legacycentral bbs

Hey everybody, been awhile. I need some electrical guru help. Troubleshooting a new project wagon and finding some strange issues.

Car is a '91 L wagon M/T. Won't idle unless the IAC connector is disconnected. TPS v is normal.
Idle switch wire has ~10.5 v from the ECU. I tried three different ECUs (one known good, one known bad) and find the same thing. Checked for shorts, pulled the wire from pin B56.6 and checked v at the pin. I get ~10.5 from all three ECUs.
From my understanding, this pin is supposed to be the idle switch input from the tps connector, which should have 0v throttle closed and ~5v throttle open.
Anyone know or have any ideas about what could be causing the ECU to have power at the idle switch pin?
I'll keep working on it, pouring over FSM diagrams and troubleshooting guides. But if anyone has any experience with or insight into this issue, please share.

You've got things incorrect.

Source voltage is battery voltage, so the voltage reading should read around what the ECU is reading.

Your readings sound fine. Here's a diagram

http://www.main.experiencetherave.com/s ... g_diag.jpg

Sounds like you've got another issue regarding the idle.

Actually, in that very manual, the idle switch troubleshooting guide (pg.86, sect. 2-7) states 0v closed and ~5v open. Also, measured batt. v is ~11.3. Which makes the measured v at the ECU pic that much more confusing. I also isolated the signal from the ECU, eliminating any other possible voltage input.

You can check just the switch by checking continuity between pins 1 and 2 with the TPS unplugged from the harness.

10.5 volts could be correct (even with 11.3V to the ECU) because of the voltage divider in the ECU (The internal resistors shown in the circuit diagram).

I'm not sure what to tell you. The manual has been known to be incorrect.

However, the idle switch shouldn't cause the problems your are axperiencing.

I don't think the manual is incorrect. That pin is supposed to receive either a 5v or 0v depending on throttle position. I'm reading v from the ECU. With the connector disconnected at the TPS, a voltage reading of ~10.5v is present at the harness connector.
I'm thinking that whatever is giving power to that pin of the ECU is also causing the other problems.