tj90

Lately Ive been having driveability problems in my 993. It started out as my HIDs not firing and low dash light brightness. Finally I put the pieces together when I was driving to work, ABS light came on, car started stuttering and finally getting the Christmas tree on the dash. I was able to get the passport plugged in the cigarette lighter and put it in voltage mode on my final drive. Got a solid 11.9V before "low voltage" was displayed.

Battery tested fine at 12.6V when I recharged it. Held charge fine under load. Oh crap.... another bad alternator? Im expecting the most expensive fix....

To make a long story short, I pulled the alternator and discovered the source of my problems. It was the NEG wire leading to the alternator. As you can see in the image, I was getting 0.3-0.5 Ohms over that short span of wire as a bent it. It looks like either the wire insulation was repaired using electrical tape or the wire was spliced? Either way, that was probably my handy work since I had the harness recall and that definitely was not OEM.

I replaced the bad wire with some new 10 gage wire and used shrink wrap to splice in the new wire outside the alternator housing. I also took my alternator down to the shop for bench testing and they found that my alternator was outputting 100A at 14V. Brushes looked good and bearings were solid.

Got everything together and the final image is the beautiful result of fixing that faulty wire. 13.9V right where it should be.

The ground and power paths are critical for proper operation. I you spent anytime on RL you quickly learn that the neg battery cable is critical, tight electrical connections are critical. Knowing this and still thinking bad alternator

cgfen

whoa
sexy probes dude.
glad you got it sorted and Welcome Back!

Craig

kjr914

Simple but true, and often misunderstood: current flows in a closed loop. The ground is as important as the power.

Your issue reminds me of a question I had when working on my alt. ....why IS that "gnd" so small compared with the pos side. The only thing I can think of is that either:
1. The gnd goes quickly to the chassis somewhere and is very short, so the wire gauge can be smaller. (This assumes max current is not the limiting factor on wire size, resistance is.)
2. It is expected that the alternator is gnd'ed via the metal-to-metal contact with the housing (a 2nd gnd). Despite my recent install/de-install/install/deinstall/install/... of my own alt, I can't recall if the gnd connection of the alt is actually connected to the housing or not... Not highly likely, as the gnd wire plus the metal-metal gnd (dual gnd loops) is not something a German engineer would do...

Does anyone have an answer to this?

Trying to figure out TJ's burned gnd wire:
If 2 is true, it might suggest that TJ is missing the gnd strap to the engine and all the current went thru the smaller gnd wire, causing its destruction... But not as likely the cause. Leaving me to think...

If #1, when he had a short at one time in his headlight (IIRC, insider info here), it caused excessive current flow that exceeded the capability of the wire. The short would cause the alternator's regulator to try and do its job (bring voltage back up) and more current flow from the alt, eventually exceeding the wire's capability. Since the power wire is much thicker, it can handle more current. (TJ, This is my vote for the strange wire melt down cause.)


TJ,
Nice, clean wire splice and shrink wrap job. Glad you got it sorted (sorta) quickly!

Garth S

To the issue of lesser gauge wire on the ground, it is the return side from whatever energy consuming load that is fed by the supply (+12V red) .... ie., if feeding a blower motor on full tilt AC, those components 'consume' the energy (watts) from the feed line - leaving 'less' to return via the ground loop;
yeah, I agree that it is a somewhat misleading description of DC circuitry, but hopefully close enough to illustrate the deal - at least the way I understand it, but remain open to correction.

Same deal in one's home 220V AC panel .... look at the gauge of the two feeds ( red & black) vs the SINGLE white neutral/bond wire. Code allows a lesser gauge bond wire for that reason.

IXLR8

An issue that so many seem to ignore.

I made a ground cable for a local 911 owner a couple of winters ago using aircraft grade/multi stranded cable crimped and then soldered at the ends. I do not have much use for "crimps" when the cable is located in the elements.





Alternator Cable Thread

Specs of cable used.

BTW, 13.8V is on the low side IF that is your battery voltage. If your detector is plugged into a lighter socket, then it may not be reading battery voltage.

cgfen

Nice ground cable fabrication Alex!

tj90

Agreed - nice fabrication job Alex. I soldered my splice job as well but I had to break out a torch to overcome the thermal conductivity of the 10 gage wire. You always wonder if its good enough but Im always nervous with open flame in the engine compartment soldering in an area that is limit access.

My escort is measuring 13.6-13.8V at the cigarette plug. I will measure the battery when I get a chance although the alternator shop said that my alternator was outputting 100A at 14.2V.

Nice discussion on the reason for the negative to be so much smaller than the feed line. BTW, I did not mention this in the original post: My DMM could not measure a resistance drop from the new alternator lug to the battery NEG strap. I was reading 0.2-0.3 Ohm but I had to use a 75 ft extension cord to close the circuit. The leads on my DMM just cannot span the distance from the alternator to battery strap.The extension cord was most of the resistance that I was measuring. The resistance contribution of the car was so much smaller than the extension cord, I could not read it with the limitations of the fluke or using a 4 point measurement approach.

JM993

Hey Garth,

Would another way of saying this is that the negative lead carries less current?

Cheers,
Joe

IXLR8

Our alternators are spec'd 115A at 14V. The voltage regulators in our alternators regulate to 14.5V (at the alternator).

This is what my on-board voltmeter reads that is connected directly to the battery terminals. On a cold day, it'll read 14.3V. On a very hot day, 14.0V


BTW, lets assume that the alternator is not grounded via the fan housing and that all there is, is that ground strap. That alternator ground cable that I made measured 0.000714 ohms using an Agilent 4-wire meter. The voltage drop across that cable would be negligible at any current in our vehicles.

Garth S

Hi Joe,
You are absolutely correct - and far more concise ... my 'essay' was an attempt to explain why the drop in current occurs.

Cheers,
Garth