Hello,

I am converting my Turbo 420a from the stock ECU over to Megasquirt 2 Extra for fuel and spark.
(Wasted Spark and Batch fire fuel if that is relevant)

I am curious on a few issues...

One, I have been looking for a GM IAT (air temp) sensor, but I have not had luck finding the thread in type at the junk yard. I found lots of rubber press in style, but no thread in ones... I think I'll buy one instead of junkyarding it, but that led me to wonder... why use the GM one and worry about threading it into the manifold when the stock setup already has a sensor? Has anyone used the stock sensor on megasquirt? Is it suitable for Turbo application? If so, are the calibration values available somewhere?

Secondly, and somewhat related... Can anyone let me know what ECU functions still remain on the stock ECU if I convert over to Megasquirt with full fuel and spark?
I believe it controls the alternator field, the air conditioning pressure sensor/clutch/fan, and (unless I rewire it) the speed sensor and speedo, and the Cooling Fan(s).
What other tidbits does the stock ECU handle? Does the stock ECU still use the IAT sensor for anything? (I realize it uses the CLT sensor for the fans)

If I steal the crank angle sensor, will I lose the built in Tach, or is that run from the cam pos sensor? I'll eventually run a tacho output from the MS, but just curious if the stock ECU will still have RPM after I steal the crank angle sensor for MS (My guess is no)...

Thanks in advance.

Originally posted by Tired2
One, I have been looking for a GM IAT (air temp) sensor, but I have not had luck finding the thread in type at the junk yard. I found lots of rubber press in style, but no thread in ones... I think I'll buy one instead of junkyarding it, but that led me to wonder... why use the GM one and worry about threading it into the manifold when the stock setup already has a sensor? Has anyone used the stock sensor on megasquirt? Is it suitable for Turbo application? If so, are the calibration values available somewhere?

You can certainly use the stock IAT sensor, but the sensor cannot be shared between the MegaSquirt ECU and stock PCM. There are calibration values for it here:

http://kb.symtechlabs.com/megasquirt:temp_calib

Originally posted by Tired2
Secondly, and somewhat related... Can anyone let me know what ECU functions still remain on the stock ECU if I convert over to Megasquirt with full fuel and spark? I believe it controls the alternator field, the air conditioning pressure sensor/clutch/fan, and (unless I rewire it) the speed sensor and speedo, and the Cooling Fan(s). What other tidbits does the stock ECU handle? Does the stock ECU still use the IAT sensor for anything? (I realize it uses the CLT sensor for the fans) If I steal the crank angle sensor, will I lose the built in Tach, or is that run from the cam pos sensor? I'll eventually run a tacho output from the MS, but just curious if the stock ECU will still have RPM after I steal the crank angle sensor for MS (My guess is no)... Thanks in advance.

The MegaSquirt ECU won't control the alternator, cooling fans, A/C clutch, and tachometer by default. You can add circuits to it to control the cooling fan relays, A/C clutch relay, and tachometer, though. You can also use an external alternator voltage regulator to control the alternator. The speedometer isn't controlled by the stock PCM, but the vehicle speed and crank sensors are powered by the stock PCM; so if you remove the stock PCM you will have to connect these sensors to an alternate power supply (12V works).

Thanks!!

Out of curiosity, why can't the sensor be shared?

I always wondered on stuff like this.. since the input of the ECU is high impedance, and the input of the MS ECU is high impedance, wouldn't 'tapping' in to the sensor be similar to putting a volt meter on it?

I guess the problem is the ref voltage, since you can't really tie the ECU's 5v ref to the MS 5v ref, etc. Maybe I answered my own question.

Either way, not worth the headache, just curious why. Thank again.

Originally posted by Tired2
Out of curiosity, why can't the sensor be shared? I always wondered on stuff like this.. since the input of the ECU is high impedance, and the input of the MS ECU is high impedance, wouldn't 'tapping' in to the sensor be similar to putting a volt meter on it? I guess the problem is the ref voltage, since you can't really tie the ECU's 5v ref to the MS 5v ref, etc. Maybe I answered my own question. Either way, not worth the headache, just curious why. Thank again.

The stock PCM switches between two different bias resistors for added resolution, but the MegaSquirt ECU can't detect when the stock PCM does this. In other applications, where the stock PCM uses a single bias resistor, it's possible to share the sensor as long as the bias resistor inside the MegaSquirt ECU is removed.

The more you know. Thanks! I ripped out the boost controller and afc neo tonight, no turning back now.

You won't regret it!