Hey Guys,

I've done a crap ton of DIY on my '99 s70 (n/a fwd), and I have yet to be this stumped...

I drove on the freeway with my car the other day, and I was flooring it to only go ~30 mph. My friend came to jump me, which solved the problem for a little while.

I figured it was the alternator, so I bought a new one. Yet still, the battery voltage drops to 11.8V when the car is started (battery is good, ~12.6V when not running).

The alternator came with a new internal regulator, so I know that isn't the problem.

An ammeter reported only a 50 mA draw when the car was turned off, so I don't believe something is shorting.

My next thought was that there was a bad ground or other wire. I checked the wiring diagrams, and found no other resistive components between the alternator node and the battery + terminal, so I decided to short the alternator output bolt to the + terminal with a jumper cable, which yielded no increase in voltage across the battery. Since that didn't work, I touched a jumper cable from the alternator body itself to the negative terminal, which still didn't help. Granted, I only held them there for a couple seconds, is it something that would require me a while to see a difference in?

I get codes for all crankshaft and camshaft position sensors, and low system voltage. I think the sensor codes are faux, as the car runs absolutely fine when the battery is well charged.

Is the alternator body itself not a ground? I'm a total noob with electrics... any help is appreciated!

 

The s70's have a couple of strange electrical quirks. -

Your voltage reading of 12.6 volts with the car off - is that of a fully charged battery. If the alternator is working the voltage should go up to ~13.5 volts with the engine running, and with a few accessories on. If not either that "new" alternator is bad or there is really high resistance in the cable between the alternator and the battery. You can do a voltage drop test from the alternator to the positive post on the battery. Set your voltmeter to volts - put one lead on the alternator output post, the other on the + battery terminal. The reading should be zero if the wires/connections are perfect. Any voltage shown is your voltage drop - and over ~.3 volts could be a problem. Always check for voltage drop with the systems under load, engine running.

Two areas that have problems - where the large + cable is clamped into the + battery terminal - a poor connection develops. It generates at lot of heat - check the temp (with a infrared thermometer or your finger) of the positive terminal after it's been running for a while. Careful, they can get really hot. That usually creates starting problems also - as in the starter seems to turn the engine slowly.

Another area - the 4 red medium sized wires attached to the + battery terminal go to the fuse box under the hood.You can easily do a voltage drop test between the battery+ and the bolt in the fuse box - again have the engine running.

That bolt with a 13mm nut on it - undo the nut and pull the wires up - and be amazed if they are not melted at that 90 degree wire terminal. That can create low voltage to the fuel injection system, and the rest of the car, especially your throttle body and cause your lack of MPH the other day.

Speaking of throttle modules - how old is yours? I trust you are aware of the problems with the 99-01 throttle bodies.

 

Hey hoonk,

I just went outside to test your recommendations. I found a zero voltage difference between the alternator output and the (+) battery terminal, with a 1 ohm resistance. I measured the voltage drop between the battery (+) terminal and the fusebox bolt, which was 0V with 1 ohm resistance. So those are not the problem.

I attached the battery (-) to the frame using a jumper cable, which did not help.

Using a test light, I checked to verify that the little cable to the regulator was active. The light turned on, so I figure it's alright.

The only oddity I noticed was a 76000 ohm resistance between the alternator output and the bolt with the arrow on it. Since there was zero resistance between the alternator and the battery, I figured this didn't matter.




This all leads me to believe that the new alternator I got is faulty. I'll watch some youtube videos to see how to test it out-of-car.

Thanks a bunch for your help man - I'm really stabbing in the dark here.
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Ended up caving in and brought it to a professional shop. They agreed with me that the new alternator was faulty... go figure.