C3 Battery Guage Fluctuation

[Larry M.]

The battery gauge needle in our '72 has begun to hover on the minus side of the gauge scale during normal operation, instead of staying on the zero index mark in the top center.

Operating with the headlights on causes the gauge to read more negative, and directional signal blinking causes the needle to bounce a bit. This might be entertaining if it didn't mean I need to add another chore to my already lengthy to-do list.

The alternator is a non-correct rebuilt replacement over 20 years old. I replaced the brushes approximately 15,000 miles ago. The battery is a 5 year old restoration battery, corrrect for the car. The car is driven (other than 'not enough') about 1,000 miles per year.

Although the starter motor spins the flywheel just fine, I suspect that, in the absence of any action on my part, that situation will change at some time in the future.

My initial thought is that the alternator is somehow at fault (BTW, the alternator belt is not loose), but since the brushes have relatively low miles on them, I don't know what would be the most obvious alternator component to replace. I'm also wondering if the low usage on the battery has caused it's plates to build up corrosion or deposits which prevent it from accepting a full charge.

Does anyone have any experience with this situation, or suggestions as to what my first repair attempt should be?

Considering that headlights and directionals have the most evident impact, maybe I need to confine my driving to daylight hours and straight roads (or drag-racing).

Thanks in advance,
Larry Maher, # 10731

[Jack H.]

Re: C3 Battery Guage Fluctuation

Alternator's internals provide 3 pairs of diodes to convert AC to DC, probably to skirt Chrysler's patent on alternator back in '59 era where a 4-diode, full wave bridge rectifier circuit topology was claimed. The symptoms you report are typically the result of one of THREE problems:

(1) One 'leg' of the diode bridge is 'blown' causing internal leakage.

(2) In '69 and later cars, the voltage regulator was reduced to a solid state function and moved inside the alternator. If the voltage regulator is 'shot' it'll NOT tell the alt to output current and charge the battery....

(3) This one happens more often than you think and can cause novice troubleshooters to pull their hair! There's a side-tap plug on the alt bussing two wires to it. One of the wires is power for the internal regulator and the other is the internal regulator's 'sense' lead (what is charge state of battery--should I output current or not?). These wires can become brittle and fail to make solid contact. If the alternator's internal regulator doesn't get power or 'see' the battery's state of charge, she tain't gonna work right! When you pull the alternator to take it into a shop for bench testing, you unplug the side-terminal connector and a connection that wasn't good before can suddenly be flexed and start to make connection again (Jeckyl & Hyde effect).

[Larry M.]

Re: C3 Battery Guage Fluctuation

Jack -

Thanks for your response. Judging by what you told me, it sounds as if the alternator, and not the battery, is most likely at fault. I'll confirm the contact on the two-wire connector, and pick up a regulator and diode trio.

Thanks again,
Larry

[Duke W.]

Re: C3 Battery Guage Fluctuation

That bloody side connector has caused all kinds of problems on my Cosworth Vega. The second order unbalanced vertical shaking force really beats it to death.

I've had broken wires at the terminal junctions a couple of times. The most recent episode really threw me for a loop. The terminal connections were okay, but the 10 gage (red wire) terminal was loose on the spade. I kept pinching the terminal with pliers to tighten it up, but the problem kept reoccuring.

I finally decided to pull the alternator and replace the regulator as I figured there was some intermittent internal bug. When I removed the old regulator one of the spades looked funky. I measured the width with some calipers and realized that the spade was worn about .030", and this is why the terminal would not remain snug. The vibration had literally caused the spade to wear, and the new regulator with proper spade dimensions solved the problem.

Corvette V8s don't have any unbalanced shaking forces or rocking couples below fourth order, so they are a much more benign vibration environment than a high revving four cylinder (13g peak, 233 Hz at 7000 revs on the CV), but the side connector regulator spades and harness terminals should always be carefully checked.

You should check system voltage with an accurate voltmeter. If it's normal- 13.5 to 14.5 volts - then the problem could be the body connector that carries the ammeter circuit as a little bit of extra resistance will cause flaky ammeter readings.

Rebuilding a 10-SI Delcotron is easy. The parts are readily available and you don't have to remove the pulley to replace any of the electrical components, except the rotor. - just separate the frame. Remember to pull the paper clip out of the brushes before you reinstall it. I almost forgot that once. Fortunately I spotted it before I turned on the ignition, so I didn't find out what shorted brushes do, but I'm sure it's not pretty.

Duke

[Craig S.]

Re: C3 Battery Guage Fluctuation

Duke - I use a round wooden toothpick I break off in the middle, and insert the thick center part of the toothpick into the brush hole. That way, when I do forget, no electrical issues....Craig

[Terry M.]

Re: C3 Battery Guage Fluctuation *TL*

Larry - The automotive electrical storage (battery) and charging (generator - alternator) system is closely coupled and a failure or reduction in performance in one part of the system can affect another part. Almost any automotive shop (certainly one worth patronizing) will poses a charging system tester. Sun makes what they call VAT [volt, amp tester] that is almost the standard of the industry. The modern testers, even in the hands of an unskilled person, can quickly determine the condition of the alternator and battery in less than five minutes without removing anything from the car.
You have received wise advise from the other posters here, and I don't wish to diminish their advise, however a quick and simple series of tests will determine the state of your battery and function of your alternator.
The cost should be minimal, and may even be free if you talk cars with the technician. I can understand your reluctance to put your car in the hands of another, but you should be able to get the job done while you wait so the car will never be out of your sight.

[Duke W.]

Re: C3 Battery Guage Fluctuation

Thanks, excellent method. I'll use a toothpick instead of a paperclip
the next time I do an alternator job.

Duke

[Larry M.]

Re: C3 Battery Guage Fluctuation

Gents -

Thanks all, for your suggestions and advice. I'm a good parts replacer, not a diagnostician, so if and when the failed component is obvious and I can make the repair myself, I'll do so. I do have a good relationship with a local business, the owner of which is heavily into 60's and 70's musclecars because he knows and enjoys them so much. Guess it's time to take the car to him, or consider a correctly dated replacement.

Thanks again,
Larry

[Craig S.]

Re: C3 Battery Guage Fluctuation

Duke - anohter item that I was thinking of trying but haven't yet is the red tubes you get on brake cleaner and WD40 cans...I have a pile collecting. If they are stiff enough, they would be the right size. I am not sure if the spring force would cause a problem since they are hollow, but they seem pretty stout....worth a try...Craig