Rod Underwood

The problem with we newbys is that we have lots of questions both out of ignorance and from being inquisitive about our new babies.

My voltmeter was reading consistently less than 12 volts around town and a little above on the road so I just stopped at my local auto parts place that does free alternator checks. I understand it is likely that my voltmeter is just getting tired.

The verdict was that it is actually charging 13.65 volts - good, but... it is putting out 188 amps! Their recommendation is a new alternator before this one destroys my battery. Can someone who knows something tell me is this is indeed too high or are 928's such power eaters that they need an alternator that puts out this much amperage?

Thanks, and have patience. I do have a set of new shop manuals, but have not yet "internalized" them.

Thanks again
Rod

docmirror

Wow 2.6 KW from your alternator. You should be proud. I think that's the total output of my Geo engine at full song Seriously, there's something wrong with their test setup. Depending on your model year, the max output of your alternator is about 85 amps. There is a model that is rated at 135 amps @ 8500RPM for > 2 minutes, but I think it's only on the GTS.

So, the indications are that your car runs fine, but the voltmeter registers less than 12 Volts. But yet, when checked at the shop it reads 13.65 at the batt terminals. Do I have that right? So, it seems that the point you're measuring from the car, is different than the batt terminal voltage. You may have a resistance in the circuit drawing amperage somewhere and this is where you need to do some more troubleshooting. I susggest some more investigation before getting spendy at the local alternator shop, that would like to sell you a new alt and batt, and regulator, and cables, etc(you get the idea).

Do you want to fiddle with it yourself, or would you rather take it to a shop? If you want to DIY, get a multimeter and one of the clamp-on ammeter accessories. www.fluke.com has the best, but others are avail. It must measure DC AMPS! You should also invest in a small battery charger, not the big high amp ones, just something to slow charge.

Since you are a member, you can also edit your user control panel with a signature, please include the model, year, and modifications of your car. We would also like to know where you are, but it's up to you. Let me know what your plans are and we can go from there.

Doc

Rod Underwood

I'm willing to do a lot of the work, I am an old timer drag racer who has been reborn into my 928 dream car. I have a multimeter, but not a clamp-on. I'll get one tomorrow. I have several battery chargers.

Which Fluke do you recommend - the 322?

I have a new interior light relay - apparently bad - coming and the ABS light is on - have a new relay coming for this as well.

I think I've edited my signature, so fire away and thanks.


Rod

docmirror

I recommend the 7X series for auto appliction. These are pricey items, so you may want to shop around for something similar. I have Fluke equipment and would never use anything else. Anyway, as you may have read in the sticky posts at the top of the 928 forum, our cars suffer from many and varied electrical issues. Your's is not all that different from some of the others.

Set the meter aside and find a Pep Boys Hydrometer to check you're battery electolyte. It's simple and fast. If the red ball floats and the green doesn't you're battery is weak. Put the charger on it for a while, and then recheck. Start with a fully charged battery! 928s are tough on batteries.

Once the battery is up to snuff, turn off all your interior courtesy lights and lets use the meter.

Check the voltage at the + battery post and the bolt for the ground lug behind the tool tray. It should be around 12V. Leave the meter in place for the next tests.

Turn on the key, but do not start all the engine. Watch the meter, it will show a gradual discharge, but should not drop more than half volt in half a minute. Ignore the dash voltmeter.

Now start the engine, the voltmeter will drop a lot while cranking, but should begin to recover voltage once the engine is on an the alternator light is off. The ALT light is off, right?

At this point, if the voltmeter is showing a rise and gets to about 13.5V or so, your charging system is working in principal. The job of the alternator is to supply current to the car systems and recharge the battery to maintain a voltage sufficient to restart the car. Let the car run for a few minutes, or drive to the store or something. Don't shut it off.

Now the car is warmed up and running. Recheck the voltage at the battery terminals. Remember that number. Move around to the hood, and locate the jump start terminal on the pass fender. Check the voltage at the jump start point, and ground on the engine. Is the reading very close to the one from the battery? Within .2 Volts? If so, the charge line from the jump point back to the battery is good.

So, at this point, what does the voltmeter on the dash say? If the Fluke meter says things are alright, and the dash meter says 11-12V, I think you know where the problem is. Taking the dash out and testing the lines to the voltmeter is a fairly big job. Now you can check the output of the alternator.

Use the clamp meter around the cable to the + battery terminal. Try to keep it away from the battery case, and hold it perpendicular to the cable direction so you get a good reading. This is the recharge current. If the battery is in good shape, this should be fairly low after a modest drive. If it shows more than about 7-10A, the battery might be just worn out.

Now, take the Fluke meter and clamp around to the front. Locate the big charge line, coming from the cable harness that passes in front of the engine, and attaches to the jump lug. Clamp around the cable from that harness, before it reaches the jump lug. This is the current output of the alternator. At idle, with no acc on, it should probably read about 15A. Now rev the engine so it's stays at about 2700-3000RPM. Watch the meter, and start turning on acc. Start with the lights, then the wipers, then the AC, then the window motors up and down.

At this point the current should read about 45-55 Amps, maybe more. Leave all this on, disconnect the clamp ammeter from the Fluke, and check the voltage at the jump lug(2700-300RPM). It should still read around 13 volts. If it is much lower voltage than the resting voltage, the alternator may be weak. Leave all the stuff on, and go around to the batt. Connect the voltmeter to the batt + terminal and the ground bolt behind the tool tray. Again, around 13V, and close to the reading on the front of the pass jump lug.

I think we've gone far enough for now. Report back with your numbers and we'll take it up again depending on what you find. We may be looking at an uncalibrated dash voltmeter, but it's early to speculate too much. Remeber, during your tests, you are looking for trends, not absolute numbers. If the voltage at the jump lug is much different than the battery, that charge line to the back is a problem. If the current at the jump point really is over 100 amps with all the acc on, then something is wrong, and you will have to turn off curcuits one at a time to figure out where all the current is going.

Doc

Rod Underwood

Thanks, I need to get the clamp on meter and can look at it this weekend.
Your instructions are appropriately specific for me. ;-)

Rod

sublimate

Before you pull your pod you should check your voltages at the fuse/relay panel. My dash voltmeter was reading way low (off scale) so I checked with my multimeter and found good voltage at the battery and jump start terminal. But at the fuses I'm getting only 11.3 volts for the constant power circuit (30) and only 10.5 volts for the switched power circuit (15) - which is what the dash voltmeter is reading.

Time for more troubleshooting.

Rod Underwood

Yet another check... thanks. ;-)

SteveG

Pls put your MY in your sig. Might try the search function.

Doc thanks for that post, very generous and helpful.

Rod Underwood

I think I've done that, unless I have misinterpreted your message. I knew the voltage information, but may have missed the amperage, and have since updated my signature.

In any event, thanks to everyone for their help. I know when I first started messing with computers, a couple of friends helped enormously and it took me about three or four years before I could actually help solve their problems. I hope it doesn't take that long with the 928, but bear with me. I really appreciate any and all suggestions.

Thanks
Rod

docmirror

This is, slowly but surely where we're heading. I want to start with the major current systems and then work down to the detail oriented probing. I will eventually have him checking the circuit 30, and circuit 15 voltages. A that point, the WSM is required, and we'll be delving into ground points, and power busses. Step by step, working from macro to micro is easier for the novice. Doing it on my own, I'd be at the circuit 15 pick line, and work outward in one direction. Linear is slower, but more revealing to the novice.

Your problem is a high resistance in the circuit 30 buss. Check multiple locations, and then start working backward from the relay panel up to the big charge line at the jump point. You could disconnect the batt, and clean the jump point connectors religiously. There is a little know power buss on the drivers side also(on my 90 GT anyway). Behind and partially hidden by the PS pump reservoir. BEWARE, lots of current in use over there too. Only work in that area with the batt disconnected. Use the meter on low ohms scale and look for resistances > 3-4 ohms. Ground points can also affect voltage drops and are disturbingly hard to understand. Just realize that the voltage must work through the chassis from the ground point of the circuit, back to the negative lead of your meter. Those two points can be significantly different in terms of voltage drop(resistance).

Problems in this area are called 'ground loops'. Large, distributed computer systems suffer from this all the time. Autos are not as large, but develop resistive problems from corrosion, road salts, water, dirt, and the ever popular galvanic action of dissimilar metals. The treatise on ground point cleaning and restoration should be well heeded.

Doc

Ketchmi

Hmmm, the factory rating on the Bosch S4 alternator is 115 amp.

We are currently testing a rebuilt unit rated at 160 amps that is doing extremely well. They will be for sale priced at $249.99 if anyone is interested.

Rod Underwood

Slowly and methodically will be appreciated with me at this particular point, especially until I learn my way around this particular engine and car.

I just ordered the Fluke and it may be here by the weekend - hope so. In the meantime I'll check everything else that you indicated and get acquainted with the nuances.

Thanks a lot for your knowledge and patience. I'll get back to you as soon as I can get into it.

I just received a replacement magnet for the oil drain and a stater and power steering pump from Jager - just insurance - part of a long stream of UPS trucks to my house. I'm still waiting for car cover, matts, service cover, cross overs, chip, relays, etc... This is really my Christmas for the next few years, probably, but at this point my wife understands and smiles patiently.

It all makes no real economic sense, but a friend of mine says "need?" what's that? At some point in your life "need" is not the operational word.

Looking forward to a long and happy relationship with my toy and the fine folks on Rennlist. I've been reading constantly since I joined and am impressed/amazed by the general attitude of the group.

(I'm obviously still on the honeymoon, fire or no fire.) ;-).

Rod

(Incidently, Doc, your response is a permanent addition to my shop manual set.)

SteveG

Rod: Sounds like you are ahead of many of us in wrenching experience. I have found a new hobby/love affair in the shark which I thought was impossible in this life. Maybe it is b/c I can only get to the car on alternate weekends and sometimes not then; it has lasted 3 years and counting; looks terminal. As a side note I have printed at least 2 reams of paper from here, nichols and pirtle's sites. Also almost anything by Wally Plumley is worth looking at. He has a primer he will email on general maintenance all MY and a white paper on the A/C. I'm sure you will be adding to my print outs.

WallyP

docmirror gave some good advice, but there is one minor problem. He said:
"Locate the big charge line, coming from the cable harness that passes in front of the engine, and attaches to the jump lug. Clamp around the cable from that harness, before it reaches the jump lug. This is the current output of the alternator."

Unfortunately, it isn't that simple...

The power wires from the alternator vary by year model, but most years send the battery charging current from the alternator to the starter solenoid on two moderately heavy wires. From there, a heavier wire runs up to the jump start terminal.

On some years, a moderately heavy wire also runs from the alternator to the Central Electric Panel.

Bottom line, the only place to check alternator output is at the alternator in most cases. You will need to look at both the wiring diagram and the actual physical wires on your car to be certain of where to test.

Rod Underwood

Thanks,

Hope to receive my new clamp meter tomorrow.

Rod

I think you're the famous "Wally" referred to here: "He has a primer he will email on general maintenance all MY and a white paper on the A/C." If you have the time, please email me a copy of your maintenance info and white paper.

Thanks rod@rod-underwood.net