I had to test my alternator recently (see back posts on "electrical intermittent" and "electrical intermittent now permanent") and thought what I picked up from the alternator rebuild guy might be of some use to others. I didn't see it in the 928OC tips so maybe it's a new bit (although I'm sure others know how to do it). Any corrections/cautions welcome since I don't want to pass along bad info. NOTE: Thanks to Wally Plumley who pointed out that:
-This test will not detect intermittent due to vibration/temperature
-This tests the alternator/voltage regulator combo as a unit. Past posts have pointed out failures/failure to replace old regulator on re-builds. If you're going to a local rebuilder, make sure they test the voltage regulator, either separately or on the rebuilt alternator before you take it.
I had a problem where my pod voltmeter read (accurately) about 11V or so and the discharge light came on. The OXS light also came on, but the consensus opinion was that this was probably an anomaly due to the low voltage condition and that the alternator was the likely culprit. I took the alternator off the car, took it to a local rebuild shop and it tested ok (regulator also). The guy gave me some advice on how to test it on the car that I'd like to pass along (and he didn't charge me anything. He just wanted my business in the future. You bet!).
I cleaned all the battery and alternator connections, re-installed the alternator, and then went through the following tests. Everything worked fine, voltmeter around 13+ and discharge light OFF, so I either had a bad/dirty connection or (shudder) I have something loose that is OK for now but may fail again in the future.
On my '82 5 spd, the Paris Rhone alternator has two terminals, a "-"
that connects the exciter coil to the battery via the Pod discharge light (which has a parallel 68 ohm resistor), and a "+" that has the heavy red cable on it that goes to the battery/starter and supplies the charging current.
When the car is turned on, current flows through the discharge light and resistor to the exciter coil which causes the alternator to juice up to ~ 14V and charge the battery/circuits. When the alternator is functioning, this "-" terminal voltage rises to 14V and, since the voltage is the same on both sides of the discharge light, no current flows and the light is OFF.
- A voltmeter (optional but useful)
(a) Test jig-easy to make up.
-Buy a 'side marker' light socket at your local auto place, a simple socket w 2 leads, and a compatible bulb of 3 W or so...not too critical. A 3W bulb is roughly equivalent to 60 ohms at 13.5 Volts
-Two clip leads and wire...solder/crimp the clip leads or alligator clips to the bulb socket wires. About 1' is more than enough
(b) Two wires with connectors (circular rings that will fit on alternator terminals or you can use a good alligator clip to grab the "+" terminal...about 3' for each is plenty. Rings/clip on one end and I used alligator clips on the other ends
(c) One jumper wire with a clip lead/alligator clip on each end, 1 ft is enough
-Prior to removing the alternator or connecting any other wires to it,
Disconnect the battery by removing the negative terminal connection
(Wing nut on mine)
-I removed the alternator and its shroud so I could get at the terminals
-Attach one wire [(b) above] to the "-" terminal and the other to the "+" terminal
-Remount alternator w/o shroud leaving OFF the wire that normally attached to the "-", I put some tape around its connector just so it didn't inadvertently touch the other wires. Re-attach the heavier, red, cable to the "+".
-Route the wires away from the engine toward the rear to keep them out of harm's way and attach the other ends clips to a small piece of cardboard/wood (not touching each other). This keeps them from shorting together
-Re-connect the battery negative terminal
-With the car off, I measured to make sure that battery voltage was present on the "+" terminal. (Measured at the clip lead to the + terminal). If not, you most likely have an open somewhere between the red cable and the battery (I assume there's a fusible link between the alternator and battery but didn't have to check this so if you have this problem you'll need to check the wiring diagrams in the manual)
-Start the car and attach one end of the test jig (a) to the wire you connected to the "-" terminal. Briefly touch the other end of the test jig to the wire you connected to the "+" terminal. The light in the test jig should light briefly and then go out indicating that alternator is working and the exciter terminal (-) has come up to alternator output voltage
If it stays on, you likely have a problem w the alternator (alternator is not functioning and the jig light is getting current from battery to ground through a defective exciter coil). Also, double check to make sure your clip on the "-" terminal didn't short to ground somehow.
If it doesn't come on at all, you again may have an alternator problem (open in the alternator exciter) or a bad clip lead connection. Again, you should double-check it.
With the car running, you can measure the alternator output w your meter by connecting to the clip lead going to the "+" terminal, and ground or the jumper connection under the hood. Should be around 14 volts. (At least greater than 12.6!)
-At this point, you may want to re-try the test with a different battery just to be sure your battery isn't bad. If the battery is OK then it's time to replace the alternator.
Once you know the alternator works:
To check that you have a good connection and components from the pod (discharge light and resistor) to the exciter coil:
-Turn the car off
-Remove the test jig
-Take one clip lead of the wire jumper (c) and connect it to the thin wire that normally goes to the "-" terminal (this is the one you left off earlier and taped).
-Connect the other end of the jumper (c) to the wire you connected to the "-" terminal. (This restores the
"Normal" connection from the pod light/resistor to the alternator "-" terminal)
-Measure the voltage on the + terminal by connecting one end of meter to the wire you connected there and the other to ground. If its greater than 13v or so the alternator is running OK and your connection is OK.
-If the alternator is not functioning you have a bad connection from the pod AND:
1.IF the discharge light is OFF, you either have an open in the wire run from the pod or the resistor/discharge light are bad (a double fault?..Not very likely)
2.IF the discharge light is ON, you may have a short to ground from the pod
TO CHECK THE WIRE from the pod to the alternator:
-Disconnect the jumper (c) from the wire on the "-" terminal. Make sure it is not touching anything (tape it if you have to)
-If the discharge light is on, you've got a short to ground between the pod and the alternator exciter wire.
Very Primitive text drawings of equipment /test connections
| + <----One wire 3'(b)---->alligator clip
|- <----One wire 3'(b)---->alligator clip
Test jig (a)
| |-------> >------->Clip lead/alligator clip
| |Side marker
| |Socket w bulb
| |--------> >------->Clip lead/alligator clip
Jumper wire (1 ') (c)
Alligator clip<---------->alligator clip
by Jim Morehouse