97 Boxster with Intermittent Brown Out

[Bush Pilot]

I’ve got a 97 Boxster with 50k mi. with an intermittent electrical brown out condition where the radio resets, car stumbles, lights dim etc. especially when an electrical load is put on the system. I measure 12.6V at the battery during the brown out with the car running. I measure 14.5V at the battery when not in the brown out. I can stop and restart the car and it will usually go away. I’ve replaced the alternator and starter and fuel pump relay with no success. I found rat damage to the knock sensor but who knows what else was damaged by a rat. I don’t see other damage.

[JFP in PA]

Originally Posted by Bush Pilot

I’ve got a 97 Boxster with 50k mi. with an intermittent electrical brown out condition where the radio resets, car stumbles, lights dim etc. especially when an electrical load is put on the system. I measure 12.6V at the battery during the brown out with the car running. I measure 14.5V at the battery when not in the brown out. I can stop and restart the car and it will usually go away. I’ve replaced the alternator and starter and fuel pump relay with no success. I found rat damage to the knock sensor but who knows what else was damaged by a rat. I don’t see other damage.

Run a voltage drop test across the primary battery cables; they are known to develop internal corrosion which leads to resistance issues, particularly when warm.

[Bush Pilot]

Originally Posted by JFP in PA

Run a voltage drop test across the primary battery cables; they are known to develop internal corrosion which leads to resistance issues, particularly when warm.

Thank you but I believe that a fault like that wouldn’t show up as an intermittent brown out. Also, the car is an Arizona car and all those connections appear as new.

[JFP in PA]

Originally Posted by Bush Pilot

Thank you but I believe that a fault like that wouldn’t show up as an intermittent brown out. Also, the car is an Arizona car and all those connections appear as new.

Don't bet on it. The corrosion issue is internal to the cable, not just at the connections. Porsche has a TSB about it and improved replacement cables because of it. And a voltage drop test only takes about 10 seconds to run.

[Bush Pilot]

Originally Posted by JFP in PA

Don't bet on it. The corrosion issue is internal to the cable, not just at the connections. Porsche has a TSB about it and improved replacement cables because of it. And a voltage drop test only takes about 10 seconds to run.

OK. Thank you. I'll give it a try. That said when you say "primary" do you mean the high current cables from the battery (+) terminal to the alternator and starter? If so, any thoughts on what voltage drop is acceptable? I suppose another method would be to measure the resistance of the cable and compare that resistance to what an analysis would yield for how much resistance should be there based upon the cable gauge, type (stranded/solid) and length.

[blue62]

Originally Posted by Bush Pilot

OK. Thank you. I'll give it a try. That said when you say "primary" do you mean the high current cables from the battery (+) terminal to the alternator and starter? If so, any thoughts on what voltage drop is acceptable? I suppose another method would be to measure the resistance of the cable and compare that resistance to what an analysis would yield for how much resistance should be there based upon the cable gauge, type (stranded/solid) and length.

"Primary cables" = both battery cables.
Acceptable voltage drop.= It should be very little... a few mili amps.
Go to Utube and look up voltage drop tests.
You will see how to do the tests properly and what to expect for voltage drop.

Edit: I said mili amps I ment to say mili volts.

[JFP in PA]

Yes, the primary ones are the large diameter cables. Maximum allowable voltage drop would be about half a volt. Because cable lengths vary from vehicle to vehicle, and their original starting internal resistance is unknown, voltage drop is quicker and more accurate. If you have access to a Power Probe electrical diagnostic tool, it has a built in voltage drop capability.

[Bush Pilot]

Originally Posted by JFP in PA

Yes, the primary ones are the large diameter cables. Maximum allowable voltage drop would be about half a volt. Because cable lengths vary from vehicle to vehicle, and their original starting internal resistance is unknown, voltage drop is quicker and more accurate. If you have access to a Power Probe electrical diagnostic tool, it has a built in voltage drop capability.

I can simply make a voltage measurement across the cable with one lead at the battery + and the other at the block where the alternator and starter tie in, located towards the front top of the engine. I don't see this potential fault causing an intermitten issue like I'm seeing though. But it's an easy measurement.

[JFP in PA]

Originally Posted by Bush Pilot

I can simply make a voltage measurement across the cable with one lead at the battery + and the other at the block where the alternator and starter tie in, located towards the front top of the engine. I don't see this potential fault causing an intermitten issue like I'm seeing though. But it's an easy measurement.

Finding any fault is often just a process of elimination...............

[blue62]

Originally Posted by Bush Pilot

I can simply make a voltage measurement across the cable with one lead at the battery + and the other at the block where the alternator and starter tie in, located towards the front top of the engine. I don't see this potential fault causing an intermitten issue like I'm seeing though. But it's an easy measurement.

Your issue sounds like a possible bad ground to me.
This video will show you how to test for voltage drop on the ground (negative) side.
https://youtu.be/qdaOSiScEu8
Keep in mind there are multiple ground points on these cars.

[JFP in PA]

Originally Posted by blue62

Your issue sounds like a possible bad ground to me.
This video will show you how to test for voltage drop on the ground (negative) side.
https://youtu.be/qdaOSiScEu8
Keep in mind there are multiple ground points on these cars.

This is where the right tool makes your life easier: A Power Probe can do either positive or negative voltage drops from one tool in a couple of seconds.

[blue62]

Originally Posted by JFP in PA

This is where the right tool makes your life easier: A Power Probe can do either positive or negative voltage drops from one tool in a couple of seconds.

You just about have me ready to purchase one just to have it in the tool box

[JFP in PA]

Originally Posted by blue62

You just about have me ready to purchase one just to have it in the tool box

Look into them, very flexible and useful tools. We have both a PP Four and a Maestro, both of which are in constant use.

[blue62]

Originally Posted by JFP in PA

Look into them, very flexible and useful tools. We have both a PP Four and a Maestro, both of which are in constant use.

Thank you for the heads up... I will check them out

[Bush Pilot]

It could pay for itself quickly when the Porsche Dealer is charging $225/hr for labor.

[Bush Pilot]

I found the issue. The battery + cable was wedged very tightly between the hard plastic cable guide coming through the firewall and the battery tray. The sharp edge on the battery tray was occasionally making a bad electrical connection to chassis by cutting through the insulation on the + cable. A good electrical connection would cause the battery to fail catastrophically and there are signs fo that happening in the past. Not sure if this happened during assembly or if it was done by a heavy handed mechanic.