I had the transmission swapped from auto to manual, and the ROM was also rewritten for a manual transmission.
The engine idles smoothly at 650-750 rpm, same as before, but as I drop the rpm slightly below it, like 500 rpm, when engaging the clutch, the voltage drops easily and the engine tends to stall.
My question is, is it common to have a low voltage easily enough to make launching sensitive, or is there any problem with the alternator, battery, or the wire harness?
By the way, the clutch plate is not sensitive at all.


Any advice is welcome. Thank you.

 

No.

 

So, what do you think is the problem?
I think an old harness can cause a low voltage and this kind of problem is often seen in old cars.
But am I wrong?

 

Why do you think voltage is the issue? You let the clutch out, at or under 500 rpm, and wonder if voltage is causing a stall? As kindly as I can say this, you need to learn to drive a manual transmission. Give it more throttle as you release the clutch.

 

I forgot to say this, but the engine tends to hunt once the voltage drops at less than 750 rpm.
This hunting never happened when it had auto.
I've been driving a manual transmission car for many years, and I know a 5 liter engine car should not have such sensitivity.

 

Have you measured the voltage when the engine is running below 750 RPM? It might help to know how low it goes, but in reality, it should not drop below battery voltage. If you are seeing a big drop when the alternator output drops off (low RPM), have your battery checked and clean and tighten all your ground connections and check your battery to alternator wiring also. HTH!

 

Applying an engine load sufficient to move the car, when the engine is at 500 rpm is going to stall it every time. Fluctuating voltage is not a likely cause here unless it is dropping severally low. I doubt it's a charging system issue because if everything else is ok, you could even disconnect the alternator and the engine would still run until your battery dies. But, let's try a few other things.
What is the voltage dropping to? Use a volt meter to measure this at the battery, you can't rely on the gauge to be accurate. Is this a stock engine, any mods other than the transmission swap? Did you miss any ground straps when you put the manual transmission in?

 

I've checked the voltage.
A digital meter is inside the car, and it shows:
ignition on, engine not running: 12.0v
engine at idling: 14.0v
engine hunting: 12.0 to 14.0v

BTW, voltage at battery: 12.5v

The engine is a stock one, and has no problem except hunting.
Ground straps have been increased in number.

Is it ok that the alternator doesn't produce any voltage at a very low rpm, like less than 500rpm?

 

Yes, it is OK, because your car doesn't run or idle at 500 rpm, that's simply too low of an idle speed. What are you using to know the engine RPM? Tachometer in the cluster (maybe your gauge is inaccurate), or a laptop or scan tool reading ECM data? I think we are taking the long way around what is really wrong here.
Are you trying to fix a stalling concern, an engine rpm fluctuation, or something else. Perhaps you can post a video of what is wrong?