Electrical Stalling....Vss signal check?

[Kid wita 5oh]

Working on a 89 with a 92ish harness & A9L ( Was put together with a newer harness not positive on the year)
306 w/ Edelbrock heads, cobra intake, E cam, pro-m with stock injectors, base timing at 14*

Having a stalling issue when coming to a stop with AC on, and/or slow speed turning. Then will have a hard time staying running for a few tries after stall, unless I give it throttle.

Have done the entire surging idle check list, fixed a few vac leaks and did base idle reset(a few times). Replaced the TPS and IAC (and swapped known good IAC from my car), and swapped in a stock MAF (and tried various clocking positions) all with no change. Voltage was right at . 9-1v at idle with either MAF and all positions

Car will idle at 850, then drops 100-150 rpm with AC on but stays steady

What type of signal/voltage should be output from the Vss?

and what voltage should I see across the IAC circuit ? Seeing between 3-4v at idle (more makes idle go up, right ? )

Have a BOB hooked up so I can test easily while running

I haven't opened the computer to check caps yet. Are these symptoms something that could be caused by that?

Open to any ideas

Thanks

 

[Dan02gt]

I fought those issues for years before giving up and getting a tune. Some cars run perfectly fine without one but mine didn’t. The tune modified the dashpot settings so the anti-stall would stay active for a few seconds after coming to a stop. In other words my RPMs hang at 850-900 until my car completely stops for 1-2 seconds then it drops to 650. I also add 100 RPM to the base idle when the AC is on. There are power steering compensator settings too but I’m not familiar with them. Those settings have been a game changer for me. I have a A9L with a Quarterhorse running on a custom tune from EFI dyno tuning based on A9LV2 and it was worth every cent.

 

[Kid wita 5oh]

Just opened the computer. 2 caps are blown, with some board corrosion. ...

Off too ecu exchange it goes

 

[Praetorious]

Sounds like once you get your ECM overhauled it should run real good, those caps look bad just like on my 91, is there a website or article or anyone that knows what every component in these ECMs does and what functions they are responsible for? I would love to see something like that.

 

[Mustang5L5]

Starting around 10:30, I identify some of them. It's not a complete ID of components, but does show what does what a little bit.


View: https://youtu.be/p73aWm9OXbU?si=Az7mJ5XMYkXf3eH5&t=629

 

[Praetorious]

Thank you, once I get home from work I'll check it out.

 

[Kid wita 5oh]

It's not my car but yes should be a big improvement
Apparently the power supply was faulty and it had a few burned traces as well. I'm surprised it ran at all.

 

[Praetorious]

Yeah mine runs fine but it has this persistent minor idle issue and everything has been checked replaced and triple checked and I opened up my PCM and some of the yellow Chiclets have corrosion plus the capacitors look bad so that's my next step, might do it beginning of the year .

 

[Kid wita 5oh]

UPDATE
Installed the repaired computer. Did a base idle reset.
Idles great at about 775-800. When I turn the AC on the rpms still drop, but it idles steady.(I feel like the IAC should be bumping it up, no?)

On the test drive everything was great until I turned the AC on. Now it stalls as soon as it's taken out of gear to coast, using the clutch or not.( It's fine during a shift though)
The rpms just drop straight past idle and stalls(I can catch it with the pedal but have to stay on the pedal or turn the AC off

Ideas?

EDIT... this is at ANY speed, not just coming to a stop

 

[Mustang5L5]

If you unplug the IAC with the engine running at idle, does the engine stall?

Any codes now on the new ECU?

 

[Kid wita 5oh]

It will almost stall but stays running

I haven't rechecked the codes, I will today.

 

[Mustang5L5]

I’d open the idle set screw just a tad. Still set it for a very low idle so the IAC is maintaining proper idle, but just lift it a little above the almost wants to stall rpm.

 

[Kid wita 5oh]

Pulled codes
96 c - fuel pump secondary circuit fault (I don't think this has anything thing to do with the issue)

91 c. - O2 sensor signal put of range

@Mustang5L5 I think I found an issue . . Does this O2 harness look wrong to you? tryin to compare to your write up but want to be sure

 

[Kid wita 5oh]

I guess it's correct for a 91-93 harness?

 

[Mustang5L5]

Would be correct for a 91-93 5-spd. Is that the complete engine harness you are using?

 

[Kid wita 5oh]

Can't say for sure, don't know the history of this car prior to the current and former owners

 

[Kid wita 5oh]

I bumped the idle up about 50-75 rpm and did another reset, no change in the issue

So just something to add(doesn't help at all just an observation). I unplugged the compressor and took it for a ride, no stalling with the AC commanded on.

So what's causing the computer to not catch the idle with the compressor engaged ?

 

[Mustang5L5]

Load of the compressor on the engine. . The IAC should open more. Have you cleaned it lately or can you give it a cleaning? It might be sluggish/sticky

 

[AeroCoupe]

Is the car an original 5.0 five speed car?

 

[Kid wita 5oh]

Load of the compressor on the engine. . The IAC should open more. Have you cleaned it lately or can you give it a cleaning? It might be sluggish/sticky

The IAC was replaced, and then swapped with mine to verify. No change

Is the car an original 5.0 five speed car?

The vin is a 4cyl car