I think I may have an alternator issue in my 88 vert. It's done this since I bought it a year ago, but have been wondering what the root cause is.

Symptom: when I drop it down to a low RPM, like at a stoplight, or during a long lazy upshift, the electrical power seems to sputter. my radio and dash lights seem to flicker (sound cuts in/out during the sputter). When I RPM it up a bit, the juice seems to come back. Also, after the RPMs drop to a normal idle, the power seems OK too.

I am wondering if there's a point in the RPMs where it cuts over from charging/running on full alternator power to running off the battery. The dash guage doesn't seem to be jumping around during all this either.

So...

Is it failing brushes in the alternator, or is there some circuitry that cuts it between alt and battery that is failing? or both?

Mucho thanks.

 

The alternator is always charging when the engine is running. Grab a multimeter and put one probe on the alternator post, then put the other on the negative battery terminal. With the engine idling, you should hopefully see 14+ volts. It may be a little lower since the S4 alt. is so weak, but at least ~13.6V is needed to charge the battery. Below that, there's a problem and the alternator will likely need to be replaced.

Other things to consider are whether the engine is idling below spec (7-800 RPM), or if you're using an under-drive pulley (larger than stock). If the alternator spins too slowly, the voltage output will drop.

 

Additional tests you could try.

http://mazdarx7.iougs.com/electrical.shtml

 

I also think my dashpot is bad or going bad. If I decelerate quickly (like rev it up and drop foot off accellerator) it seems to bounce down below normal idle then come back up. it seems to be this period below 750 or so that electrical sputters. I'll watch the RPMs next time. I might also swap alternators with my SE to see if the problem goes away.