I have a 1989 Vette. When I start it cold, the vehicle charges normally around 14.0 to 14.4 volts. After it gets hot (about 10-15 min) the volts steadily decrease to 11.5 to 12.5 volts on the dash reading. When I drive the car it charges about 13.4 volts at 2000 RPM. I verified the dash reading at the alternator and the dash guage is accurate to about .5 volts, meaning I lose about .5 from the alternator to the dash.
So far I have replaced the alternator 3 times (AC Delco), replaced the battery, voltage regulator plug at the alternator, ohmed the wires from the alternator to the block behind the battery and cleaned all terminals. I even ran a separate ground from the alternator the frame. Still the same thing, I get around 11.5 to 12.0 volts at the dash at idle and 13.4 while driving. Sometimes the dash will go completly dim and the volts drop to 8-9 volts, then the car wants to quit. I did notice that the car has an aftermarket hypertech chip in the ECM, but I'm told that that should not be a factor since the charging system does'nt run though the ECM. I'm used to GM cars charging at 14.6 volts, what could be the problem? I'd appreciate any advise.

 

My 86 is bang-on consistant at 13.7 volts. Any idea what your AMPS are doing?

 

Unless you have an unusually heavy current draw while idling, your alternator should put out 14.7 volts cold and drop to 13.7 volts hot. The alternator cannot generate rated current at low idle speeds and heavy current draw will cause the alternator output to drop. Normally there is 0.3 volts drop across the ignition switch contacts and the dash voltmeter reads 0.3 volts low and it will read even lower if the cluster is supplied with below 13 volts. One thing I would check is the voltage difference between the alternator output terminal and the positive battery terminal, engine running. The alternator has a fusible link in its output terminal to the battery and if this link has been stressed, you could have unreasonable resistance (like 0.5 to 1.0 ohms) from a damaged fusible link causing voltage drop and low charging voltage. If you have a high power audio amp and have it cranked up while idling, this could be the cause of your low idling volts.
Another thing to check is to measure the battery terminal voltage (engine and electrical loads off) after it has set overnight. This will tell you the state of charge. 12.0 volts and lower, discharged. 12.9 volts and higher, charged. A discharged battery will draw large current from the alternator and could be your cause of low volts. If discharged, charge your battery up with a battery charger for at least 24 hours, DO NOT charge the battery with the alternator as that will shorten the life of the alternator.

 

My fan is hardwired to run as soon as the key is turned...bypasses the ecm.

 

The fan motor draws 14 amps which isn't enough to drop the alternator volts at idle.

 

Not trying to hijack thread but you should really consider putting a switch on fan(s), that is if you trust yourself to monitor temps. Not so much because of current draw but there are times like in winter that your car needs to warm up to optimal temps without fans running. If not they would've just hard wired them from factory and not wasted time with all the relays and sensors and such. Just my .2

 

I did consider putting back to stock wiring, but being up here in the great white north, she goes into hibernation in November and wakes up in May...so warm up isn't really an issue.

 

I did some tests today on the car. The battery volts cold are 12.7 volts, all good there. After I verified that I began to check for battery draws. I pulled all the fuses to try and see a voltage spike and the only one that made a difference was the ctsy fuse. I looked into what is on that circuit and unplugged the blower motor. I saw a voltage spike again, about a full volt from 12.5 to 13.5. I then checked the wiring to the blower motor by looping it with a multimeter. It blew the 10 amp fuse immediately. Wadda you think? Short in the blower motor causing a draw? By the way, no crazy amps hooked up to the stereo.

 

Your 10A VOM fuse hit the silk because the fan motor draws 14 A running and the startup transient current is higher than 14A. Check and make sure that the previous owner may have installed a smaller pulley on either the crank or the alternator to reduce the parasitic load HP which will make the alternator turn slower. What is the voltage on the alternator output terminal engine idling? and what is the voltage on the battery positive terminal engine idling?
Your dash voltmeter readings appear normal except at idle speed. See if you can borrow a clamp on dc ammeter and measure the alternator output current engine idling. If you can find one, also measure the current out of the battery when you unplug the alternator, engine idling.

 

I know a guy with an ampmeter, so I'm gonna borrow it today. How many amps should the alterantor normally pull? Yesterday the car cut off on me completely 2 times when I got on the gas. Just shut off like someone turned off the ignition. Then it starts right back up. Then I saw the SES light come on while driving. I'm gonna see if I can pull a code, I don't know if one was set because it came on then went off quickly. Could these problems be be related? The alternator volt output starts out at around 14.4 and the positive terminal reads like 13.9 the dash reads like 13.5 but then all three readings steadlily decreases as the car warms up. Isn't it kinda weird that when I unplug the blower motor everthing spikes? I checked all the pulleys, there all correct for the car.

 

I had loose connector (Bolt and nut) in my alternator wiring.

 

You mean the wires that run from the alternator to the junction block behind the battery?

 

What do you mean by, "everthing spikes when you unplug the fan motor" ?
Also, there is no specific alternator current that you should expect when you measure the alternator output with an ammeter. When you first start the engine the current will be high because of the transient high discharge from the starter motor current, then the current will taper lower until the alternator is supplying all the car electrical loads. I will measure my 87 and report what my alternator current is.

 

I'd like to hear what the solution to this is... I've been having weird alt problems lately. The volts have gone up to almost 18 while driving (replaced volt. reg. twice). and when I ran a second ground for the alternator, the volts dropped to a constant 11.8...

 

I meant the nut on the backside of the alternator. There is a cable, and the nut was loose. My car was 1985 and it had SI-type alternator.

 

I meant that when I unplugged the alternator, I saw a 1.0 volt increase in the alternator output. I thought there might have been a short in the blower motor, so I went ahead and replaced it. It did'nt help, still the same thing. The car seems so run fine otherwise so I really would'nt care, but I just hate to see the volt guage on my dash drop down to 11.9 volts. I still gotta run that test with the dc meter you said to get, so I'l let you know how that goes.

 

The battery voltage goes UP 1 volt when you unplug the alternator?
If so, then you have a defective alternator. Sounds like a shorted diode in the alternator, but if so, you would experience a very hot alternator and a growling alternator and a discharged battery overnight. Unplugging the alternator turns off the alternator. The alternator is supposed to maintain 14.7 volts across the battery (engine running of course) and unplugging a good alternator will cause the battery to supply all of the electrical current to the car and the voltage should drop to 13 volts for a fully charged battery down to 12.0 volts for a battery that is discharged. Be certain that when you measure the alternator output current, that you know if current is flowing OUT of the alternator or INTO the alternator. A bad diode will cause current to flow INTO the alternator.

 

sorry I screwed up in my last reply. In meant to say that I unpluged the blower motor and that's when I saw the volts jump a full amp.

 

The fan motor draws 14 amps and unplugging it shouldn't do anything measureable to the alternator voltage, the voltage regulator in the alternator should still keep the output voltage at 14.7 volts (alternator cold). I still suspect the alternator unless you measure that it is delivering its rated current, approximately 100+ amps. The clamp on ammeter measurement will tell if the alternator is heavily loaded or not.

 

Here's a dumb question. The alternators I've been getting are rated at 105 amp. I read a reply in someone elses thread that the alternator should be 125 amp. What does the car actually call for?