[T3] Charging issue still

[Jim Adney]

On 12 Jan 2011 at 11:39, Tim Keohane wrote:

> I have finished re-assembling/wiring my '73 T-3  F I Automatic.
> It sat for 30 +/- days and fired right up. The volt meter wired  
> directly to the battery went 12.4 v to 12.1 v when I turned the key on  
> and the pump ran, then went to 12.4 v again. Engine turned over  
> quickly and fired within a few revolutions. While idling cold and R P  
> M's elevated the voltage went up to 13.9-14.0 v. As i loaded the  
> system with lights fog lamps etc the voltage dropped to 13.0 13.2 v  
> under full load. I drove the car for about 3 to 4 miles and as the  
> engine temp. approached normal the voltage on the battery dropped.   
> When I completed the 3 to 4 mile loop the battery charge would barely  
> read 12.4 at 40 mph and no load. The battery is 3 years old and seems  
> to hold a charge. ( We left it for three months connected ( clock only  
> drawing power ) it turned over and fired off as if it had been sitting  
> for a few days.) I'm thinking the generator has an issue that is temp.  
> sensitive.

It's appropriate to check your generator installation, to make sure 
the cooling hole is pointing straight down, which happens when the 
dash on top points straight up. When correctly installed, the 
terminals will point slightly to the left of center.

You're correct in thinking that these numbers are too low, assuming 
that you aren't loading the system with more current load than the 
generator is rated for. The generator is rated at 30 A. If you exceed 
30 A, the system voltage will sag since you are now draining the 
battery. In a properly operating system, the generator will provide 
all the operating load, plus a little bit extra to top up the 
battery.

With low load and the rpm above 2000 rpm, you should measure above 14 
V at the battery. The voltage that you reach and hold at, as you 
increase the rpm, is the regulating voltage and is controlled by the 
voltage regulator. Voltage regulators wear out, allowing the 
regulating voltage to drop. Once this happens, the only cure is to 
replace the voltage regulator. Don't, however, confuse low voltage 
due to a worn out VR with low voltage due to excess current drain.

Note that the voltages with the engine off, with the key on but the 
engine stopped, with the starter cranking, and with the engine at 
idle may tell you something about the battery, but they tell you 
nothing about the charging system.

To check the charging system, measure the voltage with normal loads, 
with the engine warmed up, and at medium rpm. That's the only voltage 
you need to measure.

Your mention of fog lights makes me wonder if your test was a fair 
one, because it's easy to add loads to our cars which can overwhelm 
its electrical system.

Do you already have my charging system faq, Tim?

-- 
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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