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Re: Caught in traffic, Voltage start dropping to 11.3V Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Thu, 6 Mar 2003 08:16:10
In Reply to: Caught in traffic, Voltage start dropping to 11.3V, Dean Tobani, Wed, 5 Mar 2003 19:00:41 Members do not see ads below this line. - Help Keep This Site Online - Signup |
A few basics-
The purpose of the battery is to start the car, and on occasion, provide a cushion for occasional current load spikes. But it's not there to supply current when the engine is running.
When the engine is running, the alternator should supply all the current requirements of the car - engine electronics, fans, headlights, radio. It also needs current left over to re-charge the battery, to recover the power used to start the engine.
The battery will only charge if the alternator voltage is above around 12.6 volts. Below that, and you are pulling current from the battery. You can do this for short periods, but you can't do it for long.
The Voltage Regulator is temperature compensated. Shoving current into a battery heats it up. If the battery is hot (say underhood on a hot day), you don't want to charge it as fast as when it's cold. So the VR adjusts its output somewhat so as not to 'overcook' the battery. The VR doesn't know the battery temperature - it just knows its temperature, but it assumes the battery is about the same. So if the car is sitting in traffic, the underhood temps will increase, and the VR will drop the output voltage.
BUT - on a cold day, the VR will put out 14 volts or so. That may drop into the high 13's (13.8?) on a nice spring day. Maybe 13.0 on a scorching hot July stuck in traffic. But not below 12.6, and definately not 11.3 in winter.
At idle, the alternator has to work much harder to put out current, because it's turning slower. In winter, with defrosters, seat heaters, headlights, sitting in traffic can present a BIG electrical load.
Lu brings up a good point - the EDU is NOT a precision electrical device. I've seen as much as 0.3 volt difference between the EDU and the Battery voltage. What's more, that can vary with electrical load, because the EDU shares some load paths with other systems. The best way to tell - open the hood and stick a good Digital Volt Meter (DVM) on the battery terminals, and compare that voltage with the EDU. Do this with very few electrical loads, and do it again with everything you can think of - defroster to HI speed, seat heaters, rear window defroster, headlights, fog lights, everything on. Now you know what the real voltage is, and that's the one that counts.
When driving, you should see voltages in the 13 volt range - preferably upper 13 volts. At idle, it shouldn't drop below 13 volts, and should never drop below 12.6 - otherwise you're using the battery to run the car, and it can't do that for long. 11.3 Volts is a warning sign - if you check with a DVM and see that's corrrect, you need to fix it NOW.
Low voltage IS NOT the battery's fault. The battery can't supply more than 12.6 volts. It's the job of the alternator to supply that current. If the alternator can't put out enough voltage, don't replace the battery.
You said you 'rebuilt the voltage regulator' - I assume that means you put in new brushes, but kept the old VR electronics. The problem probably lies in the VR/Alternator. Step 1 is to put in a new VR. This is the cheapest solution. It may not solve the problem. The brushes on the VR run on metal pickups on the armature. If the brushes were worn, so are the pickups. If they're shot, you can't pass much current through them.
If you can't get voltages above 13 volts under any conditions, then you may have a bad diode in the alternator. You can replace just the diode, but replacing the whole thing is best at that point.
If replacing the VR doesn't fix the problem, find yourself a rebuilt alternator. Make sure you get a 115Amp alternator, not a 80Amp. 115 should be stock for '92, but I don't assume anything. I've gotten 115A quality Bosch rebuilds from Sobstory for about $130-$140 after core charge - I'm sure you can get similar prices from any of the other folks that advertise on this BB. Don't pay the dealer $600 for the same part.
posted by 192.249....
, Thu, 6 Mar 2003 08:16:10 <-- Viewing This Message, Fri, 7 Mar 2003 08:59:09
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