I think I know whats going on...AIT is giving me grief

djandruczyk · Post by **djandruczyk** » Tue Mar 07, 2006 4:00 pm

mops wrote:yes, I agree for turbo motor, iat shoulc be after intercooler, preferably just before or after throttle body.

i'm still N/A.... and having hard time...

I can tune and engine responds, but as the temp sensor soaks.....

best restults i have is when I disconnect temp sensor alltogether, then ms thinks it's 21*C and then air density correction is 100% and it all works great

but i'm sure that's not the right way...

Since this issue keeps coming up for some users, and has since the dawn of the MegaSquirt's release, I wonder if there's an error in the MS's fuel calcs in relation to the order in which the calcs are done. (as I think it's broken up into stages due to the 8 bit limitations )

I think that a firmware addition in order to disable IAT correction during cranking, ASE and possibly during other modes (rpm /map ranges?) might solve this issue.

mops · Post by **mops** » Sun Mar 12, 2006 1:24 pm

well, over the weekend i went for few longer drives and i was watchign IAT gauge. note ambient ~20*C

sensor is positioned on the airbox intake side, which is like 5 inch away from outside....

on high speed cruise (60+ MPH), it reads approx ambient temp, which is understandable

on low speed cruise (~40mph) it reads 5*C more than ambient (i wouldnt expect that because on the location of the sensor is nearly outside....)

in low speed city traffic it';s at least 20*C over ambient, and it's climbing fast when I'm stuck on the traffick lights... like... on the long trffic lights engine barely runs and IAT reading is 60*C and climbing.... aswell it takes a while to cool it down...

so what I tried doing is to rev the engine (to force more ait throught the sensor), and guess what... no change.... IAT still climbs even if i rev the engine.... my iat reading therefore is speed dependednt, which is pretty useless......

goddamn, i sooooooo want to find working IAT

apart from that my car is pretty good

i drive without laptop all the time... starts a bit rough, but still every time i want it (sometimes i have to stab the throttle to enforce longer pulsewidths, but thats due to bad IAT reading)....

yeach, last problem is a bit or rpm spikes/dropouts at high rpm, but i'll fix that as soon as i sort out this IAT... i'm still not sure hot to do that tho...

jonfx4com · Post by **jonfx4com** » Sun Mar 12, 2006 2:22 pm

Mops,

I may be off target here but that sounds like a symptom of the IAT being open circuit I think (or the wiring being open circuit to it) . It may be worth a check with a meter.

Jon

mops · Post by **mops** » Sun Mar 12, 2006 2:27 pm

nah, i think the circuit is ok... once i hit the highway again the temp eventually goes down to near ambient, but it takes a while....

Boris740 · Post by **Boris740** » Sun Mar 12, 2006 5:26 pm

I have been using this thermistor from Digikey

KC005G-ND about $3

It is a size of DO35 diode and has a time constant of 7 seconds. I am still looking for something with even faster response. You cannot go too small because of the dissipation constant http://www.betatherm.com/dc.htm

There is also a murky issue of radiant heat effecting it.

If you choose to use it here are three temp points

-22Â°F 65,429 Ohms
77Â°F 2786 Ohms
149Â°F 545 Ohms

Data sheet http://www.thermometrics.com/assets/images/ge10_1.pdf

mops · Post by **mops** » Sun Mar 12, 2006 5:35 pm

yep, that KC005G surly looks great, however I just checked... it's not available locally and shipping from digikey will cost me something like US$30....

Boris740 · Post by **Boris740** » Sun Mar 12, 2006 5:41 pm

I have a open order with Digikey that I will be closing in few days. Paypal me the cost of postage and the part and I will mail it to you.