'63 ammeter showing discharge at 750 rpm idle speed

[G B.]

Problem(?): The amp gauge on my injected '63 shows a ~15 amp discharge at a 750 rpm idle speed. If I increase the idle speed to 900 rpm, it shows a slight charge rate. This is without any electrical loads other than the ignition. My alternator is a rebuilt "628" rated at 37 amps.

When I start the car cold, the charge rate is perhaps 30 amps for the first few seconds during an initial 2000 rpm fast idle. It quickly comes down to perhaps 5 - 10 amps while still on fast idle.

I checked the alternator diodes with a multi-meter. All six show one-way directional current flow with ~480 ohms resistance. The brushes look fine. I cleaned the contact surfaces. I replaced the good original voltage regulator with an NOS Made-In-America Delco regulator. That made no difference. My Delco battery is one year-old and holds a charge well for weeks.

Am I just looking for trouble where there is none? I thought the alternator should still show a slight charge rate at idle even with the headlights on.

[John P.]

Re: '63 ammeter showing discharge at 750 rpm idle speed

Hello and Merry Christmas to all, check the diameter of the alternator pulley. Several rebuilders install a larger diameter pulley causing the alternator to rotate slower. A smaller diameter pulley will rotate the alternator faster and possibly correct your problem. John

[Tim S.]

Re: '63 ammeter showing discharge at 750 rpm idle speed

Is the charging voltage where it should be? In other words, check the battery with a DVOM while running.

Tim

[Brian T.]

[G B.]

Re: '63 ammeter showing discharge at 750 rpm idle speed

Yes, I had the gauge checked recently and it's working correctly.

[G B.]

Re: '63 ammeter showing discharge at 750 rpm idle speed

The pulley is stock and original for this application.

[G B.]

Re: '63 ammeter showing discharge at 750 rpm idle speed

The battery is charging correctly above 900 rpm.

[Rich G.]

Re: '63 ammeter showing discharge at 750 rpm idle speed

I agree. I didn’t think an alternator based charging system could show a discharge unless the alternator/regulator is putting out less than 13 or so volts, which it can only do if it’s defective. In my alternator equipped airplane the only time the gauge goes negative is when I forget to turn the field switch on!

As above, measure the voltage at the battery at idle and higher. My 66 with the 37 Amp alternator reads over 14 volts at the battery at idle. A fully charged 12 volt lead acid battery should read 13.2 volts. Therefore the alternator has to put out more than that to make the amps go into the battery. Outside chance the bulkhead connector has a bad connection and the additional resistance is confusing the shunt/gauge. I’m not even sure that goes through there, but if it does it’s another place to look.

[G B.]

Re: '63 ammeter showing discharge at 750 rpm idle speed

My alternator is putting out less than 12 volts at 750 rpm, Rich. It does put out 12 volts plus above 900 rpm, however. I was trying to find out if other 37 amp alternators put out 12 plus as low as 750 rpm, and it sounds like yours does. Thanks.

[Duke W.]

Re: '63 ammeter showing discharge at 750 rpm idle speed

As you probably know the "ammeter" in a '63 is really a voltmeter that uses the slight difference in voltage at two points in the system as a surrogate for whether the battery is charging to discharging.

The real test is to use a voltmeter between the battery positive post or alternator output stud and ground. At idle with nothing on other than the ignition system it should measure the normal 13.5-14.5 volts, hot.

So the problem is likely either the alternator or regulator. Maybe it's time to disassemble the alternator and do all the other electrical checks including the stator and rotor coils. Also, it might be the VR adjustment. The procedure for this and the complete alternator overhaul procedure is in your 1963 Corvette Shop Manual.

Duke

[Thomas R.]

Re: '63 ammeter showing discharge at 750 rpm idle speed

If it turns out to be an alternator problem, I have a decent one #1100628 37A dated 3H9. Also have a 63/64 non A/C pulley #GM 3790356. Send a message if interested upon diagnosis.

[G B.]

Test Results?

Am I just looking for trouble where there is none?

Unfortunately, I'm still unsure. My test results follow.

Cliff's: Maybe there's a short in the stator windings.

Long version:

Alternator "BAT" terminal Voltage measurements: 12.6 with engine off. 14.6 during fast idle. 12.5 during 750 rpm idle with only ignition load. 11.6 at idle with headlights on.

Alternator "GND" terminal Voltage measurement: Zero at all times.

Diodes: As previously stated, all six diodes are one-directional when checked with an ohmmeter.

Rotor: Slip rings are not grounded or open or shorted when checked with an ohmmeter.

Stator: Winding leads are not grounded when checked with an ohmmeter. Windings are not open because leads checked in pairs show 2 ohms resistance. Ah yes... but the kicker from the shop manual: "A short in the windings is difficult to locate without special test equipment due to the low resistance of the windings. However, if all other electrical checks are normal and the generator [sic] fails to supply rated output, shorted windings are indicated."



12/25 Conclusion, sort of: I really doubt the windings are shorted since my low voltage output is only during idle speed. I'll now turn to the mechanic's solution of last resort, just start replacing crap. I've already tried a new regulator with no joy. I plan to buy a rebuilt 42 amp Delco alternator #334-2108 tomorrow for about $70. Unless someone comes on here and says "Hey, my '63 with a 37 amp alternator shows a discharge during idle, too", I'll report back in a few days with the result of an alternator replacement.

12/27 Conclusion, Final: Today I paid $74.96 for a rebuilt 55 amp Delco alternator #334-2110 at the local Delco warehouse. That price included 10% sales tax and a $5.50 core charge. It came with a small diameter, narrow-groove pulley and 8 pages of instructions including important safety warnings (MAY CAUSE CANCER IN CALIFORNIANS; DO NOT INGEST!). I replaced the pulley/fan/nut with my originals, removed a few Godless metric nuts, and counter-clocked the rear housing 90 degrees before starting the engine. Problem solved: I measured 14.6 volts during fast idle and 14.6 volts during a warm 750 rpm idle. Maybe someday I'll transfer the good internals of this alternator into my original "628" housing, but I kinda doubt it.

Thank you all for your input. I do appreciate the advice.

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Is nothing sacred?

[Danny P.]

Re: '63 ammeter showing discharge at 750 rpm idle speed

My Fuelie Idles at 850 rpm where it should be for a fuelie and charge rate is 14.3 , at 750 rpm of course it will charge less but if you want to set your idle at 750 rpm just adjust the voltage on the regulator at the proper voltage you like at 750 rpm

[G B.]

Re: '63 ammeter showing discharge at 750 rpm idle speed

Thanks for the suggestion, Danny. The "correct" idle speed for an injected '63 seems to depend on where you look.

The First Edition of the '63 owner's manual says 700 rpm on page 44 (photo below). The '63 GM shop manual doesn't mention it, but the '64 supplement manual says 850 rpm for the 1963 injected cars. The 1965 Form 1514C, FUEL INJECTION SPECIFICATIONS sheet says 800 - 900 rpm for 7375 units, but 700 - 800 rpm for the same engine in 1962 with a 7360 FI unit.

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