Can alternator diodes "leak" and discharge the battery?

[Lil bastard]

Originally Posted by J Tinsby

... It would be great to use a real meter and not a digital one, if you can find one today... Good luck!

No it wouldn't! An analog gauge should NEVER be used on microelectronic circuitry because it draws too much current to actuate the gauge and can damage the circuitry which wasn't designed for that amount of voltage.

Wet cell battery technology is well designed and understood. Each cell is specifically designed to produce 2.1-2.2 VDC. You have 6 cells in series, so the total voltage on a healthy battery s/b 12.6-13.2 VDC. Any more/less, and you have a less than perfect battery. Anything under 12 VDC s/b replaced.

To use a DMM, disconnect the battery and measure the voltage across the battery terminals, looking for that 12.6-13.2 VDC discussed above, in any case record the value.

Now, reconnect the terminals and measure again. The difference is the draw by the car's ancillary systems. If greater than 1 VDC, you have an issue.

Now start the car and let it idle 1 minute. Check across the battery terminals again and you should now see 13.5-14.5 VDC (that's the rating on the Bosch alternator). If not, the weakest link in the charging system is the diode pac, so this is likely the culprit.

The up side is that it is both cheap and easy to replace with the alternator in situ because it's external.

Cheers!

[Dale_K]

Tell me more about replacing the voltage regulator with the alternator in place. I'm pretty sure I'd want to do this since the part is pretty cheap and especially if you're saying it's not difficult. Do you have a link to a DIY? What part exactly am I looking for and where can I get it?

[Dale_K]

Update - I see Pelican has the VR available for order so I know where I can get the part but I'm not sure about being able to replace it with the alternator in the car. i didn't find any DIY that speaks to that. They all say you have to take it out first.

Another update - Just got back from O'Reillys and had them do a battery and charging system test. They said everything was OK. Charging system voltage at 13.6, which seems a little low to me. They could be right though if there's a parasitic drain with the car off. I have a digital multimeter with a miliamp range so I might be able to check the basic system if I can find a way to clamp the test lead probes to the cable and terminal. My meter doesn't have alligator clamps, just sharp probes.

I'm not too sure about how to test the car since the interior lights will be on when I open the door and I don't see a way to disable them. They'll go off after a long time but it will take forever if I have to wait 2 hours each time I pull a fuse.

[JFP in PA]

The alternator has to come out of the car to replace the voltage regulator.

[J Tinsby]

Originally Posted by Lil bastard

No it wouldn't! An analog gauge should NEVER be used on microelectronic circuitry because it draws too much current to actuate the gauge and can damage the circuitry which wasn't designed for that amount of voltage.
Cheers!

I wouldn't put an analog voltmeter in a circuit smc's in it. But the gauges on the dash are analog and magically they don't hurt anything. Putting the meter in line like an ammeter shouldn't effect anything, at best it would be a floating ground connection, you might lose the meter in a worst case that's all.

No worries looks like he is replacing the regulator anyway.

Regards,

[Dale_K]

I've ordered the new VR and sure hope it fixes the car. Yesterday with the AC on the accessory voltmeter I bought occassionally showed less than 12v at idle.

About the gauges - I don't know about Porsche specifically but most modern cars don't operate the gauges from directly connected sensors. The gauges are controlled by the ECU, which decides what position to point the needle based on the sensor reading but the ECU is programmed to avoid alarming the driver with mildly high readings (temp) or low readings (oil & volts). The ECU puts the needles in pleasantly reassuring positions unless the underlying system gets really bad.

[JFP in PA]

One point worth considering is that the dash displays in these cars are notoriously inaccurate to begin with. The coolant temp typically displays near 180F, while the DME says the actual temp is more like 210F; this same display is also non-linear in accuracy (it is further off at some temperatures than others).

[Dale_K]

I haven't received the voltage regulator yet but I did perform a parasitic battery drain test today. The car had been sitting for many hours and I was careful not to open a door or do anything to "wake it up". I saw about 17-22 milliamps when I disconnected the battery cable and hooked the battery post to one side of the meter and the cable end to the other side of the meter. So I'm assuming I can rule out parasitic losses.

[JFP in PA]

That is correct; normal draw can be as high as 40-60, so if anything you are on the low side.