Is my CDI box gone?

[L999]

I'm new to the forum and have owned aircooled Porsches for 10 years.

I need some help diagnosing an electrical problem with my 930 and hope someone can help.

I recently replaced the battery in my 930 and it went dead in about 3 days so I figured there must be a parasitic current draw somewhere. I measured about 22miliamps with the car off. Some articles I have read have suggested that this is a normal amount of parasitic draw but I'm not sure?  The new battery now does not hold a sufficient charge (after recharging and going dead a few times) to start the car.

I haven't immediately replaced the battery because I figured I would be wasting new batteries until I fix the parasitic draw.  What I did do however was buy a booster pack from Repco so I could start the car and drive it.

I checked all the fuses under the front hood and could not locate the parasitic draw so I figured it must be coming off the main red lead from the battery which runs to the ignition system in the back.

I also noticed that my tachometer was bouncing at random while the car is running.  This made me think there is either something wrong with the alternator (voltage regulator) or the CDI box.

I don't know how old the alternator is in the car so I have bought a refurbished one (yet to be installed).

One thing I did notice however is that if I leave the booster pack on and drive the car, the car runs significantly better than if I turn the booster pack off after starting the car and driving it.  I also do not get the tachometer bouncing if the booster pack remains on whilst driving the car.

This is making me think that the problem is with the CDI box not the alternator, possibly a weak spark which gets stronger if the booster pack is on?? Can anyone confirm??

The CDI box is making the piercing noise when the car is on ACC but I'm thinking it may be producing a weak spark and is need of a refurb or new capacitor.

I would appreciate hearing from you if you have some experience with this.

 

 

[GUT]

I'm not an expert but I thought the CDI box either works or does not work. But it can come back from the dead.

My tacho on my 930 started bouncing just before it stopped dead. It happens when the charge gets too low. My alternator was stuffed so I got that fixed and everything is fine.

I don't know what's drawing on the battery but check the alternator to make sure it's actually producing charge.

 

[Redracn]

Hi L999 and welcome

22mA is a little bit high but acceptable( Less that 10mA is desirable ) This is about 16AmpHour per month or around 1/3 of the typical car battery capacity. Flat in 3 days at 22mA indicates a capicity of about 1.6AmpHour.

From your description you have a dud battery. CDI and most ignitions draw their current in short sharp bursts and it is the battery that supplies this current burst not the alternator. Batteries can even seem ok and crank the engine but have problems in supplying short sharp current pulses which the CDI relies on to operate correctly. It is the CDI causing the supply voltage to drop in response to its current draw that the battery can not adequately supply that is most likely causing the tacho issue. To quickly chech the alternator turn on the headlights and measure the battery voltage while running the engine at around 2000-3000rpm should slowly get to over 13.5V. 

[Glom1098]

Sounds like a problem with the alternator regulator-lots of posts on the us forums about this causing the bouncing tach problem. I had the same problem with the tach of a different cause (when the fresh air blower was on-there was shorting across the contacts). Checking charging current for a voltage drop is a good idea

[L999]

Thanks for the replies. Appreciate it.

So I think i found one source of the current leak. Im measuring 22ma if the car is locked and 11ma if the car is unlocked. When the car is locked the central locking light in the centre console is illuminated so there must be something wrong with it. This light was never illuminated in my previous 87 carrera when locked except if the car lights were on.

Off a cold start i measured 15.3v fluctuating to 15.7v on idle. This started to setlle at about 15v. If i turn the lights on whilst idling the voltage drops to about 12.8v fluctuating to 13v. At about 2000rpm i read 15.5v with and without lights on. It sounds a bit high and inconsistent to me.

It might be time to replace the alternator?

[Redracn]

22 or 11 mA is a side issue as both are acceptable and will not flatten a fully charge car battery in less than a month. You may also have more than one issue but I believe that the battery is a problem. The fact the voltage is easily raised to 15.7V indicates the battery is not performing properly it should take a lot if amps and some time to get a fit battery up to this voltage. Also when the battery is high impedance you are no longer measuring a DC voltage as there will be a significant amount of AC present and the type and capability of your multi- meter will affect the reading. You may still have an alternator issue in that the regulator is not functional and the alternator is providing maximum output but I doubt it as at low RPM the max current the alternator can output is nowhere near enought to get a good battery to 15.7 V. A lot of places that sell battery's have a special tester that measures the battery performance under load it may pay to get it check/tested. It is only possible to check an alternator in vehicle when attached to a known good battery. 

[Harvs11]

I'm far from knowledgable on electrical problems, but here's my 2c.  The bouncing taco needle prompted my memory of a problem I experienced a few years back in my SC.  The speedo needle was bouncing when the ignition was OFF.  No problem whilst the car was running.    I can't recall the clue which helped solve the problem but it was caused by a faulty relay diverting current to the speedo circuit via the chassis earth.

The relays are a common failure so worth checking their condition before going down a more expensive path.

[GUT]

Where are you located? If in Melbourne, I've got a spare good battery that you can whack in and see how it goes.

[L999]

I'm in sydney but thanks for the offer!

I'll buy a new battery and see how I go.

Cheers.

[JV911]

Even newer 911s are known for killing batteries if left to sit for a while (most run trickle chargers) but 3 days is unusual

Bouncy tach will be the voltage regulator. If it's overcharging it'll cook the battery (notice a sulphur smell?) and undercharging is self explanatory. 

[L999]

No sulphur smell at the moment. My 87 Carrera overcharged and blew up a battery once so I have never forgotten that rotten egg smell!!

I suspect the alternator is on the way out and will replace it at next service.

I did open up the battery an noticed that the water level was low (which I believe is also a sign of overcharging). The battery was less than 3 months old. 

 

[Redracn]

When it comes to alternators overcharging is pretty much the result of a faulty voltage regulator or wiring depending on the location of the regulator (external or built in). The alternator brushes, diodes, field/armature wiring etc can not cause overcharging but a fault with them will cause a lower/no output situation possibly resulting in not reaching a high enough voltage after supplying the vehicle systems to adequately charge the battery. In general most batteries are ok up to 14.4 volts for short periods and 13.8 for long periods but this is specific to battery chemistry and temperature. Taking the multi-meter for a drive should show any overcharging. Intermittent faults are common but not likely to wreck a battery by overcharging without being easy to detect with the meter.

I am still of the opion that the battery is faulty (high impedance) but can not rule out over charging or flattening it  causing the faulty condition. Once a good battery is installed diagnosing over/under charging is fairly simple. 

Hope this helps get the issue sorted as P cars should always be ready to roll.?