SECS PSI3 - Throttle percentage reading
 

anyone else find that the throttle percentage will only ever read a maximum of 81%?

just something i noticed today while sitting in work carpark wating on key holder opening up.

with just the ignition on, pedal to the floor and only reading 81%, off the throttle it will read 7%

the car will rev to the redline just interested in this reading from the PSI3 to see is it common.

car is a 99 UK turbo

Throttle needing recalicrated maybe or psi3 not reading correct...

is that you just guessing or do you have a psi3 that reads 100% on throttle percentage?

First of all, I've never had (or studied) a SECS monitor up close so I don't know how it derives its throttle position information. However, that said, there are a few suggestions I can make here.

There are two ways of measuring the throttle position via the select monitor datastream. The first is to read the throttle position sensor voltage. This should range between around 0.48-0.50 volts at idle, and around 4.28-4.3 volts wide open (both numbers plus/minus a bit). The external monitor would then, ideally (see below) do some simple maths to determine whereabouts the throttle is relative to those minima/maxima and derive an opening angle or percentage.

The other way an external device can read the throttle position is via the select monitor's own throttle opening angle parameter. This is the value actually used to look up the various throttle position related maps. It is calculated inside the ECU in more or less the manner I described above. The ECU keeps a note of the idle and max available throttle voltages, and then performs a calculation to work out whereabouts the throttle is relative to those two numbers.

The throttle angle parameter should produce a value that ranges between 0 (at idle) and hexadecimal FF (i.e 255) at 100% throttle opening. As such, if the SECS was using this, you would expect it to read 0 at idle and 100% at WOT. Clearly, this is not what you're seeing.

It can, however, read strangely either immediately after an ECU reset (because its min/max numbers have been erased and need to be re-acquired) or if you have a dodgy/misreading TPS pot (again the min/max numbers can get f**kd up and thus the position calc goes wrong).

If the SECS is deriving its throttle data from the select monitor opening angle parameter, it might be worth trying an ECU reset (via the black/green plugs/drive car sequence) to see if this alters the readings in any way.

However, again, bearing in mind I don't know how the SECS manages this particular parameter, there is one other thing that may explain the readings you are currently getting. If all SECS 99/00/5/6 owners report the same sort of numbers via the throttle position parameter, I would suspect this is the reason:

All the Impreza's analog sensors (MAF, TPS, knock sensor etc) run at 5 volt signal level. This means that the ECU's analog to digital converter measures their output relative to a 5 volt maximum. The 7% and 81% min and max throttle positions you are quoting would, measured against a 5 volt scale, equate to 0.35 volts (7% of 5 = 0.35) and 4.05 volts (81% of 5 = 4.05).

This would give a sweep between min and max of 3.7 volts, which happens to be about right (3.7-3.8 volts is what it should be). As such, if others report similar numbers on a SECS with 99-00 ECUs (and the same PSI3 firmware revision), then what you are seeing is a measure of throttle voltage relative to a theoretical 5 volt maximum rather than, as I suggested above, performing a calculation based on the idle and WOT sensor values and creating an opening percentage from the ratio between the two.

Make any sense? :D

Ive found you dont get 100% if the battery is low, try it again with the engine running or charge the battery for a few hours and you will find it reaches 100%.

If you've found that, it sounds as though there's something very wrong with your car, or your monitoring unit.

Cheers.

The ECU hasnt been reset recently, nor is the battery down on power it was reading 12.2v on ignition only reading again from SECS

ill get the multimeter out and see what the TPS voltages are.

There have been threads on this in the past. the low reading on the classic seemed not uncommon, unfortunately, I can't recall the details. The PSI3 in my 03 STi shows the full range 0 - 100% as does the AVC-R

JohnD

Correct, don`t see if the throttle reads 100% when the cars not moved for 3 weeks, the battery was low but I still did a little head scratching until I tried to start it,
Once the battery conditioner had been on for a hour or 2 it was fine, I did the same test with the engine running and it was fine,

The car isn`t used a lot and I did the same test a month later to check my findings.

It's Gambit's thread so it would be a bit "off" to take it too far away from topic, but it would be interesting to hear exactly what type/year of car you have and exactly what device you were using to generate those throttle position readings.

If the battery was low enough (i.e. less than 7 volts or so) to muck about with the sensor power supply, chances are that it'd also be too low to run the ECU in the first place.

Bottom line is that provided there is enough voltage in the battery to start the car, there will be enough to supply the sensors properly, because, as above, they get regulated 5 volts via the ECU (and so the battery needs to drop below around 7 volts to affect the sensor supply). If your battery was so low that you couldn't start the car without charging first, that might explain your odd readings - but it would also clearly be irrelevant to Gambit's question.

If you try that, and find that the readings are similar to those 0.35 volt idle / 4.05 WOT values I speculated on earlier, make sure you let us know, as there's some follow-up advice I have for you under these circumstances. :thumb:

as per my answer, I didnt realise the battery was low until I tried to start the car, then it was pretty obvious,
Once it had been charged/conditioned for a while it was fine and again when the car was running it was fine,

My original comment stands if the car had been stood for a while tbh as explained.

also making sure you have a good battery voltage would save a lot of other hassle for something that is easy to check.