Slow battery discharge on 67

[Bob Mitchell]

My 67 is experiencing a slow battery drain (from full charge to zero) over a period of 10 to 14 days. Its a real shame I can't drive it every day because of job duties and much travel. Does anyone have any ideas? To the best of my knowledge the alt. bat. and regulator are ok. Two weeks ago I seperated the bulk head connectors for an inspection and did not notice any thing unusual. Any help will be much appreciated.

Thank You for any help you may provide. Bob

PS All swithches are off and the clock appears to be the only draw.

[motorman]

Re: Slow battery discharge on 67

disconnect your ground cable from the battery,put a multi amp meter between the cable and the battery. see if you have a draw. pull fuses till the draw goes away.that will be your problem. if fuses do not do it take the wires off of the voltage regulator

[Joe Fisher]

Re: Slow battery discharge on 67

Bob, battery drain problems can be a real pain to detect. First you have to determine if it is a drain, a bad battery not holding a charge or a poor charging system. The battery can be eliminated by disconnecting it for the 10 days, if the battery is dead, well you have a bad battery. The charging system should be checked, a simple voltmeter will do. With a fully charge battery, at 1,200 to 1,500 RPM, with accessories on, the battery voltage should be between 13.8 an 14.5 volts. If it is a drain, you have to determine what circuit it is in. If it is a large enough drain, you can remove a battery cable, and put a test light in series between the cable and the battery. If it lights, start removing one fuse at a time, but be sure to depress the door button for the interior lights. If the light goes out when you remove a fuse, use your wiring diagram to trace it down. If no fuse turns the light out, the drain could be in the alternator, disconnect the two wire connector to it, if the drain goes away, you could have a leaky diode. Most people disconnect at the regulator, but if the light goes out it is the alternator feeding back to the regulator. If all this shows nothing, I will give more suggestions. Joe

[Jack H.]

A few more words....

Advice given by others is EXCELLENT, especially the battery integrity test (internal discharge rate)....

On the fuse advice to troubleshoot things, advice given is nice but NOT sufficient to guarantee sucess. Why?

Well, what you're looking for is a load that's active when the ignition is OFF and that points to unfused/un-switched circuits AS WELL AS those branch circuits that are fused/switched....

Example 1: the clock runs with the ignition off as well as certain interior lights (glove box is one that's hard to tell if it's really turning off because you can't see behind a closed door). These branches/loads ARE fused and the advice given will help you fault isolate, but you also need to know how much current is normal and it's timing (mechanical clock only draws current when points close and its wind-up solenoid fires).

Example 2: certain engine compartment branches/loads are NOT fused (depends on year and circuit topology) so simply pulling fuses won't get you to all the critical areas. Say, one of the diodes in the alternator's 3-phase bridge has seen electrical overstress and suffered junction puncture. It's entirely possible for it to be reverse bias leaking and causing the discharge condition AND for the system to charge good enough during normal operation to 'appear' correct/normal (depends on how 'fried' the diode/diodes is/are)....

So, have fun in the hunt, but always be on the lookout for things that aren't obvious and/or result from a prior owner's 'tinkering' or a 'wierd' fault condition. This is part of the 'fun' of having a vintage car!

[Doug Flaten]

Re: I had a similar problem on my 61

When I bought it, it had an alternator. The slow drain took out several batteries and lifetime warranty alternators and regulators. I could not find the short. When the stores said they no longer would honor the lifetime warranty on the car since I was getting the best of them, I broke down and bought a new harness and re-wired the car. I went back to original generator setup and have not had any problems. I think Jack is right, locating some of those shorts are tough.

[Bob Mitchell]

Re: Jack, Joe & Motorman - test info.

Some test results: 1) Light in series with battery produced no light. Maybe a test for a real small current would be better? 2) The battery is new (doesn't mean its good ie internal short) however produces good voltage. 3) Checked the alternator output with a cheap idiot light tester. This indicated 13.0V @ 2000rpm (poor) with no accessories running. Also after this test while at an idle (700rpm) I turned on the lights, this indicated a significant discharge rate on the amp gauge as did running the headlamp motors up and down. It took running the engine to plus 2000rpm to overcome the amp discharge to zero. Item # 3 seems to indicate potential alternator problems. If so, could the diodes be causing this condition? Should I do some more testing before replacing or should I look into other areas?

[Bob Mitchell]

Re: Slow battery discharge on 67

Posted additional info under Jack Humphrey post (A few more words..) Thanks

[Bob Mitchell]

Re: Slow battery discharge on 67

Posted some additional info under Jack Humphrey post (A few more words..) Thanks

[Jack H.]

Re: Jack, Joe & Motorman - test info.

13V @ 2000 RPM (measured where/how?) seems suspicious. But, remember it's the voltage regulator that 'senses' the battery state and tells the alternator (or generator) to turn ON, pump current, and drive the battery back to full charge condition....

If the voltage regulator (early electro-mechanical jobbies like 515 were adjustable) is out of alignment and/or it's 'sense' winding is 'seeing' the battery at/near 'normal' state of charge (battery can be freshly charged and/or over-charged, plus can be higher than normal 'load' in series with battery like flakey, high resistance, battery cut-off switch whose IR drop is pushing battery voltage above or below ground depending on which side the switch was mounted, same goes for voltage regulator 'sense' lead wiring integrity) then the charging system could be fooled into doing what it's doing....

On the other hand, if your voltmeter is RIGHT ON the battery and you've JUST cranked the engine, it gave up a significant degree of stored charge and the voltage regulator/alternator OUGHT to spring to life, run to their rails (+14V range) and restore the spent energy. This will evaporate pretty quickly depending on how new/fresh the battery is. BUT, for a few seconds at least, da sucka ought to be SIGNIFICANTLY above its 12-13 VDC normal state.

Since Chrysler patented the alternator and began shipping in 1960 (think I'm right here), they put their foot down on the lowest cost/simplest circuit configuration (4-diode, full wave bridge setup). Others seeking not to license made their designs more complex and you'll find the GM version of the alternator (Delcotron) is a 3-phase system using six diodes. So, it's possible for one or two stages of the internal rectification system to be 'dead' (dysfunctional and/or shorted) causing the output response to the voltage regulator's call for charging current to be marginal....

Your shop manual gives a step-by-step debug proceedure for finding bad diodes. But, you can pull both the voltage regulator and the alternator, take 'em to a competent auto parts store and have them tested (BOTH have to work properly for the system to function). Some testers are simple idiot boxes (does it spit out a reasonable level of charging current?) while others are more rigorous (What does the charging waveform look like on an oscilloscope? What happens as a function of alternator RPM, the voltage regulator's call for current across it's rated dynamic range, and what does the system do after it internally heats and runs for 10-20 minutes?). It's possible to have 'flakey' components that only malfunction at 'just right' conditions....

[Jack H.]

Re: I had a similar problem on my 61

That's one of my pet peaves about the level of documentation we get from Chevy on these cars. The electrical stuff is NOT schematic diagrams! It's called wiring diagram....

A schematic shows any/all electrical branch and node detail. It lets you devise test(s) to troubleshoot and fault isolate.

A wiring diagram simply shows functional block level detail and defines how wires route to inter-connect the blocks. This presumes one is going to effect repair by component substitution and has access to known good functional components and/or correct original wiring that's intact.

The fallacy is, these cars have typically passed through n-tuple prior owner hands before we get them, known good original parts are hard to come by and usually quite pricey, plus do we really want to risk 'smoking' an NOS this/that only to find there's been hanky panky inside the wiring path?

Each time I contact Chevy to find where I can get my hands on REAL SCHEMATIC DIAGRAMS, I get told that what's in the shop manual/chassis service manual is all there is. BULL!!!!

Places like engine electrical, body electrical, and service research at GM had a need to know and this documentation HAS to have been available (to someone, somewhere, sometime).... Plus, things get worse as you enter the early Shark era where diode-transistor-logic (DTL) circuitry got packaged inside what looks like ordinary electro-mechanical relay packaged. Without knowing what's inside a given 'black box', it's simply impossible to devise an adequate necessary/sufficient functional test to debug a malfunctioning classic car!

[Bob Mitchell]

Re: Jack, Joe & Motorman - test info.

Jack, Last night I charged the battery because it was at a zero state. Figured I needed some place to start. Prior to starting the ole 67 I rolled open the headlight doors and turned on the headlights for a few seconds (15) turned them off and then started the vehicle. It took a few cranks since it had been setting for about 14 days. During the warm-up (high idle) I checked the alternator output at the battery with the el chepo tester mentioned in the previous post (an idiot box with 3 lights, 1 at 13V, 1 at 13.8, and 1 at 14.4V?) no lights lit on the tester. I then ran the rpm's up to 2000 get the 1st light on the tester to light indicating 13V. Also as a note the regulator is not a 515, it is an aftermarket replacement (no adj.) If I read your post right I believe I should have these areas fully tested with some high quality equipment to full analyze the charging components. Thanks Again, Bob

[Jack H.]

Re: Jack, Joe & Motorman - test info.

Something's rotten in Denmark (no pun intended for the Bard). Even if you had fully charged your fresh/new battery, the EXERCISE you gave it of rolling out the headlights and illuminating them drew a reasonable portion of its current store....

THEN, you cranked, and you mentioned having to 'gun' it quite a bit -- not tap, Varoom. There's no way in heck, in my humble opinion, a properly functioning charging system shouldn't have seen a low battery condition and, at least for a REASONABLE period, say 10-15 seconds, have pegged the charging system's rails (read this as seeing the battery 'pumped' up to 14-16 Volts) trying to restore the battery's spent energy.

The first thing I'd do (you probably don't have my oscilloscope and other test equipment) is pull BOTH the voltage regulator and the alternator off the car, take them to your local auto parts shop (forget whether or not they have a go/no-go or fully functional dynamic tester) and get 'em checked out together and individually. I smell 'odor' floating South from Denmark based on what you say and it seems to point to the charging system.

PS, if alternator is correct/numbered item, remember it CAN be rebuilt and preserved vs. swapped as a trade-in core. You already said the VR was not a Delco-Remy unit (PS same as any '67 passenger car you'll find at the scrap yard if you want to whistle and stroll) and didn't indicate the configuration of your car was such to merit a rare solid-state DR regulator.... So, the alternatives abound depending on what due diligence in the test cycle tells you. Peace be with you and anyway we can help, we're here and ready to roll up our sleeves.

PS, this is how I failed my first Performance Verification on my '65 396.... It had 'correct/original' dated diodes and I let 'er sit on the Judging Field at an NCRS National for a week. One was flakey, had suffered junction puncture, and was reverse bias leaking.

When it came time to fire the beast up -- NOTHING! Later, I'd pull the alt, take 'er into my laboratory, don a black mask, and run the diodes on my curve tracer one at a time. Yep, one (negative in this case) was FLAKEY.

Acted like a diode at certain voltages, and looked like a 'sieve' at lower voltages. After a year of walking scarp yards, I found a perfect donor alternator (same diodes used in menial Chevy passenger cars alternators as in exotic L-78/396 solid state ignition '696' 42A alternator). Vendor and date code agreed ON THE MONEY with the bridge in my alt.

Took 'er home, pressed out a good diode, replace the baddie in my rare alt, and next time around -- NO PROBLEMA! But, there aren't many EE's out there as anal as I to walk the muck of scrap yards and hand pick dated diodes from junk alternators. Bottom line, if she be discharging inside your rectifier bridge, the alternator is connected across the battery ALL THE TIME, it'll present a discharging load on the battery, AND your ammeter will NOT show a discharge condition....

[Mike Andresen (27410)]

Re: Slow battery discharge on 67

Bob, While you're diagnosing what to do on this, why not install a battery disconnect switch. They are cheap ($10 to $15 as I recall) and do the trick to keep the battery from discharging while you are off on business. Saves you the hassle of recharging the battery each time. Heck they provide crude, cheap theft protection, protection against fire AND don't even merit a points deduction at NCRS. You can't go wrong on this. They are advertised in the Corvette Restorer each time or you'll find them with most of the vendors.

Cheers, Mike

[motorman]

Re: Jack, Joe & Motorman - test info.

my test for alt. has been at higher rpm they should put out 14.5 volts, if they do they are ok, if not something is wrong.IMHO. all good alt i ever tested have put out 14.5 volts reved up in the car.