I know electrical problems are difficult and ones on swaps are even more problematic...

Ok have been working on stuffing a LT1 from a 95 Roadmaster into a 74 Monte Carlo SS. The Buick was rolled, and we swapped out everything.
It is OBDI

So it runs, shifts and starts good. Will fry the tires easily (3.73 gears) as long as the alternator connector is not plugged in..

I have a 750-ohm resistor in series with the gauge wire to sim the idiot light….

I have done the following.

At normal battery voltage (12.8 or so) it runs good, no codes, no check engine light. Check engine light works as we initially had a bad O2 sensor wire, (fixed that)

When you connect the alternator and start it up, the voltage jumps to 14.8 (like it should) then it runs ruff, stumbles and dies, we can force it to run by getting into the accelerator, but it still runs real ruff. We get no codes, or fault light as soon as you disconnect the alt plug; it smoothes out and runs great.

If you jump it from another vehicle that is running, you get the same ruff idle, and stumble and die. If all you do is jump it and the other vehicle is not running (charging) then it runs fine. So this appears to be a condition that is a result of the increased voltage.

I re-traced all the wiring again.
Replaced altenator

I’m thinking maybe the computer is bad?

Any suggestions would be greatly helpful.

 

I can't see above where you listed what make/model the alternator is from, or how many wires the alternator has. If you're using the one from a 74 Monte, I'm guessing it requires an external voltage regulator. If it's from the LT1, it might be 1-wire internally regulated, which means you would just run the one line from the battery to the alternator and that's it.

Can you clarify on that one? I'd like to give a suggestion...which would be to move to a 1-wire, internally regulated alternator, but ...

Doesn't make sense why this would happen to you when getting jumped, though, unless you were getting an unregulated voltage from the donor car.

 

I had similar symptoms twice during my swap. The first time was after I connected the headlights. It ran fine before then did exactely as yours does. I did not have one of the headlight ground wires grounded well enough. I fixed that then it ran fine. A week later I started it again but the battery was dead so I jump started it with the charger, it started then did the exact same rough idle and die again. So I let it charge for 15 mins then it ran fine. Don't know if this helps at all, but all I can say is to check all your wiring and start with a fully charged battery and maybe its as simple as that?

 

Thanks for the replies.

The alternator is from the 95 LT1 Roadmaster, battery terminal and the two connector blade connector for the gauge wire...My understanding from the service manual was that it needed the 750 ohm load on the idiot light circuit to the blade conncetor toe excite the alternator to work.

With just the bat wire connected I'm only getting battery volts (12 or so), less when you load the electrical system with the blower or headlights

We put in 2 different full charged batteries and we still had the same problem..We used new cables.

We’ve swapped the alt from the wife’s Roadmaster into it (it also is a 95) with no change, but the alternator from the Monte works fine in the wife’s car.

We spent the weekend going back thru the wiring harness and checking for cold solder joints or connections with only a few wires connected. Everything checked good.

I just can't figure the computer going bad and not coding....

The kicker is that this is the 3rd swap we’ve done using the LT1 from caprices and roady’s and this is the only one we’ve had major problems with…..

It is frustrating….

I think we’re going to pull the computer and send it off.

Thanks again for the replies.

When we find out the cause I will for sure post it.

Meanwhile any suggestions are welcome and appreciated.

G

 

Before doing that, I would double check your grounds by putting several new (temporary) grounds in place. One from battery to frame+body+engineblock, one from frame to engineblock and body to frame (I think?) I think there are five grounds that people typically do. At least rule out it's not an issue with grounding.

 

Have you just tried using a 12volt source with a 12volt light (idiot light, not LED, one lead to 12+ source and the other lead going to the alternator) as many do for there swaps w/standalone harness? just a though to try and see what happens..

It might be over kill.. but I have 7 grounds that are engine and battery related:

3) Battery to all of these: Engine, Body and the 3rd to the Frame.
2) Frame to Engine mount on both sides of the engine.
2) firewall to the back of both heads

Also I have the PCM gronds going to the same (inside of firewall) bolts as the firewall / back of head grounds.