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Join Date: Jul 2012
Location: Park City, UT
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How to test Alternator and Voltage Regulator?

New owner and new to the forum. Bought a Garnet Red 1985 930 with 68k miles locally a few weeks ago. I had a PPI done and was happy that the big stuff (engine, trans, body, etc) was in good working order. Very few service records with the car but I pieced together that it was only driven about 10k miles in the last 15 years. I think the previous owner(s) just waxed and looked at the car and didn't bother to keep after the little issues that were starting to pop up. My goal is to get this car sorted and have a reliable summertime driver.

I knew there were things that had to be done right away. I replaced tires, CV joints/boots (3 were torn), heater motor rod ends, front and rear deck shocks, adjusted door handles, and started chasing down some of the bigger issues. First, the light switch and blinker switch were replaced due to bad rheostat in light switch and the car going black when you turned on the high beams. All lights and switches now work great. I removed an aftermarket alarm system that was inop while I was it it and really cleaned up under the dash.

The tachometer in the car was inop when I bought it. While I was replacing the CV I did clean up the trans to body ground and have cleaned gauge grounds in the front bonnet area, tach still inop. The car has a Permatune ignition that tests good and the black/purple wire to the tach has continuity and 12V when running. The lights and boost gauge all worked on the tach. I finally decided to have the tach rebuilt by our host and just got it back today with a note saying to check my alternator or I may fry the tach again. I have a solid 13+ V at the battery but I don't think this will shows spikes or other anomalies that may fry the tach. Another consideration, when I turn on the headlights, the fuel gauge, oil level, oil temp and oil pressure all jump up and remain in the new position by about one needle width. This tells me there may be a problem with the regulator, but not sure? What is the best way to test the alternator and/or voltage regulator?

I'm afraid to plug in the tach for fear of frying it again, so don't even know if all is good now. I have been lurking around finding answers to a bunch of questions I had on the car and this forum has been a great help but I am stumped now on this issue now.

All in all, I love this car, awesome color and a great driver. Is anyone in the Park City / Salt Lake area?

Old 07-09-2012, 05:55 PM
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Old 07-09-2012, 05:59 PM
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Very pretty car.

I had questions about the health of my alternator/VR (my tach was jumping around intermittently). While I sat in a parking lot the tach would work fine and the voltage would read 13.x volts across the battery terminals. But randomly while driving the tach would start jumping around for a minute or two. By the time I would pull over and haul out my multimeter, everything would be back to normal.

So I took my multimeter and some speaker wire (for length), and connected the multimeter to the battery terminals. The speaker wire ran from the battery, out the corner of the front hood and in through a window to the multimeter wedged into a corner of my dash. When the tach started acting crazy, I turned on the multimeter to find my voltage up to 17-18 volts.

Since replacing both the alternator (rebuilt) and VR (new), no further problems.
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Old 07-09-2012, 07:56 PM
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headed to park city next week to get out of this phoenix heat. the voltage regulator is relatively cheap. I had my tach jumping last year so i thought it was the alternator. took the alternator to a shop next to mine in phoenix and the guy rebuilt the alternator and said the VR was fine. i put it back in the car and it acted the same way. i pulled it out agin and put the new VR in and it world perfectly. I would pull the alternator and replace and or rebuild. i would also replace the VR. i think its like 60 or 80.00 from our host? beautiful car
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Old 07-09-2012, 10:43 PM
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Thanks for the feedback guys. Since the tach is out of the dash, I think I am going to hook the multimeter up directly to the tach leads and see if I get spikes at varying rpm (I think this will be an accurate test?). I will also run it on the battery as Noah930 suggested to see if that is any different. It sounds like the alternator overhaul and VR replacement is cheap insurance in any case and will be somewhere on the punchlist pending the results of the multimeter experiment. It may also solve the needle jump when I turn the headlights on. J930, hit me up if you want to take a ride around PC - 435.565.1288. -Chip
Old 07-10-2012, 07:04 PM
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The tach works! I did as suggested and ran around with a multimeter hooked up to the battery (it was easy to route a wire with the tach out of the dash) and saw a consistent 13.8 - 13.9V at the battery and at the tach power source. I also hooked up the positive to the purple/black tach lead and saw anywhere from 13 - 15V, which, according to Permatune is normal. I think the charging system is working as normal and suspect the tach was inop in this car for a long time before I bought it.

This car is starting to come together. WUR and AAR are next on the list to solve a low idle while cold / high idle while warm problem.

Thanks again for the help. This forum is a great resource!
Old 07-13-2012, 04:03 PM
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Quote:
Originally Posted by aerochip View Post
...saw a consistent 13.8 - 13.9V at the battery...
I don't know why more people don't install a voltmeter in their car. Next to oil pressure and temperature, its the most useful instrument in my car.

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Old 07-13-2012, 04:11 PM
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