Got a big problem.
alternator is new.
the car runs great till 60mph then shut down.
if I disconnect the battery the car runs well at every speed.

how is it possible?

 

Rather than start a new speculation thread, could you remove the air cleaner assembly in your car, and post a bunch of photos of your engine compartment?

Do you have the stock ECM, CCC, and 7-pin distributor? Any codes or check engine light? Do you know that the CEL bulb is good (it should flash at startup)?

Perhaps the wind is shorting your starter extension harness? Mine looked like this before I replaced it.

 

Standard ecm and distributor.
cel light is ok

Another test and another headache .
Battery connected: dies at60
Battery disconnected: perfect
Battery connected and ecm in safety mode( disconnecting distr or ecm): run perfect over 60

 

Did you check your alternator voltage? I wonder if there’s some over-voltage protection happening and your new alternator regulator is shot.

 

14,7v… but if was about overvoltage the problem has to be also when I put the ecm in safe mode….

 

 

Here the video

 

https://www.veed.io/view/88b2bcfe-3c...ingWidget=true

 

By the way the tach drops out I would try a new pickup coil and module.

 

Thanks, that's very helpful!

So that shows that your gauges stay powered up. Is your car an automatic?

I don't know enough about the 81 system to know what that means, but it does imply that it is not a battery ground issue, or a dozen other things.

I can't imagine why it is speed and/or battery connection dependent. Can we see a photo of the distributor and carb? Maybe there is something obvious there.

 

its standard 81 with ecm
carb is original and distributor too.
mill take the picture tomorrow.

ps if I disconnect the distrib, the ecm goes in safe mode and the car runs great over 60mph, rich but great

 

Why are you disconnecting the battery while the engine is running in the first place?? I was always taught that you NEVER EVER disconnect a battery on a running engine as the alternator and any electronics could/would get fried. That has to be a terrible thing to do to the electronic controls in your Corvette. I would monitor the voltage at the coil and see why it is turning off. Disconnecting the distributor is not something I would do to a running engine either.

Something that is monitoring the the vehicle speed and or the engine speed is doing it. Does the car have an OBD1 Port that you can attach a scanner to or Tuner Pro to watch it while it is running? On a C4 I would be monitoring the engine and wait and see what is turning things off at that point. Does the car idle forever and only die when it hits ~60 mph or what?

What does the ECM use for a speed sensor? If it goes past 60 in "Safe Mode" then it is likely something is telling your car to shut down in the normal operations part of the software. Maybe a bad ECM? I hate to start at that point but I know little about the 1981 C3's. I am very familiar with the C4's and their systems however.
I have a Bluetooth adapter that plugs into the OBD port and connects to Tuner Pro on my laptop and that would be really helpful IF you have an OBD port. I have a 1987 VW that has an OBD port in it so your 1981 "might" have one.

What software controls are speed associated on the 1981 C3's? How on earth does it run without a signal getting to the distributor?

 

You just triggered something in my brain.

https://www.corvetteactioncenter.com...icing-677.html

There IS a VSS (maybe). I don't have an 81, I know very little about it. There are codes you can read with a paperclip (no kidding), the instructions are on the Forum.

What does happen at 60 MPH? The torque converter locks up. Why this causes the ignition to drop out (if that's what's happening), I don't know. But perhaps this problem really CAN be solved by adding a third pedal!

 

tried with the paper clip, and the ecm goes in diagnose mode but doesn’t show any error.

where is this Vss?

the lockup converter will lock only at 60? Someone told me that it’ll lock in 3 and in 2nd gear at any speed, it depend on speed and rpm…

could a lockup cause this shutdown?
how could it cause the shutdown only with te battery connected?



by the way could it also be a bad ignition module on distributor?

 

There is a VSS on the 81 auto cars. It is actually attached to the back of the speedometer and runs directly to the computer.

 

could it be the problem ?
can I intercept the wiring somewhere and try to disconnect it?

 

The wires from the VSS does run a short distance to a connector from where it is mounted on the back of the speedometer, down through the center console area and then back to the computer. If memory serves me, the connector is either right under the dash or right where the wires run into the console area. There should be three wires in the harness, a brown wire, a black with white stripe and a green with black stripe.

I don't know if it can cause the problem you are seeing. I think it would be very unusual if it did but anything is possible. It does send a signal to the computer.

 

vss is lockup.

so, unhook dist all is well?

hmm, puter controls timing.
what in puter goes wonky when bat hooked up?
puter is in front of bat.

 

PARTIALLY SOLVED:
it was the vss. Disconnected and no more problem.
just one thing.

the car doesn’t go at her best.
when I accelerate she mumbles a little. As you can see from the rpm in this video.

if I put the ecu in safe mode she goes fully rich but doesn’t mumble and goes well.


https://www.veed.io/view/ee683e60-5e...ingWidget=true