ecu and idle

[mocoloco]

before I try to get a pimp (sounds bad!) I am trying to fix an issue and I need some help.
88 mustang with la2 ecu, just tuned up, big throttle body (94 5.0). will idle at 1500 rpm.
I have checked the timing belt, set timing at 10, tps is the original 5.0 not adjustable, cleaned throttle body.
issue seems to be voltage, with car on voltage is normal at 12, but the ground reads 10, (I have a 2.3 turbo ranger and running it reads 6 at idle) everything else is exact.
I tried swapping ecu, no luck.  tried swapping van, no luck.
when warm if I unplug idle control valve, it drops to 800 rpms. which tells me it is not a throttle body issue
idle is all the way down.
what could make the computer open the iac? I tried also different iacs.
thanks,
ahmad

[Stinger]

The ground should read 0 volts at all times. Any voltage on the ground side is a major issue.

[mocoloco]

well, its not voltage, it is like negative voltage, we tested it with a circuit tester. my ranger runs great and it read I guess -6 at idle, the mustang reads -10 at idle and it idles much higher.
I though that the iac would be controled based on the voltage given, but it always seems to have the same voltage, I was told by someone that the other wire (ground) is the one that somehow controls the iac opening and closing.
any of that make sense?
thank you very much for your time,
ahmad

[Stinger]

Without testing it the traditional way I'm used to (with a multimeter) I can't respond to the voltage/negative voltage scenario. Have you tried swapping the IAC from the Ranger to the Mustang to see if the problem persists (wiring or other cause) or if it fixes it/makes it way better (faulty IAC)?