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What is the correct charging voltage?

 
Old 11-23-2010, 09:07 AM
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tlsmith1999
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Default What is the correct charging voltage?

My 79 is showing 12.5 volts at the jump post with the car running. Is this correct, or a little low? It was showing 11.5v, but then I cleaned the engine ground, starter connections, jump post wires, 14 pin connector, and alt connections. The battery is new, and I cleaned the ground strap and terminal wires at that time.

I am showing 11v at the cig lighter, and the headlights are a little slow going up, so it seems to me that it needs more work. I will be R&Ring the CE panel this week, and have already done the pod and grounds behind the pod.

Thanks,
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Old 11-23-2010, 09:34 AM
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LT Texan
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That is low. Time to clean all the grounds. And check your alternator output. What is the voltage reading with the headlights, aircon (fan on high) and everything else on? Is it below 12.5v?

I'd look for a range more around 13 - 13.5v
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Old 11-23-2010, 09:37 AM
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12.5v was with no AC, or lights on and measuring at the jump post.
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Old 11-23-2010, 10:00 AM
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Yep, that is too low. You'll want to start by cleaning all the grounds. Some of them are a PITA but you need to do them. Clean off the ends with emery cloth til shiny. The apply a contact stabilizer like Stabilant or DeOxit Gold. Once put back together, slather on some dielectric grease to keep it isolated from crud.
This is one of the jobs almost no one does but everyone should.
Also, clean your 14 pin connector under the jump post cover. Be careful taking this apart, as the connector can be brittle. Clean your battery terminals, nice and shiny, the apply DeOxit Gold to them as well.

This really does make a huge difference. Goes a long way to solving electrical gremlins. Also a good idea to take your fuses out (in out 3 times) and make sure they are all seated well. Process of taking them out and putting them back in can help prevent gremlins. Do the same with your relays. The relays can be a bugger to get out so be careful with them.

Disconnect negative ground strap before you do all this cause you'll probably not want to end up shorting across something you shouldn't.
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Old 11-23-2010, 10:12 AM
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Check the alternator belt - it needs to be tight. And, since it is a V-belt, make sure that the belt doesn't touch the bottom of the pulley groove, only the sides.
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Old 11-23-2010, 10:15 AM
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Tim--

The voltage rating is usually in the 13.5-14 Volt range, depending on which alternator you have. The rating is cold, measured at the alternator terminal and the alternator frame.

Every resistance between the alternator and your load will reduce the measured voltage there. Possible resistances include the ground path from the engine to the chassis, and every ground connection between a load and the chassis.

On the positive terminal side, the cable between the alternator and the positive terminal on the fenderwall is often a problem. For instance, look at the short stub of the positive cable where it connects there by the jumpstart terminal. Many cars have cracked or missing insulation on that cable where it's exposed out of the front-of-engine harness sheath, and subsequent corrosion back into the cable itself thanks to the new access for water and other corrossives like salt or car wash soap. From that same jump-start point, wires run back to the CE panel. Those are subject to the same damage, but are hidden as they pass back under the edge of the fender.


It's actually pretty simple to find where you have significant volatage drop in the system. Your digital voltmeter and a couple long test leads will allow you to test every path and segment for voltage drop under load. Just put a lead at each end of a cable section, set your meter to read volts, start the engine and turn on your load. Might be headlights if you are checking the path to the CE panel, for instance. Remember that you can check ground paths the same way.


The voltage regulator on the alternator is designed to reduce charging voltage when the alternator gets hot. I can see half a volt or 'adjustment' in my Bosch alternator between first cold temperature and normal 'warmed up' temps. It gets worse in the summer with high ambients and high alternator loads, getting down to just a hair over 13 Volts with full fan and AC load with headlights on. Porsche recognised that the high temps woould be a problem, adding the cooling hose and shroud to the alternator so it wouldn't have to swallow hot air from the back of the radiator. If your hose and shroud are in less than great condition, consider getting replacements. The hose is a particularly vulnerable piece, since it seems to get oil-soaked from any power steering or oil leaks there at the front of the engine bay. I think Roger has an alternative hose that's a fraction of the cost of the orginal if yours is tired or torn. Use clamps, and route the hose correctly so it gets cool, relatively clean air from the area under the headllight. The cooling shroud on the alternator needs to be intact too. If yours is missing or cracked, replacements are available for not a lot of dollars. There's a foam gasket/seal between the alternator housing and the shroud that needs to be renewed. Many early cars have had the original Paris-Rhone alternator replaced with a later Bosch, and the shrouds are different. If your alternator is newer and needs a shroud, it's an easy task to fit a newer shroud. I know that they are available new or used from 928 International for reasonable costs.

Last but not least, alternators do get tired with age and use. There are brushes that ride on copper slip rings to carry excitation current from the voltage regulator to the armature. The carbon brushes wear, and may reduce the capacity if the wear is significant. A replacement regulator assembly includes new brushes, an can be chaged without disassembling the alternator. The copper slip rings wear too, but they can't be easily repaired by mere mortals; a rebuilt alternator is usually the solution. I know that 928 Intrnational carries great rebuilt units. You may find a local place that will 'rebuild' yours, but unless they replace the copper slip rings you'll find the total result less than perfect after a short while. If you decide that a new higher-capacity Bosch alternator is a good idea, note that there's a resistor in the instrument cluster that needs to be changed to get the new alternator to start charging on car start.
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Old 11-23-2010, 03:27 PM
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One thing I failed to include in the 'possible-causes' list is a worn-out battery. Batteries deteriorate from the day they are manufactured, so a battery that's older will need more current to keep it charged than a new one. You can try disconnecting the battery and charging it with a regular charger, and see how well it catches up. Be sure to check the fluid level and top it up as necessary prior to and following the charging session. Best done out of the car if at all possible.

After charging for a while (time depends on charge rate...), check the specific gravity of the fluid with a hydrometer, available at most parts places for less than $10. If charging it does not restore the correct gravity in the electrolyte, you know for sure that the battery needs to be replaced.


A tired battery can suck up a bunch of the available charging current from the alternator, causing the system voltage to drop. Think about the battery as just another load on the alternator, a big one in fact, so a high charging current requirement will mean lower system voltage. There's a snowball effect where the extra charging current requirement causes the alternator to heat up more, dropping the voltage available and slowing the charge rate some. Fun stuff!
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Old 11-23-2010, 04:09 PM
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Cosmo Kramer
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Mine runs at 14.1 with everything off and 13.6 - 13.8 with all the lights and A/C on at idle.
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Old 11-23-2010, 04:10 PM
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Tim, was out in my '94 today and the dash gauge read about 13.8V...did not measure at the alt or the posts, just looked on the dash. I don't think any of my others ever read that high, but I don't seem to have any problems with them either.
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Old 11-23-2010, 11:10 PM
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dr bob
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James--

Must have been a cold day in Austin today. Cold alternator, no cooling fans running, daytime so no headlight load.

Mine ('89 S4), with a new Bosch alternator, sits right at 14 immediately after starting. Stays there for a while then goes slowly down to a half a needle less than 14 as the engine warms up. This is in our sub-tropical winter weather today, when the daytime high was a frigid 65º. Might get into the 30's overnight though, a real rarity here. But let's get back to the charging issue. VR rating on the original alternator was 13.8 Volts. New one has a 14 Volt tag on it. It does make 14 Volts under the stated 'perfect' conditions, but the internal load/voltage curve for the alternator itself hasn't changed any from the original. The only thing that's changed is the cold voltage reading.
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Old 11-24-2010, 02:36 AM
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Tim, I am having the same issues wity my 928. I had the alternator rebuilt and purchased a new battery. If you need any help let me know....
Please keep us posted on your progress, as I plan to follow your listed steps to trouble shoot my car.
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Old 11-21-2014, 06:45 PM
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Went out to the car and battery was totally dead.

Charged it and brought in for load test - tested good.

I'm seeing around 13.4 volts at the battery with the car running and the interior lights on.

Hope I don't have a new drain somewhere.
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Old 11-21-2014, 07:10 PM
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Bertrand Daoust
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That's what I have at the jump post and everything is fine (I think).

How long since you took the car for the last time?
I guess it was locked with the alarm system on?

I know that my car with the alarm system on, after about three weeks, the battery is quite low.
But I never lock the car in my garage so I don't have any problems.
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Old 11-21-2014, 07:12 PM
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It is parked in my garage and never locked.

The day before this no-start the starter seemed low on power, but it did start. That was after a few days of sitting.

The no-start happened the next day after having driven it.
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Old 11-21-2014, 07:24 PM
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Bertrand Daoust
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13.4 is good so the alternator should be good.

Looks like a drain somewhere...

Or a week battery.
But it would be a real week battery problem to lose all the power in one day!
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