TheSamba.com :: View topic - Voltage drop to points on a 12 volt system

Thu Apr 04, 2013 7:18 pm

Visiting with a friend this evening that is a vertual encyclopedia on anything automotive when the discussion turned to ignition points.
He was saying that a 12 volt system running points should only have 6 volts at the distributor. American cars used a resistive wire from the coil to the distributor to drop the voltage. I've never heard of this before. Does VW do this as well through either a resistive wire or something internal in the distributor?

One of the reasons I ask is I just burnt up a set of points after about four months dayly driving my bus. They were adjusted at .016 per VW specs, timing is spot on. But the other day I noticed an ocssional miss, then it just quit. Luckily I had a spare used set of points and got home alright. Been spoiled with the electronic points replacement the last few VWs I've owned. :)

Thu Apr 04, 2013 7:32 pm

12v across the points is normal in a 12v car.

Thu Apr 04, 2013 7:42 pm

I remember working on older cars with points that had a ballast resistor. On Chryslers it was mounted on the firewall and on Chevy's I believe it was built into the coil to drop the voltage to about 9 volts after the car was started. The VW ones might be in the coil.

Thu Apr 04, 2013 10:26 pm

I remember my old '64 Corvar had a ballest resistor too. It shorted out one night and burnt the points up just as I was pulling up in front of my house. Wouldn't start the next day. Took me a few days to figure it out and only then after several calls to my dad explaining the symtoms. :)

I'm going to put a meter on the positive side of my bus coil in the morning and see whats what.

Thu Apr 04, 2013 10:51 pm

Its 12 volts, or more likely a bit more, whatever your battery/generator/voltage regulator is putting out. It's a direct connection from the positive circuit to the coil.

Thu Apr 04, 2013 10:55 pm

Aren't proper coils for A/C VW's internally resisted? I always thought they were, for the exact reason stated by the OP.

Thu Apr 04, 2013 11:22 pm

cdennisg wrote: Aren't proper coils for A/C VW's internally resisted? I always thought they were, for the exact reason stated by the OP.
Yes, but if you're talking about measuring the voltage at the condenser side of the coil, or at the points, that will be constantly changing. It's grounded when the points close, and varying as the charge on the primary builds up before the next spark discharges. Plus the residual from the condenser which is designed to prolong the spark discharge time.
Here's a typical primary circuit voltage waveform. Note the 300 volt peak. That's not the secondary coil spark, that's the primary which causes the secondary to discharge.

[img]www.picoauto.com/automotivetopics/waveforms/primary.gif[/img]

Fri Apr 05, 2013 10:52 am

There should be 12 volts at the coil (terminal 15). Any needed resistance is inside the coil itself. There is no difference in points used on 12 volt systems from those used on 6 volt. You could conceivably use a 6 volt coil on a 12 volt system by adding external resistance (same value as the coil primary), but I'd recommend using a 12 volt volt instead.

Fri Apr 05, 2013 6:51 pm

Measured 8.5 volts out of the negative side of the coil today. I have the Bocsh blue coil. So I guess that answere my question.

Sat Apr 06, 2013 8:56 am

If you are burning up points you might want to put in a new condenser, they are there to reduce the arcing of the point contacts as they open and close. I even had a brand new one I put in a Subaru once that was bad and the car ran awful.

Sat Apr 06, 2013 9:23 am

If your only gapping the points that might be part of your problem.i dont care what the point gap is, its the dwell that really matters.what ever it takes to get 50 deg + or- 2 degrees using a dwell meter.would you set your timing with the motor not running, static time it or do you use a timing light to set it with the motor running.using the dwell meter is the same principle, its a pain in the ass removing the damn cap several times to get it right.vw should have copied chevy's dist cap it had a little window you slid up and used a small allen wrench to adjust the points with it running..or scrap the points\condinser and go petronix

Sat Apr 06, 2013 4:07 pm

I always hook up the dwell and timing ligt after messing with pionts. I set my points at .016 to start and adjust from there. I can never remember witch way to go when adjusting the dwell number depending on point gap and usually go the wrong way before getting it right. :D

Sat Apr 06, 2013 5:20 pm

plane_ben wrote: I always hook up the dwell and timing ligt after messing with pionts. I set my points at .016 to start and adjust from there. I can never remember witch way to go when adjusting the dwell number depending on point gap and usually go the wrong way before getting it right. :D

I think it was Jon Muir who taught me that "if the dwell is too high, the points are too far open". That is how I remember it from oh so long ago.