10-SI Alternator Not Working; Possibly Has Extra Resistor

[Mark E.]

Any alternator experts out there? I need help troubleshooting 10-SI alternator on '70 Corvette.

Problem: No current output when running.

Performed the on car "condition check" per '70 CSM 6Y-18 by grounding the field winding with a screwdriver. Alternator then worked (went from battery only voltage of 10.5v to over 13.5v with headlights on). With this result, CSM says to replace the voltage regulator.

Disassembled alternator, and performed "Voltage Regulator/Brush Lead Clip Check" per '70 Chassis Overhaul Manual 6Y-25. This checks if either brush lead clip is grounded. One lead failed with a low resistance (44 ohm). I then noticed this lead has a resistor attached which is not shown in the Service or Overhaul Manuals (see photos). It goes between brush lead with low ground resistance to a screw which grounds to the case. I removed the resistor, and both brush lead clips passed the test (no ground). The resistor I removed is 44 ohms, so that's what is causing the test to fail.

While at it, I also performed the "Diode Trio Check" (COM 6Y-24). One of the three diodes failed the resistance test.

Should the 44 ohm resistor be installed? The alternator worked for quite awhile with it installed, yet the Overhaul Manual does not show it, and the regulator fails its ground test with it installed.

Is the problem the voltage regulator? Extra resistor? Bad Diode Trio?

Slip Ring End of my 10-SI showing resistor not in chassis manuals:



Photo from COM 6Y-25 with no resistor shown or mentioned in text:

[Duke W.]

Re: 10-SI Alternator Not Working; Possibly Has Extra Resistor

My '76 Vega/Monza Supplement states: "The 10-SI generator is mostly carrryover from 1974. The only difference being that a 40-ohm resistor has been added to the warning indicator circuit. The purpose of this resistor is to provide a definite warning indicator light in the case of an open field circuit in the generator." You didn't say if you have the original alternator, but this resistor was apparently not added until '75 or '76 because it's not shown in the '74 Vega shop manual. So the 40-ohm resistor is probably not necessary unless you have a warning light in lieu of a voltmeter. Beyond that it sounds like your tests indicate that a new diode trio and regulator is required. As with ignition parts, likely all the alternator replacement parts are manufactured by Standard Motor Products, so the brand name, as long as it is a "name brand" on the box is irrelevant... just buy from the most convenient or inexpensive source. You

[Mark E.]

Re: 10-SI Alternator Not Working; Possibly Has Extra Resistor

Thanks Duke; very helpful. Given the obscure reference ('76 Vega Supplemental), I'm impressed you found this information.

This is a poor man's 1100884 alternator. I found the bare drive end and slip-ring end housings separately on eBay, had them cleaned, installed new bearings, then installed the "guts" (stator, rotor, regulator, brush holder, diodes, etc.) from another commercially rebuilt 10-SI.

It worked fine for awhile, then crapped out. Given what you said, I'll replace the regulator and diodes, and remove the 40 ohm resistor. Then we'll see if it works.

[Duke W.]

Re: 10-SI Alternator Not Working; Possibly Has Extra Resistor

I own a '76 Cosworth Vega, so the '76 V/M supplement is one of four GM manuals I have to cover every service on overhaul operation. I remembered because I had trouble with the 10-SI on the CV. For several years it suffered from an intermittent overcharge problem. If one of the side connectors doesn't make good contact it can overcharge and a similar problem on the other wire causes a no charge condition. One of the connectors seems loose, so I tried to tweak it, which didn't help, and the wire finally broke off the connector. Even a new connector didn't help so I finally pulled the alternator for a full overhaul. All the electrical checks were okay, and I just happened to notice that the regulator spades didn't seem rectangular. Since I couldn't test the regulator, I just bought a new one, which was not expensive. Upon comparing it was clear that regular spades were worn along the sides, and that's why the connector didn't have a good grip. So I replaced the brushes, relubed the bearings, assembled with the new regulator, and it's been fine ever since. The problem was likely caused by the four-cylinder's second order unbalanced vertical shaking force and 5000 miles between 5 and 7K revs at Riverside and Willow Springs. At 7K the vibration is about 15g at 233 Hz. Every piece of sheet metal on that engine broke at least once at one time or another including the oil pickup tube and support. Be sure you do all the electrical checks including the rectifier bridge. Being as how you built it not long ago the brushes probably have plenty of life left.

[Mark E.]

Re: 10-SI Alternator Not Working; Possibly Has Extra Resistor

"15g at 233Hz"... What does 15g mean?

[Mark E.]

Re: 10-SI Alternator Not Working; Possibly Has Extra Resistor

My alternator may have the Mozzetta curse.

Since my last update:

- Retested the diodes and rectifier per the COM (simple ohmmeter test). Replaced the diodes; rectifier tested ok.

- Replaced the regulator. No real test for it in the manuals, other than shorting the field while running. This test suggested the regulator was bad.

- Removed the 40 ohm resistor per Duke's post above.

Note: All original internals were transplanted from another working, newly rebuilt, alternator. The only parts not used from the doner alternator are the case, bearings, fan, and pulley.

- Assembled and installed alternator.

Here's what happened: After starting the engine, the "amp" gauge immediately showed about 20 "amps". 20-30 seconds later I saw the gauge suddenly dip to slight discharge and the lights dim a bit. When I climbed out of the car, I saw my attached VOM reading about 11.2 volts... meaning no alternator output. As a test, I then grounded the field with a screwdriver, and the voltage climbed to over 13.5 volts. As stated before, the CSM suggests this means the regulator is bad.

This is the same outcome as the time before, except failure was within seconds instead of minutes.

Any insights about the root cause?

[Duke W.]

Re: 10-SI Alternator Not Working; Possibly Has Extra Resistor

"g" is the force of gravity, so 15g is fifteen times the force of gravity. I'm stumped on you 10-SI problem, but I'm thinking some kind of internal short. Two of the three screws that attach the regulator/brush block to the frame have insulating washers. Your COM should show them as shown in my '74 Vega manual. I recall these appeared to be nylon and might be hat shaped. Make sure those insulating washers are installed and not damaged.


Inherent nature of the beast with a four cylinder unless you have a balancing shaft.

[Mark E.]

Re: 10-SI Alternator Not Working; Possibly Has Extra Resistor

"g" is the force of gravity, so 15g is fifteen times the force of gravity. I'm stumped on you 10-SI problem, but I'm thinking some kind of internal short. Two of the three screws that attach the regulator/brush block to the frame have insulating washers. Your COM should show them as shown in my '74 Vega manual. I recall these appeared to be nylon and might be hat shaped. Make sure those insulating washers are installed and not damaged.

Inherent nature of the beast with a four cylinder unless you have a balancing shaft.

Well Duke, I have a modicum of satisfaction the problem has you stumped. I'm not an alternator expert, but at least I'm not asking a beginner question. I learned about the insulated screws and how to test them during the first round, so they were ok during assembly. During the next tear down, I'll go through all the tests again to see if anything changed.

The manual says to replace the regulator with this failure. But after two regulator failures, something else may be causing the regulator to fail.

[Duke W.]

Re: 10-SI Alternator Not Working; Possibly Has Extra Resistor

Inherent nature of the beast with a four cylinder unless you have a balancing shaft.

Actually, taming the second order unbalanced vertical shaking force of an inline four requires two counter-rotating balance shafts running at twice engine speed. They were invented and patented in the early 20th century by an Englishman named Lanchester.

Somehow Mitsubishi was able to repatent the idea in the seventies... I never understand how. GM hates paying licensing fees, so the Quad 4 , which was otherwise a good engine, developed a reputation for roughness. Porsche paid the licensing fees for the 2.5L four in the 944.

Nowadays, the typical 2.3-2.5 liter inline fours that are base equipment in small to mid-size sedans and small crossovers have the balance shafts. They have a typical "four-cylinder beat" at idle due to only two power strokes per revolution, but above idle they are as smooth as an inline six or cruciform crank V-8, all of which have balanced first and second order shaking forces and rocking couples.

Duke

[Gene M.]

Re: 10-SI Alternator Not Working; Possibly Has Extra Resistor

Not being an electronic wizard why not buy a good NEW alternator from Reliable supplier such as DB ELECTRONICS. They have upgraded out puts too. Take the guts of the NEW alternator and put everything in the original case. This would eliminate all the guessing of trying to detect the faulty part or parts of your unit. After all who cares what's wrong with the old parts when you end up with a functioning alternator with the correct case. Fix it once and for all.

[Mark E.]

Re: 10-SI Alternator Not Working; Possibly Has Extra Resistor

Not being an electronic wizard why not buy a good NEW alternator from Reliable supplier such as DB ELECTRONICS. They have upgraded out puts too. Take the guts of the NEW alternator and put everything in the original case. This would eliminate all the guessing of trying to detect the faulty part or parts of your unit. After all who cares what's wrong with the old parts when you end up with a functioning alternator with the correct case. Fix it once and for all.

That was the original plan, except I used a rebuilt donor alternator. My next step is to buy and install ANOTHER alternator and see what happens. If it works, then I know the problem is not external to the alternator.

The installed 1100884 with guts from a rebuilt, donor alternator charges with the field grounded from the test hole. So this leads me to believe the stator and rotor are ok. That leaves the internal connections, rectifier, diodes, regulator, and opens/shorts.

[Mark E.]

Re: 10-SI Alternator Not Working; Possibly Has Extra Resistor

I installed another complete alternator as a test if there are any problems beyond the alternator itself. It works fine.

As a next step, I'm considering using the guts from this known good one and installing them into the 1100884 case.