MSefi.com read only forum (Jan.15,2018)

Hello all:

I know about #17 in the troubleshooting section, so before you send me there, hear me out . . .

On my freshly assembled MSII v3.0 board, my coolant and air sensors read 170 when plugged into the harness on the car. The harness goes to my relay board. Everything else works fine. On the stim, both temps respond to moving the pots (32 - 215 deg F, then 170 beyond the 215 point).

Here's the catch: My MSI v2.2 behaves exactly the same on the stim, but the temps read accurately when plugged into the same harness!

Is there something different about the MSI/MSII wiring other than the IAC and ignition outputs? All ideas welcome.

Are you using different sensor calibrations on both units or are they both calibrated for the same sensors?

Matt Cramer wrote:Are you using different sensor calibrations on both units or are they both calibrated for the same sensors?

No changes to either unit's calibrations. Stock programming for GM sensors.

(I'm using v 2.891 on the MSII v3.0)

Weird. I'd backprobe the sensor and check the voltage going to the MS with both boards.

Alrighty, I think I'm onto something here. I'm pretty sure the problem is that the MSII isn't grounding pin 19, the sensor ground. Here's how I found out:

First I plugged the stim into the MSII, turned on MegaTune on the PC and verified the MSII's operation. Everything -- including temp sensors and TPS sensor -- gave rational values.

Then I made up a limited external DB37 wiring harness with only +12v (pin 28) , -Gnd (pin 10), sensor signal (pins 20, 21), and sensor ground (pin 19). I fired it up by applying voltage thru pin 28 and grounding pin 10. I started MegaTune on the computer and verified the MSII unit was working (verify MAP sensor input by sucking on the vacuum tube). All good. (temp showing 170 on MegaTune, but the sensors were not hooked up yet)

Then I hooked up the sensor between pin 21 and pin 19. Nothing. 20 and 19. Nothing. d'oh!?

So then I jumpered pin 19 to pin 18, and when I ran the wire from pin 19 to each temp sensor signal wire, I got rational readings.

Anyone see any glaring mistakes in my diagnostic? If not, it's time to figure out why pin 19 isn't grounded by my new MSII . . .

Oh yeah: So why did it work on the stim? The stim does not use pin 19 for the sensor ground.

Solved. There was an open circuit between the board and Pin 19 on the DB37. Everything working nicely now.