What led you to have the have the alternator tested in the first place? It may be as simple as the voltage regulator needing replacement. The old "roundbacks" didn't put out out much current to begin with. And what year is the car? The old "roundback" you have pictured first is typically seen in '69 and older and will have (1) terminal as the field terminal and the other terminal is grounded to the casing of the alternator and not used. '70 and newer style are the "squarebacks" and they have (2) active field terminals. Many of the rebuilt alternators today use the "squareback" cases due to a shortage of the "roundback" cases I'm told. Typically on the '69 and older application alternators the grounded terminal will be clipped so you can't use it as well.
If his vehicle is of this vintage using this style VR('69 and older), then one of the terminals should be grounded.
Great. I'll give it a shot. Thanks.
It's a 66 Plymouth satellite. I had it tested because it has an aftermarket stereo and it started cutting out at low RPM but has gotten worse. The squareback case doesn't fit perfectly and rubs the block, so unfortunately it's canted just slightly because they bushing won't fit in the mount. So I wish I had a roundback.
Check both terminals to make sure one isn't grounded already. A grounding washer will be installed under the terminal typically if so. If one of the terminals isn't grounded already then this can be done by removing the terminal and installing a washer under it as seen here. I just went through this with someone on FABO recently. The smaller arrow points to the grounding washer. Your field wire (green) will NOT be connected to this. It will be connected to the terminal that is NOT grounded. The larger red arrow just indicates how some of these new rebuilds will clip the unused terminal to help eliminate confusion.
The car is better off to get the original one rebuilt. Call around and find a local rebuild ship that can do this service for you.
So, I bought a new roundback alternator assuming it would be easiest to match the original. I installed it but it is testing 75%, bad charge by autozone. I wouldn't imagine the unit is bad... Does this means a different component is causing the issue,like the voltages regulator?
Make sure:
Did you start with a fully charged battery?
Assumption and overlooked. Very good point Don. You’re always a huge help.
When your auto parts store tested your origional unit and condemned it by saying the output was "too low", what was the actual output, in actual numbers....not just too low. What was the applied field voltage, alternator RPMs during the test, what was the applied load during the test?? The auto parts store is in the business to make a profit and a new Duralast Gold unit contributes to their bottom line.
I just put it in a trickle charger last night. Yesterday when I took it for a drive to fill the tank and have the new alternator tested, before my post, I had it running for about a half hour. The ammeter show either discharge or right in the middle without much bounce like I had before the replacement.
