Alternator problem?
Thread Starter
6th Gear
Joined: Jul 2002
Posts: 6
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Alternator problem?
could someone help me? i own an auto '81 928S which has been giving me a pain with the electrical system. whenever i have my foot on the gas the amp gauge sits right whre its supposed to be but as soon as i take my foot off, it drops to the bottom and the battery light comes on. the battery eventually drains and leaves me stranded wherever i happen to be. any suggestions to what it could be?
thanks alot.
karl schubach
thanks alot.
karl schubach
Three Wheelin'
Joined: May 2001
Posts: 1,641
Likes: 0
From: Hudson Valley NY
Corroded relays, fuses and bad grounds cause electrical phantom problems.
Low battery voltage or bad grounds, overcharging and undercharging alternator can cause an erratic computer control. Any input sensor, example, TPS, CTS (coolant temperature sensor), relay can malfunction or give a wrong reading under these conditions. With the ignition off turn on the headlights for one minute after one minute turn the headlights off; connect DVOM set to DC volts, negative lead to negative post, positive to positive. A fully charged battery should read 12.6 volts. Anything lower than 12.3 volts, you will need to charge the battery, with the main cables disconnected. With the meter connected to the battery positive-to-positive, negative-to-negative, start the engine; turn on the headlights, heater fan, and radio. This puts a load on the system. You should see 13.5 volts to 14.4 volts. Anything under 13.5 volts could be a bad alternator, bad diode, bad ground or high resistance in the positive cable. Anything over 14.5 volts is an overcharging alternator. With the car still running, lights and accessories still on, place the negative lead of the meter to the negative lead of the battery post, place positive lead of the meter to all of the ground wires. No more than 0.10 volts should read on the meter. A good ground is 0.00 or 0.01 volts. If you have 0.20 or more you have a bad ground wire. You can use a jumper wire for the negative lead of the meter to the negative lead of the battery post, so you can reach the engine compartment. Just be careful and use common sense when working in the engine compartment when the engine is running. You should check the alternator output directly at the alternator with the car running and all accessories on.
Make sure you have the decimal in the right place on your meter.ck the belt adjustment.
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
Low battery voltage or bad grounds, overcharging and undercharging alternator can cause an erratic computer control. Any input sensor, example, TPS, CTS (coolant temperature sensor), relay can malfunction or give a wrong reading under these conditions. With the ignition off turn on the headlights for one minute after one minute turn the headlights off; connect DVOM set to DC volts, negative lead to negative post, positive to positive. A fully charged battery should read 12.6 volts. Anything lower than 12.3 volts, you will need to charge the battery, with the main cables disconnected. With the meter connected to the battery positive-to-positive, negative-to-negative, start the engine; turn on the headlights, heater fan, and radio. This puts a load on the system. You should see 13.5 volts to 14.4 volts. Anything under 13.5 volts could be a bad alternator, bad diode, bad ground or high resistance in the positive cable. Anything over 14.5 volts is an overcharging alternator. With the car still running, lights and accessories still on, place the negative lead of the meter to the negative lead of the battery post, place positive lead of the meter to all of the ground wires. No more than 0.10 volts should read on the meter. A good ground is 0.00 or 0.01 volts. If you have 0.20 or more you have a bad ground wire. You can use a jumper wire for the negative lead of the meter to the negative lead of the battery post, so you can reach the engine compartment. Just be careful and use common sense when working in the engine compartment when the engine is running. You should check the alternator output directly at the alternator with the car running and all accessories on.
Make sure you have the decimal in the right place on your meter.ck the belt adjustment.
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
Rennlist Member
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Joined: Jun 2001
Posts: 6,469
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From: Ball Ground, GA
Annual maintenance:
1) Open the hatch and remove the tool panel. Remove the carpet and spare tire cover. Remove the spare, tools, etc. Open the battery box.
2) Disconnect the ground cable from the chassis. Remove the negative battery terminal. Remove the positive battery terminal. Remove the battery. Use extreme caution to avoid getting battery acid or residue from the top of the battery or cables in your eyes. Goggles are recommended. If the battery brushes against your clothes, you can end up with holes.
3) Clean the battery terminals until they are bright and shiny. Dull gray won't do. If it is possible to check the fluid level in the battery, do so. Fill to the rings. Wash the battery. Put it on a charger if possible.
4) Clean the battery box, removing all corrosion. Wash the battery box. After it dries, touch up the finish as required.
5) Clean the battery cable terminals (all three) until the connection area is clean and shiny. Dull gray won't do. Clean the connection area on the chassis.
6) Clean the speedometer/odometer drive electrical connector in the right forward corner of the spare tire well (pre-'90 models).
7) Raise the hood (bonnet). Remove the protective cap from the jump start terminal and pull the protective plastic cover from the terminal. If you are missing the cover or the cap, replace them - they are critical items. If there is no cover, water will enter the connector and corrode the connectors and wires, causing serious electrical problems.
8) If the connections are clean and tight, replace the cover and cap. If the connections show any corrosion or looseness, disassemble, clean and reassemble.
9) Jack the right front of the car so that you can SAFELY work underneath.
10) Remove the ground cable that runs from the right side of the engine to the chassis. Clean both ends of the cable, and check for corrosion in the cable. Clean the connection points, and replace the cable.
11) Check the heavy cable connection on the starter solenoid for looseness or corrosion.
12) Check the moderately heavy wires on the same connection for corrosion and breaks inside the insulation. These carry the charging current from the alternator.
13) Check the small yellow wire connection for corrosion and tightness. This wire triggers the starter solenoid.
14) Lower the car. Fold back the carpet and open the central power panel cover.
15) Check the main power connections at the top of the panel for tightness and corrosion.
16) Check the ground connections above the panel for tightness and corrosion.
17) If you are having electrical problems, remove each fuse and reinsert three times. Check the fuses for the proper value, and examine them visually for damage or corrosion. Test them if desired. Later cars have a fuse tester built into one of the relays.
18) If you feel that it is necessary, remove and reinsert each relay. This won't be easy, as they fit very tightly in some cases. Porsche sells a relay removal tool, but I am not impressed. It is basically a pair of pliers with square jaws, and will pull the relay apart. One tool that works well is a spark plug boot removal tool. It is a thin but strong sheet metal tool shaped like a "J" with the top 1/8" bent over at a right angle. You can put this angle under the bottom edge of the relay and lift it out - IF the battery is disconnected!
19) Close the panel and replace the carpet.
20) Replace the battery. Install the positive cable. Install the negative cable. Put a drop of oil on each nut. Connect the chassis end of the ground (negative) cable. Lubricate the battery box latch and close the box.
21) Lubricate the wiper drive shaft and hatch latch mechanism.
22) Clean the spare tire, jack and tools. Put a couple of large heavy plastic garbage bags in the spare tire well for emergencies, along with a tube of hand cleaner and some paper towels. Install the cover and carpet.
23) Lubricate the jack, and put it inside the tool panel. Clean and lubricate the tools. Reinstall the tool and tool panel.
24) Reset the clock and radio presets.
A couple of hours of quality time with your 928 once a year can forestall many problems.
1) Open the hatch and remove the tool panel. Remove the carpet and spare tire cover. Remove the spare, tools, etc. Open the battery box.
2) Disconnect the ground cable from the chassis. Remove the negative battery terminal. Remove the positive battery terminal. Remove the battery. Use extreme caution to avoid getting battery acid or residue from the top of the battery or cables in your eyes. Goggles are recommended. If the battery brushes against your clothes, you can end up with holes.
3) Clean the battery terminals until they are bright and shiny. Dull gray won't do. If it is possible to check the fluid level in the battery, do so. Fill to the rings. Wash the battery. Put it on a charger if possible.
4) Clean the battery box, removing all corrosion. Wash the battery box. After it dries, touch up the finish as required.
5) Clean the battery cable terminals (all three) until the connection area is clean and shiny. Dull gray won't do. Clean the connection area on the chassis.
6) Clean the speedometer/odometer drive electrical connector in the right forward corner of the spare tire well (pre-'90 models).
7) Raise the hood (bonnet). Remove the protective cap from the jump start terminal and pull the protective plastic cover from the terminal. If you are missing the cover or the cap, replace them - they are critical items. If there is no cover, water will enter the connector and corrode the connectors and wires, causing serious electrical problems.
8) If the connections are clean and tight, replace the cover and cap. If the connections show any corrosion or looseness, disassemble, clean and reassemble.
9) Jack the right front of the car so that you can SAFELY work underneath.
10) Remove the ground cable that runs from the right side of the engine to the chassis. Clean both ends of the cable, and check for corrosion in the cable. Clean the connection points, and replace the cable.
11) Check the heavy cable connection on the starter solenoid for looseness or corrosion.
12) Check the moderately heavy wires on the same connection for corrosion and breaks inside the insulation. These carry the charging current from the alternator.
13) Check the small yellow wire connection for corrosion and tightness. This wire triggers the starter solenoid.
14) Lower the car. Fold back the carpet and open the central power panel cover.
15) Check the main power connections at the top of the panel for tightness and corrosion.
16) Check the ground connections above the panel for tightness and corrosion.
17) If you are having electrical problems, remove each fuse and reinsert three times. Check the fuses for the proper value, and examine them visually for damage or corrosion. Test them if desired. Later cars have a fuse tester built into one of the relays.
18) If you feel that it is necessary, remove and reinsert each relay. This won't be easy, as they fit very tightly in some cases. Porsche sells a relay removal tool, but I am not impressed. It is basically a pair of pliers with square jaws, and will pull the relay apart. One tool that works well is a spark plug boot removal tool. It is a thin but strong sheet metal tool shaped like a "J" with the top 1/8" bent over at a right angle. You can put this angle under the bottom edge of the relay and lift it out - IF the battery is disconnected!
19) Close the panel and replace the carpet.
20) Replace the battery. Install the positive cable. Install the negative cable. Put a drop of oil on each nut. Connect the chassis end of the ground (negative) cable. Lubricate the battery box latch and close the box.
21) Lubricate the wiper drive shaft and hatch latch mechanism.
22) Clean the spare tire, jack and tools. Put a couple of large heavy plastic garbage bags in the spare tire well for emergencies, along with a tube of hand cleaner and some paper towels. Install the cover and carpet.
23) Lubricate the jack, and put it inside the tool panel. Clean and lubricate the tools. Reinstall the tool and tool panel.
24) Reset the clock and radio presets.
A couple of hours of quality time with your 928 once a year can forestall many problems.