I have a 1990 740GLE that up untill now has been very reliable. Recently, the battery will draw down after an hour or so of driving rendering the car dead.

If I recharge the battery it will run for a while and then die again. It can be jump started but once the jumper cables are remove it will die if the RPM's go below ~1200.

I took the battery and the altenator to Autozone and they both tested fine, both show perfect performance.

I checked all the fuses, belts, wiring, cleaned the battery terminals, all to no avail. It just seems like the altenator is not chrging the battery although it tested fine at Autozone.

What am I missing???

 

Get a 2nd opinion from a real shop that knows auto electrical.

If you're handy with a VOM you could do it yourself; shout back if you are.

 

Does your model have an external (not on the alternator) voltage regulator? If so I would check it. The other thing you can do if you have a electrical meter is test the system yourself. Get a meter $10.00 at radio shack. With the car running put the leads on the battery terminals and if the reading is not above about 12.5V - 13V you most likely have a bad alternator.

 

I consider myself a pretty good DIY'er so I am open to any suggestions. I believe the regulator is on the altenator.I have a digital volt meter and as soon as I get the battery off the charger and put it back in the car I will check the readings. Thanks for the tips and I will report back.

 

As gedaggett said, with the car running attach your Volt meter to the battery terminals. Ck the voltage; should be in 12 to 12.5 volts range.

Now manually turn the throttle to raise the engine to various rpm speeds (in neutral/park of course!); ck the voltage at about 1500 and again at 2500 rpm. Voltage should go up; max out at no more than 14.5V. Make sure it doesn't go back down to nominal battery voltage (about 12.5 or less volts) at elevated (above idle) rpm ranges.

What about your alt idiot light? Is it working? With the engine NOT running, turn the key to the ignition (Run) position; it should be on here, and then go off once you start the engine.

 

OK,, here are the results:

Battery after charging - 12.7v
Battery installed in car engine off - 12.7v
Battery engine at idle 11.57v
Battery engine at 1000RPM - 11.57
Battery engine at 2000RPM - 11.56


Seems like a bad altenator. Would it be wise to just replace the regulator to save $?

 

The alt could be good. Does the 'Batt' idiot light come on while you're cranking the engine?

 

Those voltages are too low. I had the same problem and changing the regulator solved the problem (warn brushes).

To be on the safe side you could always take the same measurement (at idle) between the large red wire lug on the back and the case to rule out a bad connection.

http://www.fcpgroton.com/product-exe...ategory_id/111

Some people are able to replace the regulator while installed in the car but I took my alternator out to clean it at the same time with electrical spray.

 

Yeah the battery light is on all the time. Dim at first and brighter as the battery draws down.

 

[QUOTE=bubba240;172119]

To be on the safe side you could always take the same measurement (at idle) between the large red wire lug on the back and the case to rule out a bad connection.
QUOTE]

I will check that befor I head over to Carquest for a regulator. They have them for 41$.

 

Really getting frustrated now. So I measure the back of the altenator and get 41 volts DC. I measure several times to be sure and it was that high. I then go to Carquest and get a regulator and slap that in and measur again and its 36 vdc! And after a test drive the battery still draws down, no charging going on. I'm thinking it may be time to replace the altenator.

 

Those voltages from the red lug to the case don't sound right. I am not real familiar with this model (others help me out please) but isn't there a ground wire from the alt to the engine block? Is that wire there?

By just replacing the brushHolder/Regulator assembly you're essentially betting that the stator and field windings, bearings and slip rings within the alt are still OK.

If you take out the brush assembly and can see that the brushes are worn out, then that's likely a safe bet. Were the brushes worn out?

I am a bit surprised your red charging light has been on throughout all this but you still weren't sure if you had a charging problem... did you believe the Autozone guys over the idiot light?

Bottom line: sounds like either bad alt and/or some serious wiring issues.

 

The ground wire on my car is blue, it goes from the very bottom of the alt case to the engine block.

 

Do they have a complete alt rebuild kit for that car. If the new alt is not too cost prohibitive just slap a new one in there. It will eventually need replacing anyhow and you could just consider it a preemptive strike.

 

Yeah the ground is connected correctly. I will grab an altenator later today and see what happens. The wiring should be fine, no changes to the car or anything else that would impact wiring.

 

Another something to verify - make sure you have battery voltage on the small red wire post with the key on. This provides the excite voltage to start the charging process.