I am using a dry cell battery and a Classic Instruments voltmeter. The gauge is definitely faulty and reading 1v to 1.5v lower when checked against a multimeter. My auto electrician says the gauge does not cause other problems. With the multimeter I check the battery and get around 12.8 volts. On initial start I then checked on the multimeter and it was charging at 14.2 which is OK also. Turn on lights and air con and charge drops to 13 or so volts. When driving with air con & fans on the gauge drops to 11 volts and you can even see it flicker up/down with the turn signals operating. After a run last week with air/fans on I got home,parked and then shortly after needed to move the car and battery flat!! So I guess it was not charging sufficiently and was running on the battery some of the way and not enought charge to keep it up to 13v. With it charging correctly on the multimeter when first started the alternator must be kicking in OK but then or some reason the charge drops off. Could the battery be causing this problem and not accepting charge or something faulty in the wiring/connections. I do recall some months ago working under the car and tightening something and accidentally hit the power wire at the same time the screwdriver was against the chassis and shorted out. Got a good spark off that but not sure if that would damage anything internally. As said earlier still charges correctly at least at start up but something happens paticularly when air/lights/fans turned on.
You could have blown a diode when you did that sparky thing but as the alternator seems to be charging right on the desired 14.2 volts and you have 12.8 sitting there with a fully charged battery I am wondering if your alternator just flat doesn't put out enough amps to keep up with the demands of all the stuff you have on the rig. That is unless it kept up in the past and this is a new thing. The diodes are easy to check if you pull it off and pop it apart and you could check the brushes at the same time. Or pull it off and haul it down to a parts house or auto electric shop that can put it on test bench and check it.
Sounds like the alternator is only charging half of its output. You may have blown a diode when it shorted, it can not keep up when a load is put on it. I would have the alternator tested.
Something is obviously wrong with your charging circuit. Have you checked the voltage regulator? It is normal for battery voltage to drop as current draw increases, but not down below 13V. Have you checked all your grounds? A bad ground somethings is the culprit or even just loose battery terminal fittings.
The dead short could damage a diode, but you really should get the battery tested first. Testing the charging system doesn't mean anything if the battery is faulty. If it is okay, you need to check both the voltage and current output of the alternator. With the engine running at about 1200 to 1500 rpm and all accessories on, you should see around 12.8 volts or more at the battery and within an amp or two of the rated output of the alternator on the alternator output wire. An inductive clamp on the alternator output wire is the easiest way to test. Typically alternators should be sized to carry all accessory current with 5 amps left to keep the battery charged.
Jaracer is partly correct. You have to understand that the battery is the heart of the charging system and needs to be able to carry the load of the electrical system, the alternators job is simply to keep the battery healthy and if the battery is fully charged it will then run the system. At full load, cooling fan, ac and whatever else you have going, the alternator and battery are working as hard as they can, you need a battery with some built in reserve to allow the alternator to keep up, I'd load test the battery and if you can put a bigger one in.
Trying not to sound like Capt. Obvious here , but are you SURE the belt is tight and not slipping ? At one of the shops I worked at we called fan belts " rubber regulators" LOL
You can go to pretty much any parts store, Advance Auto for one, and they will check it for you. They can test the battery, alternator and starter in a few minutes. Might even charge the battery for you. It sounds like a diode in the alternator, by the way. But the tester will know and you will not be buying parts to replace perfectly good parts. Sent from my SGP561 using Tapatalk
I am sure the belt is OK. Not a V belt but running a pulley system with wide belt and plenty of wrap on the alternator. Will get the auto electrician to check the overall condition of the battery and CCA. I hadn't mentiond the screwdriver eposode to him as that only just resurfaced from my 71 year old memory. I think it is a slow drop off of the charging capacity after everything heats up and the alternator just can't handle what it needs to do. All this stuff is hidden down low in the Hudson and time consuming to get to. Thanks for all the input.
No. The battery's job is to start the engine. That's it. (It does act as a filter or "flywheel" to the charging system, smoothing spikes and sags) It is the alternator's job to provide all of the power for ignition, lights and heat/cooling and accessories. When the OP mentioned he's reading 11.x volts cruising down the highway with the AC on, the alternator is not providing enough juice, and the battery will be discharged. There's yer problem. Anything less than about 12.6 volts, I'd expect a battery is getting smoked hard. Recall it takes about 2 volts above and beyond 12.6 to reach a full charge, and that's at 77° F.
Might not cause anything, but it may be a symptom. When checking the volt gauge against the multimeter, are you measuring at the same points as the where the volt gauge is attached? Or are you measuring just at the battery with the multimeter? Need to measure at the same points to verify if the volt gauge is reading correctly. The gauge may just be demonstrating the voltage drop, not an error of the gauge itself. I would prefer to see 13.5V on a system supporting several large electrical loads. You can get away with 13V on basic systems(non a/c, no electric fans, pumps, etc) If this is happening after the car has warmed up, it is most likely the alternator is getting heat soaked. Or is simply not up to the ampacity needed. Too small of an Alt will not keep up. External Regulator alternators don't have the low speed charging capabilities of an Internal Regulator alternator. Even a lowly GM 10SI 63A IR Alt will spit out 35A at idle and peak amperage off idle. A similar 61A ER Alt will be barely spitting out 10A at idle and peak amperage is at 2500rpm. There may also be a current draw when the car is off. Remove the negative battery cable, place the multimeter in series of the cable and battery, turn it to AMPs and see if there is a load being drawn. After a vehicle is started the alternator will immediately go to a higher charging stage as the draw from starting the engine can suck up quite a bit of juice. Usually takes about 10-15mins of driving around just to replenish the battery from starting. So the voltage will be high initially, as the battery is being charged the voltage will taper down to normal running voltage. This will change depending on additional loads. Yes, the only way to know would to load test the battery itself. Most places have meters that can read the internal resistance of the battery and simply call it good. These are mostly reliable, but nothing beats a good, old style, carbon pile tester to stress test a battery. High resistance connections(loose, painted, dirty, incorrectly spliced, undersized) will cause voltage drop and power issues. Not a bad idea to go over the basics. Bad grounds and bonding connections are a big issue. Older External Regulator vehicles that have charge issues always have horrid connections at the regulator, or the regulator itself is not even bolted(bonded) down properly. If it was simply a main power feed to the frame, you probably didn't hurt anything. If the alternator was damaged it would be noticeable. A bad diode/rectifier would be fairly noticeable, you would get a bit of pulsing from the headlights/dashlights from seeing the AC affecting the system. Those are heavy load items, if the alternator itself was not originally used in a vehicle that had those same loads, it will most likely struggle to recharge, let alone maintain, the battery, and is simply undersized. What alternator(model/amperage)? Charge wire(from back of alt) size? Electric fans alone would demand quite a bit of juice, an alternator rated ~100Amps minimum would be needed just to maintain a balanced electrical system. Heat soaked alternators will reduce output due to resistance caused by the heat. Alternators cool by bringing air from the back and blows the air out of the front fan. If the back of the alternator is up against the block or a header, it may just be overheating itself. If it is a GM 3wire unit, it needs to be properly wired up as a 3 wire. A 1wire setup is poor for anything beyond a tractor. Also the 'sense' wire from a GM 3 wire unit needs to have the sense wire attached to the main power splice/buss. Attaching that directly to the Alt BAT terminal or battery will be incorrect, it needs to detect actual system load.
Air conditioning? In February? Wow, you must be on the other side of the world! yes, Mike described the situation very well. What alternator are you using?
When you checked the gauge did you apply the multimeter at the same place the gauge is hooked up? The gauge may be right and you have a voltags drop if not checked in the same place.
My avatar car was suffering from basically the same problem. With a known good, fully charged battery and with the headlights on bright and two electric fans running the volt meter would drop to around 12V and fluctuate between 11 and 12V when I turned a turn signal on. I upgraded to a 150 amp alternator and doubled the output wire size. Problem solved.........
Then it should be able to handle the load, if it's in good condition. What size wire did you use to connect the alternator, battery, and main feeds that power the fans, etc?
I have a car that was doing all kinds of crazy electrical stuff. Previous owner put a new high amp alternator and battery, did all kinds of tests and stuff but it didn’t help much. I bought the car and immediately pulled on the positive battery terminal and it pulled loose in my hand. Fixed it and problem solved. Just sayin...
Customer had a flathead with a gm alternator, I could drive it with no problems, when the guy took it back to the city he had problems. The flatty had a 4" crank pulley. That was the problem.
BTT. Well maybe some problems solved. Got into the back of the instruments and found the earth wire to all the instruments that terminated at the back of the tacho was loose. Got that tightened up and the voltmeter is now reading more correctly at 13.5V. The gauge does still drop away a bit when lights and air are on so maybe still an alternator issue to be checked. Checking at the battery with the multimeter gives me a good reading on charge with a drop if lights and air on. The earth was shared by all the gauges with it hooked up from speedo across all of them and the final that is the ground to the car was on the bracket holding the tach in. Now wondering if the loose earth was having any effect on the other gauges. Fuel gauge would tend to fluctuate a bit and I thought that was from the sender. Oil and temp always seemed OK. While I was at it I pulled the Speedo out as the odometer has not worked forever. Still reads 550km and then stopped. Apparently this is a problem with these Classic speeedos. They are electronic but use a gear system from needle to run the odometer and sometimes the gears get stuck. Should have that back next week. So a lot happier at the moment.
thanks for the report. The common ground, that is not really ground, will cause all kinds of fun issues as each gauge gets a "funky" voltage for ground, from all the other gauges. It's a constantly changing resistance network, and the results can be quite unpredictable.
Bet you a buck Squirrel answered your question in post #18, regarding wire size. ALL of the wiring harness kits I have ever purchased have a small charge wire from alternator to battery (#8 and even #10). With fans, water pumps, headlights, AC if you have it, and everything on, there's no way the alternator can keep battery charged. Run a heavy wire jumper cable directly from the single wire alternator to your battery, turn everything on, and see if you're not 14.2 Volts at idle vs 13.5.
#10 is good for continuous use at 30 amps, and for shorter duration at higher current. It's common to see #10 wire used on older cars that had 55 or 63 amp alternators, but they've moved up to larger wire sizes on newer cars and aftermarket systems.
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