How can I tell if my alternator is failing? Options

[andreic]

A week ago I had trouble starting my 914-4. This was after driving it for an hour, and then leaving it in my garage for half a day. I started it by pushing it. Drove it for an hour and parked it in the underground garage.

Today, tried to start it, again same issue. Won't start. Took it to a battery shop, where they measured the battery and said it looked good, but that my charging voltage is low (they measured 13.1 volts, even with me revving the engine). Drove it home, measured again at home the voltage at battery with the engine revved up, 12.6 volts.

Tried replacing the voltage regulator (had a spare one at home), same exact result.

Before I launch into replacing the alternator, what else should I check, and in what order? What are easy steps of things to check?

Thanks,
Andrei.

[andreic]

P.S. My red light on the dash is not on, and certainly there is *some* charging going on. After driving the car for 30 mins, the battery is charged enough that the car starts easily.

[r_towle]

try removing and cleaning the regulator that is on the relay board.
Clean all the little plugs and female sockets.
I use gun cleaning wire brushes and spray electrical cleaner.
Every 40 years it helps.

Otherwise, you should be seeing ~14 VDC when idle charging.

Rich

[andreic]

That's what my problem is -- it does not show anywhere near 14v. I can try cleaning the contacts at the regulator, but I suspect nothing will happen, replacing the regulator was supposed to do the job...

[914Sixer]

Feel the alternator harness for a Hard spots. Make sure it is soft and pliable. Do you have another relay board to test with? Let me know and and I will send you a restored relay board to play with.

[porschetub]

That's what my problem is -- it does not show anywhere near 14v. I can try cleaning the contacts at the regulator, but I suspect nothing will happen, replacing the regulator was supposed to do the job...

Most likely regulator but have you done a high load test,chuck everything on and meter your battery,if the system tips over and the voltage drops rapidly its most likely your regulator isn't sensing voltage drop.
With altenators they seem to work or they don't unless your drive belt is loose/worn ?
I'am no expert but this has worked for me for years,good luck.

[Mikey914]

Don't rule out the battery itself, check the date it may be long in the tooth

[Spoke]

Does the GEN light come on when you turn the key to ON before starting the engine?

Does the GEN light go out when the engine is started?

Check out the diagrams below. It helps to make some measurements to determine what is working in your system.

Note in the first diagram the signals D+, DF, D-, and B. Measure all 4 of these signals with respect to chassis ground. There's a stud behind the relay board and one behind the battery. Get chassis ground there.

D+, DF, and D- are on the alternator cable coming from the engine. Signal B is the battery positive. Measure all 4 with engine running. Let us know what those readings are.

D+ should be 14V
DF will vary between 5V and 12V
D- should be 0V
B should be 14V

Also do this test: Remove the VR. Short D+ and DF at the VR connector. Start the car. This should provide maximum flux and maximum voltage output. The voltage from D+ to chassis and Battery POS to chassis should be around 16V.

[second wind]

It is always the battery.
gg

[oldschool]

Don't rule out the battery itself, check the date it may be long in the tooth

(IMG:style_emoticons/default/agree.gif) ..I just had the same problem with my Boxster, I got a new battery problem gone.

[GregAmy]

I wouldn't be surprised if it's a flat battery.

The VR on our cars is kinda strange; it likes to charge at around 1.5V over battery rest voltage. For example, I have a LiPo battery in my street car (Autocraft LFP-7) which sits at around 13.5V with the car off. The VR charges the system at 15V.

When I swap in a Miata AGM battery to test - 12.5V at rest - the same alternator/VR charges at around 14V.

Bring your battery to a FLAPS for load testing.

[stownsen914]

OP mentioned that the batter tested good.

If the battery is good and swapping the VR didn't fix the issue, that leave wiring and alternator. Checking the connections is worth doing as noted earlier. Next step is to remove the alternator and have it tested.