78Silver928

Guys- I posted a week ago about a new Bosch alternator I bought that could not be installed with the original cooling shroud. I had the original Marchal unit rebuilt at a local reputable alternator/ starter rebuilding service. Got it back $100 later looking brand new, turning smoothly with all necessary parts replaced. Installed it today, and the Volt gauge is steady at about 8 Volts, even after a nice long charge on the battery. Is there a trick I'm missing, or should I take the alternator back to the shop?

SwayBar

With a fully charged battery, your gauge should show 12-ish volts, not eight, unless there is a short somewhere or the gauge is wrong.

Do you have a multimeter to check/verify actual voltages?

dr bob

VERIFY that you have excitation current flowing to the alternator. It normally flows through a resistor in parallel with the charge indicator bulb in the dash. Does the dash light come on with key on? If not, look through the flow path for that excitation, including the 14-pin connector in the engine bay and the actual connection at the alternator. There's more upstream that will take a hard look at the wiring diagrams, but the basic idea is that the 15 circuit from the key passes through the light-and-resistor pair on its way to the 14 pin and the alternator. Ring each section to verify circuit integrity. It's all pretty low current flow, so cleaning the engine bay connections would be a good start.

Mrmerlin

FWIW if you at any time had the alternator hanging from its wire harness,
then the chances are good that the blue wire has sheared about 4 inches up from the connector at the back of the alternator.

TO check this remove the blue wire connection at the alternator and see if you have continuity to the 14 pin connector at the hot post junction.

NOTE dont rely on the volt meter on the dash for anything other than trending,


Use a real Multi meter to measure voltage at the hot post and the battery terminals, standing voltage SB about 12.7

if this system is charging properly then you should see 13.5 to 14.2 at 1500 rpm
if it stays under 13.4 then the chances are good one of the the diodes has blown in the alternator.

NOTE alternator damage can occur when jumping the car with battery charger quick jump this can spike the electrical system,
it can also damage the engine computers.
also trying to start the car with a depleted battery can damage the relays as they wont get full power to close and thus will arc.

dr bob

Expanding on Stan's guidance:

Check for voltage at the exciter terminal at the alternator. With key on (position 2, normal engine run) but engine not running, you should see at least a few volts there. Go back to my question about the dash charge light too. With the broken exciter wire that Stan describes, the warning light won't be on.

Another thing... The alternators are "self-exciting" after initial light-off. So long as the alternator is spinning, once it sees enough excitation after it starts charging, it no longer requires external excitation. Used alternators typically have enough residual magnetism in the rotor to get going even without external excitation. Owners report that a quick rev to 4000 engine RPM is usually enough to get things started even without the external exciter connection. This is an easy test if everything is still assembled and connected.

78Silver928

dr bob-- THANK YOU. Just started the car after charging the battery overnight... was showing 11.5 ish Volts at idle, not enough. Revved the motor over 4000RPM, and suddenly the gauge jumped to 13.5 V and stayed there. Turned the headlights on, and it continued to stay above 13V. Problem solved.

Now onto the fuel sender and getting the fuel injection perfect.

dr bob

Silver --

The problem isn't solved, you just helped identify it. You need to actually fix the excitation issue that's blocking the current to the alternator so it lights off correctly. Work your way backwards from the alternator, through the front-of-engine harness, through the 14-pin connector in the engine bay, the CE panel, the instrument cluster, to the ignition switch.

Alan

And assuming your measurement tools are accurate - you really need to get a new battery. 11.5v just after starting on a freshly charged battery says there is a problem there (even with no alternator).

Alan

78Silver928

Update: I have to rev the car above 3500rpm to get the alternator to charge every time I start it.

I get no battery light in the cluster ever. Not even at start up. Could this bulb be the issue? I also cleaned the 14 pin connectors and noticed a very crusty single wire that looks green/ blue heading up the front of the motor on the passenger side... haven't traced it all the way down to the alt, but it has a disintegrated plastic connector on the end and some of the insulation is cracked/ missing on the wire. Could this be the exciter wire? Maybe I put the wire back on the wrong post of the alternator? I did replace the battery with a brand new one that still shows only 11V on the voltmeter gauge until I get the alternator to light off. I guess I need to start checking current with the multimeter at different locations?

Mrmerlin

Did you read post 4 ?
Do you have any comments?


google the year and make and model of your car and add 14 pin connector diagram

Alan

The bulb not illuminating is a problem. You should get illumination during bulb check regardless of the exciter connection - so yes investigate that (in the pod).

I'd also be checking the rest of the connections while you are in there (alternator exciter terminal to pod bulb gives you the full loop.

Did you get new battery? - or at least take it in for load testing?

Alan

dr bob

On the 1978 (and all years really) the exciter circuit for the alternator passes through Pin 1 of the 14-pin connector at the right fenderwall forward under the hood. The top section is the engine side, stationary/bottom section is the car side.

The current flow path through the gauge cluster is from the 15 bus (hot when key is in RUN or START positions. Flow is through CE Panel connector at H7 to the gauge cluster, through the bulb, then returns to H8. Current flows then through an internal CE panel jumper to O8, then on to pin 1 in the 14-pin connector in the engine bay. From the engine side of that connector, current flows to the D+ terminal on the alternator via the KS2 (Front of Engine) harness.

Diagnosing hints:

-- A digital voltmeter connected anywhere in the circuit should read battery voltage with the key on, while the alternator D+ is disconnected.

-- Grounding the circuit at H8, O8, Pin 1 of the 14-pin, or the wire normally connected to D+ at the alternator, should cause the dash warning bulb to illuminate with the key in RUN or START position.

-- With the key in RUN or START position and the alternator D+ disconnected OR the 14-pin connector apart, you should be able to read battery voltage at H7 and at H8 and at O8. If you see no voltage at H7, there's an internal CE panel fault from the 15 bus that may be related to the ignition switch. If you see voltage at H7 but not at H8, the bulb has failed or a connector in the gauge cluster is open.

-- With the key in RUN or START position and the alternator D+ disconnected OR the 14-pin connector apart, you should be able to read battery voltage at H7 and at H8 and at O8. If you see voltage at H8 and O8, test at the CAR side of pin 1 in the 14-pin connector in the engine bay. If no voltage is detected, there's a fault in the wiring between O8 in the CE panel and pin 1 in the 14 pin.

-- With the key in RUN or START position and the alternator D+ disconnected and voltage detected at the car side pin 1 of the 14-pin in the engine bay, reassemble the 14-pin connector and move to the alternator. Test for battery voltage at the wire that is disconnected from the D+ terminal. If not battery voltage, the excitation conductor from pin 1 in the front-of-engine harness has failed.

-- With the key in RUN or START position and the alternator D+ connected, the 14 pin connected, Test for battery voltage at the wire that is connected to the D+ terminal. You should see at least a few volts there. If not, there's a poor connection or conductor. If yes, your alternator will work immediately on engine start, or the excitation isolation diode in the alternator is faulty.

-----

For those following along at home, be aware that the CE panel terminal numbers are NOT always consistent year-to-year. Specifically, the central electrics terminal and connection numbers shared here apply ONLY to the 1978 cars. Consult the factory WSM current flow diagrams for your particular year when troubleshooting the charging system.