Connecting to computer on the bench but not in the car
 

I have built an MS3 + MS3X ECU for my JDM 1.6 1992 MX5 (Eunos Roadster) following Frank's guide: http://westfieldmx5.devocht.com/megasquirt/ms3/90-97-ms3/

On the bench using a car battery I have connected to the laptop and flashed the ECU with the newest firmware.
Tuner Studio also communicates with the ECU on the bench.

The problem is when I hook it up in the car the computer doesn't communicate with it. It detects something on the USB but doesn't connect to Tuner Studio.

The car runs fine on the stock ECU and I read 11.5v between the 12v and Ground. I don't see why the MS3 doesn't seem to be powering up on this.

I'd really appreciate your advice.

Are you using the same cable to hook it up both times? It seems that a lot of connection problems come from the usb cable and or the drivers for the cable. It should be cheap and simple to swap another cable just to rule it out.

How are you grounding it when on the bench vs. when it's in the car? A vast majority of electrical problems in cars has to do with grounding.

If it's through the cable, then swapping as Miater suggested is an excellent place to start.

I'm using the same USB cable all the time.

I can set it up in the passenger foot-well using the stock harness and get no signal to TS. Then take laptop, cable and ECU to the boot of the car and hook it up directly to the battery and get a signal in TS.

Thanks for the input - I checked the grounds but didn't find a problem so tried a few other things.

After a lot of messing about I found that when plugged into the car, the MS3 only communicates with Tuner Studio when the TPS is not connected.

Now, Frank's method says that the stock TPS would cause a floating input but I didn't expect it to prevent the MS3 from communicating with TS...

Anyway, hoping it was a problem with the TPS and not something inside the MS3, I tried the BMW TPS and I got some sensible readings in TS. On the back of this I installed the BMW TPS and calibrated it just fine in TS.

If you have any idea why the stock TPS would prevent the MS3 from communicating with TS I would be interested to know.

I hope this will help some other newbie in the future!!

When you hook up the stock TPS, you're shorting VREF to ground and shutting down the processor. There is no reason to wire the MegaSquirt to the stock 1.6 TPS; it's not really a position sensor, just an on/off switch.