Intake Air temp strange behaviour

[Checkwise]

Ms1 v3 supplied by Megasquirt.com on 1500 midget. fuel and ignition.
Hi Guys. I need a little help with a intake air sensor issue.
setting up the MS for first time.
Have GM sensors.
Finally got car started, although poorly. set TPS , ignition timing at 10 degrees..
I have still to set VE table correctly as I have just played around idle area at moment. need to set up W.U.E. and A.S.E.
Problem I have is I have an issue with the reading TS see's for I.A.T.
When you first turn on ignition but not start , the CLT is @ 60F and IAT is 133F. (both gm sensors).
I disconnect CLT and gauge drops to zero as it should , however if I disconnect IAT sensor the gauge still reads at 133f.
I disconnected the wire at the 37pin MS connector it still reads 133 so its not in the wiring to sensor. and no continuity from pin 20 to those surrounding it so connector seems good.
I re joined the wire to pin 20 but did not plug the sensor back into the loom.
when I started the engine you have to blip the throttle gently to keep it running until it warms. (where I still need to tune).
Strange thing is that when you blip the throttle the IAT gauge blips as well. up to I guess 160F.
Its almost as though its getting a signal from elsewhere.
I Know the wiring is good so figure I must have something incorrect in settings.
Any one able to point me in right direction, so I can complete my Tune please ?

[Checkwise]

Hi Attached the wire diagram I installed from if that helps.
Not using IAC just on/off idle valve.

[Matt Cramer]

Do the sensor inputs behave correctly on a Stim?

[DaveEFI]

Obviously, on a car left say overnight, both sensors should read ambient at switch on.

So it's safe to assume it's the IAT one which is wrong?

They are generally easy to remove, so I'd check its actual resistance at three known temperatures. A good DVM with a temperature probe is well worth having for many things, as well as this.

Put it in the freezer compartment of a fridge etc for a couple of hours. Check the temperature of the freezer with the probe and measure the resistance of the IAT sensor. Do the same at room temperature. And again in an oven set to approx 100C - again check the true temp with the probe.

Enter those figures on Tuner Studio for the ATS. If it is now still incorrect at ambient, you'll need to check the MS input circuitry.

I'm saying to do this as I recently bought an ATS said to be GM spec - and it wasn't (or rather different from the specs I have). But works fine after calibration.

Since I'm in the UK and may have bought the same sensor, these are my readings:-

-8.4C 16000 ohms
21 3300
123 230

[Checkwise]

Unfortunately I Do not have a stim to try..
Everything else seems to behave normally.
I have checked and double checked the wiring. Both CLT and IAT sensors are calibrated the same. Also if I unplug the coolants connector and plug the IAT sensor into that connector it reads correctly on the coolant gauge so I guess the sensor itself is good.
I cant understand why even when disconnected at the MS connector pin 20, I still show 133F.
The MS inatallation was only completed about 3 weeks ago. The IAT read 133F the first time I connected to tunerstudio. But I could not get the car to run on wasted spark. so was side tracked into sorting that first. (thanks for your help with that Dave)
Now the cars starting and I have read up a bit more to feel confident to start the tuning process, Thought best to get accurate temperature inputs first. Thinking it was just a calibration issue. Not till I looked deeper that I found gauge reads even with the sensor disconnected.
I read on similar posts where people have had no reading what so ever, and that re flashing the firmware has cured the issue.
I,m hoping my problem may be a setting as I am unsure about the re flashing process. and the MS was professionally built.

[DaveEFI]

As you say if the CTS gauge on TS reads the same with either sensor, the sensors must be be OK.

If there were a short on the ATS circuit, I'd expect it to read maximum. Open circuit minimum. Not 133F

With the CTS plugged in, but ATS unplugged and the MS powered up but engine stopped, are you seeing 5v from ground on one of the pins of the ATS connector?

[Checkwise]

Thanks I'll try to check this after work Tomorrow.
When you say seeing 5V from ground, do you mean between chassis ground and signal pin in connector or between the two connector pins, i.e sensor ground of MS and signal ?

[DaveEFI]

Between engine block and one of the two pins of the sensor connector. One of the pins has 5v, the other is the input to the MS.

[Checkwise]

Well I popped into the garage on the way home, and yes I do get 5volt at the IAT connector.
However I also checked the sensor returns back to pin 19. I'm positive that I had no continuity between pin 19 and any of the engine ground pins when I tested with the assembled loom plugged into the MS unit off the vehicle. Now however I do. It seems that perhaps a diode has failed ?

[DaveEFI]

I may have confused you as I confused myself.

One pin of the sensor should be grounded to the sensor ground point. The other will show approx 5v with a DVM, with the sensor unplugged.

With the MS unplugged, there should be no ground connection - the sensor grounds go to true ground via the MS PCB.

Sorry for the silly mistake.

[Checkwise]

I have had a chat with Eddie Walsh at Megasquirt uk and he says I can post the ECU to him and he will check it out for me.
so hopefully he will be able to tell me what the problem is.

[Checkwise]

Ok, so I am in a bit of a pickle.
Sent the MS1 back to be checked and they confirmed the fault was still showing when used with a stim.
They had not seen this fault before so have changed components related to MAT and the board chip but the fault is still there.
Unfortunately I'm told that they don't know where the fault lays (possibly board traces) but they cant repair.
So Two things.
Can someone suggest someone in the UK who might be able to repair my MS1 or Is it possible to configure the MS1 to ignore MAT and solely use the CLT for mixture correction?

[DaveEFI]

PM sent.

[Checkwise]

Many thanks to DaveEFI for repairing the board.
Connected it up today and what a difference to the running and starting of the car.
Still need to fine tune but at least its starting and idling reasonable with correct temperature readings.
Thanks again Dave. your a star.

[DaveEFI]

You're very welcome. Wish all faults were that simple to fix.

[knastenrad]

what was the fault on the board?