help a bit for some folks.

My one comment (I'm trying a resistor out myself - have been for a couple weeks. But ambient temps are only in the mid 90's, which is too cool for when my issues arise):

My chrome CAI heat soaks the IAT. I've checked calibration at various temps on the bench and it's about as accurate as I care to interpolate. But when the IAT is in the CAI, it seems to heat soak BAD. If I can narrow down how much heat soak there is over a given range of effective temps, I might simply use a resistor inline with the stock thermistor (installed in the CAI) so as to reflect more accurate intake air temps. I think if I had the IAT in a stock CAI, it wouldn't get heat soaked like it is, ergo I wouldn't need to do any of this crap. My engine temps stay at 190* (even in 110*F temps) but my IAT is the sensor that skyrockets as a function of time at said operating temps (SN95 underhood temps are rediculous. Add chrome to that [CAI] and all bets are off).

For the hard start folks: I have noticed that it takes an addition 1-2 'whirs' for the motor to catch. I'm not concerned as this is the worst temp delta we will see (as we head for Fall, the resistor value will correlate closer to actual temps). I think I'll go back to the stock plumbing/wiring once temps get below triple digits for most of the days (I live in AZ).

Just some random thoughts to toss in the pot. Good discussion. I applaud folks for not turning this into a pissing match, as most folks who bash such an idea haven't even tried it in its true dynamic to accurately comment. I have to say that I was a bit surprised by what I saw. Previously I had wondered if the puter would richen things up a bit once it saw IAT temps skyrocket, and doing the resistor have the opposite effect of what I desired. I'm still lookin for definate info on that but this seems to work for some folks in any event.

Was waiting for you to contribute, hissin
How much resistance are you running? You think your car is harder to start when its cold because of the resistor?
I'm trying to put all of this together in my head

- When ACT temps are very high, the computer will dump fuel.
- This was happening to me previously. Horrible performace and gas mileage, black smoke out of the tailpipes, after switching to a resistor, this problem alleviated.
- I have hard cold starts, with the ACT and with my 43k resistance.
- The computer looks at the ACT before you start the car, and if its cold (high resistance) it adds fuel. My cold start should have improved, but it has not. The resistor I am using is true to the outside temperature when I go to start the car first thing in the morning. ~65*
What I really have to do is measure resistance at the computer KNOWING there should be 43k ohms, or reference that to voltage and measure that.
I'm thinking I might see different than what there is, and this may be causing both of the problems.
But this is just my problem. I'm really liking where this discussion is going as well with the tuning aspect.
You would in theory be able to richen or lean things out depending on your need, I have experienced this first hand; and that one sensor really does make a huge difference.

MLC Stang · Aug 11, 2006

Guys,

When trying to work-around this stuff, remember the following:

ACT sensor reports the incoming combustion air temperature to the PCM (as measured in the intake ducting typically just before the throttle body).

ECT sensor reports the engine operating temperature to the PCM (as measured by sensing the engine coolant temperature).

They both work together to modify A/F ratios according to those two specific operating conditions. A stone-cold engine just starting up in the morning is much more affected by the ECT input. A fully warmed up engine running when it’s either really hot or really cold outside is more affected by the ACT input.

final5-0 · Aug 11, 2006

Modding a sensor to do one thing is gonna effect MORE than that one thing.

If you poke around in the pcm's programming, you'll see MANY references to the act and how it effects things ... other than spark.

If we just stick to the act sensor and not any others .............

I'll list a few I can think of or easily find in the t4m0 (GT pcm) Strategy.

btw ... as JT pointed out ... some may OR may not be active

I make mention of that as we seem to be trying to throw ALL Stangs in the same category.

From a tuning perspective ... That ain't how it is

anyway ...... the list

minimum adaptive control
maximum adaptive control
border line spark table
egr multiplier
isc adder
ol fuel multiplier
spark retard

Might not be all of em as I made a very quick pass through the values

In no way am I trying to discourage a mod like this

Just trying to share some info that may help evaluate the mod .....

other than

I think it works
OR
What Happened

Grady

tmoss · Aug 11, 2006

95Vert said:
TMoss, since I'm doing an a9l swap, will I see/feel a difference from replacing the act with a resistor? I know the timing pulled comes in at higher temperatures in an a9l(thanks DON). And wouldn't 2.80 or 3.84 ohm resistor work better since the best temperature for our engine is right around 196*F?

You might see a little until the O2 sensors sensed a rich or lean condition and tuned it out, I wouldn't put much effort into it personnaly.

those are "k"ohms or thousand ohms,

i.e. 2.80 = 2,800 ohms

95Vert · Aug 11, 2006

Thanks MOSS.

HISSIN50 · Aug 11, 2006

Very interesting stuff guys. A few of us are in the same boat - we have an issue that's causing headaches and are willing to take a hit in another aspect of EEC performance if it helps with the bigger issue. Grady, cool info as always. I ain't smart enough to predict how things will work, so I just try 'em out.

I'll reply to Joe here, for comparison's sake.

Synned said:
Was waiting for you to contribute, hissin
How much resistance are you running? You think your car is harder to start when its cold because of the resistor?
I'm trying to put all of this together in my head

I have a 40K resistor. I ohmed it out and it read about 46K ohms however. No biggie.
I know correlation isnt causation, but as soon as I did the resistor, the car would turn over an extra 1-2 times when bone cold (in 90-95* ambient temps) before catching. Is it the resistor? Grady, Tom and others can say better than I. Am I willing to have that for this experiment - you bet. I can get a bit lost when thinking of how the EEC adapts previous OL runs and all that mumbo jumbo that the smart cats in here understand.

Synned said:
- When ACT temps are very high, the computer will dump fuel.
- This was happening to me previously. Horrible performace and gas mileage, black smoke out of the tailpipes, after switching to a resistor, this problem alleviated.

When my IAT temps were up (ECT was still the same), I was getting some low load pinging in OD (Joe and I have AODE's). If I had a stick, I'd kick it down a gear obviously, and without the converter, I wouldnt have the issue in the first place (mine likes to stay locked in OD at low rpm, which = bog).

Synned said:
You would in theory be able to richen or lean things out depending on your need, I have experienced this first hand; and that one sensor really does make a huge difference.

That's where we get into the adaptive limits and so on. I dont know how this will do in the long haul. The puter wants to keep things stoich, but since I was running lean to start with, it wasnt cutting the mustard on its own. Hence my little nudge in the butt with sending it sensory illusion.

OrangeMustangGt · Aug 11, 2006

it makes sence. the computer will adapt for cruising, a/f wise, but at WOT does it use the ACT?

tmoss · Aug 13, 2006

At WOT it uses the last adaptive strategy correction factor entered into KAM by the O2 sensors.........

95Vert · Aug 15, 2006

FYI, I replaced my ACT with a 47k ohm resistor and noticed no difference. I thought my rpms were a little more stable but that wasn't the case. It just just like it had teh ACT, maybe a lil worse. It's been on for 3 days now, so I'm taking it off.

1TallMF · Aug 18, 2006

Back to the top on this discussion..

I tried a 33K ohm resistor in place of the IAT last night in an atempt to crack down on my low-rpm/high temp miss and intermittent pinging issues. It seemed to have made a really noticable difference in power with the resistor, and pinging was almost gone, but after driving it for about 1/2 hour, the dreaded low-rpm boggin and hesitaion was back, just as bad as it was before.

The unplugged IAT was showing 10K ohm after a good heat soak, which seems normal, but when I checked the resistor, I noticed it was reading 20K ohm when plugged into the harness. I took it out, and it read 33K ohm, as it should. So I plugged the resistor back in, and lo and behold, 19.9K ohm. I am left scratching my head here. I would expect the resistance through the harness to be higher if anything else, not lower. Anyone know why this might be?
I'm willing to give this another try tonight, with whatever combination of resistors needed to see ~50K ohms through the harness...

Using a potentiometer instead of IAT

tmoss

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Synned

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HISSIN50

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