Large voltage drop

[Jonty]

Hi all

Just wondering if any one can help. I’m noticing that with my lights and blower in I’m seeing a large voltage drop (down to the first line on the volt gauge). If I turn my lights off the Volts slowly increase to just below 13 and then if I turn off the blower it’s back to normal. What’s also interesting is my idle rpm goes up to 1000 when the voltage is on the first line as as soon as I turn things off the rpm drops back to normal.

The first thing I’ll do is check the ground by the drivers front wheel but anything else please let me know

Edit: I also note my alternator has sticky goo on it so wonder if it is on the way out?
Thanks

[Jason]

Hi all

What’s also interesting is my idle rpm goes up to 1000 when the voltage is on the first line as as soon as I turn things off the rpm drops back to normal.

Does the idle really go up or is the RPM meter wrong? If the voltage to the instrument cluster drops too low the RPM meter will start reading high.

[Jonty]

Does the idle really go up or is the RPM meter wrong? If the voltage to the instrument cluster drops too low the RPM meter will start reading high.

Actually I don’t hear the idle increase so you must be right. Why does the low voltage affect the rpm needle exactly?

[cdrusn]

One of the diodes in the alternator is on it's way out. Take the alternator to a local rebuilder and have them test it and rebuild if necessary.

[Jonty]

One of the diodes in the alternator is on it's way out. Take the alternator to a local rebuilder and have them test it and rebuild if necessary.

Thanks - if it’s an orginal Motorola is there any reason not to just get a new alternator rather than bothering with a rebuild?

[Michael]

Thanks - if it’s an orginal Motorola is there any reason not to just get a new alternator rather than bothering with a rebuild?

There is a starter/alternator rebuild shop near me that has been in business for at least 25 years. I would have yours rebuilt IF you can find a shop like that. I went through like 4 alternators over 10 years before I realized a rebuilt alternator is only as good as the rebuilder, and I have had several new alternators before I realized that the rebuilders down in Humble TX aren't my first choice.

Once you know for sure your alternator is good, there are several grounds that if dirty, will improve your volt gauge readings substantially once cleaned.

[Elvis]

You probably have voltages drops on several points.
just getting a new alternator can not fix a high resistance (for example due to corrosion or loose wire).

Not only GNDs get bad !!! the positive wire sees the same current !

Get a multimeter and measure the voltage drops with the car running and the load must be on of course.
The rest ist patience and your will to search.
Some creativity is also needed - how to increase the length of your test wires...

As a nice sideeffect you learn a lot about your car.