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Tue Dec 03, 2019 4:20 pm

So I did a stupid thing and left my ignition switch in the Run position with the engine off for about an hour. I know this was stupid, and I'm beating myself up for it, but it's done. Coil was quite hot by the time I found it, and although the car will start, it idles rough and my spark (which was quite bright, long, and blue a few weeks ago) is now short and yellowish. This led me to check my voltage at the coil, and I'm perplexed by what I'm finding.

Ignition on but car not running
Battery: 12.3 V
Terminal 1/15 voltage: 10.8 V (why isn't this the same as the battery?)

Car running:
Battery: 12.5 V
Coil terminal 1/15 voltage: 4.6 V (this seems far too low)
Alternator: 13.7V

Did I fry my coil? Any chance I did damage to anything else? I have an 034 distributor with points, and I've read that I might have damaged the points too.

Tue Dec 03, 2019 4:26 pm

virusdoc wrote: Ignition on but car not running
Battery: 12.3 V
Terminal 1/15 voltage: 10.8 V (why isn't this the same as the battery?)

Is that with the points open or positive wire at the coil disconnected?

Tue Dec 03, 2019 4:32 pm

Eric&Barb wrote: virusdoc wrote: Ignition on but car not running
Battery: 12.3 V
Terminal 1/15 voltage: 10.8 V (why isn't this the same as the battery?)

Is that with the points open or positive wire at the coil disconnected?

Not sure if point were open (I'm new to this distibutor--Bill Fowler just sent it to me a few weeks ago); this was with all wiring to the distributor in place. I guess if points were closed that could be voltage drop across the distributor?

Tue Dec 03, 2019 4:36 pm

Resistance of coil (terminals 1 to 15): 3.6 ohms
From distributor conductor (center) to either terminals 1 or 15: 9.2 k ohms.

Tue Dec 03, 2019 5:09 pm

You probably want to check the condition of the points as well — they might have gotten “fried” a bit (?). If so, you could try cleaning / filing them down. Or just replace them. Then set the appropriate dwell & timing, and see if that helps your car run correctly again.

Tue Dec 03, 2019 5:26 pm

Eric&Barb wrote: virusdoc wrote: Ignition on but car not running
Battery: 12.3 V
Terminal 1/15 voltage: 10.8 V (why isn't this the same as the battery?)

Is that with the points open or positive wire at the coil disconnected?

Regardless of non-running voltage, doesn't the 4.6 V at the coil seem far too low when the car is running? I'm not having any luck figuring out what that voltage should be.

Tue Dec 03, 2019 5:34 pm

virusdoc wrote: .....Terminal 1/15 voltage: 10.8 V (why isn't this the same as the battery?)
....Car running:
Battery: 12.5 V
Coil terminal 1/15 voltage: 4.6 V (this seems far too low)
Alternator: 13.7V
The voltage between #1 & #15 is low when running because it's an interrupted circuit, your volt meter can't keep up with the change between points open and closed values that fast, engine stopped it should be real close to battery voltage if the points are closed fully (depends where the engine stops, points open should be 0V).

There's plenty of places to lose voltage in a 40+ year old electrical system, a slightly dirty or loose connection at the starter, or fuse box, or one of the many other joints can add up to a significant drop once you add all the .0xx V's here and there up. A weak ignition switch can also cause a significant drop, especially once it's been on a while and warms up.
It's always time well spent to remove both battery cables (both ends) and the ground strap between the trans and frame for a really good cleaning and reinstallation on a clean surface with a little grease or vaseline to head off corrosion. Same deal for all the ring terminals on the starter, and the big spade junction under the LR seat (if you have one). Fuse box connections can also corrode internally but look fine, or get a little loose, that's the forward junction for everything in the car, remove and clean every terminal and spade, if they seem at all loose invest in a ratcheting crimper with the dies for uninsulated terminals and replace them. Voltage has to go from the back of the car all the way up to the front, then swirl around behind the dash before finally making the long trip all the way back again, that's a lot of feet of wire and connections, anything slightly weak in the circuit isn't helping.

I'll bet your alternator light glows faintly while driving in the dark, it's telling you there's a difference in voltage between the front and back of the car.

Tue Dec 03, 2019 5:45 pm

busdaddy wrote: virusdoc wrote: .....Terminal 1/15 voltage: 10.8 V (why isn't this the same as the battery?)
....Car running:
Battery: 12.5 V
Coil terminal 1/15 voltage: 4.6 V (this seems far too low)
Alternator: 13.7V
The voltage between #1 & #15 is low when running because it's an interrupted circuit, your volt meter can't keep up with the change between points open and closed values that fast, engine stopped it should be real close to battery voltage if the points are closed fully (depends where the engine stops, points open should be 0V).

There's plenty of places to lose voltage in a 40+ year old electrical system, a slightly dirty or loose connection at the starter, or fuse box, or one of the many other joints can add up to a significant drop once you add all the .0xx V's here and there up.

I'll bet your alternator light glows faintly while driving in the dark, it's telling you there's a difference in voltage between the front and back of the car.

Thanks for this excellent, informative reply. I bought an old car partly because I wanted to learn how these things work, and I guess f&%@ing something up is a powerful emotional stimulus for learning!

You are absolutely correct about my alternator light glowing faintly at night!

And the interrupted circuit explanation for the low voltage makes perfect sense. I assume if I had a fast oscilloscope I'd be able to see that tracing--but I'm only seeing the best average my old voltmeter can provide.

Tue Dec 03, 2019 6:12 pm

I opened the points and took a photo, below. There is a white crescent on the upper left aspect, but no scorching. I’ve never had a car with points before so I’m not sure what to expect. Does this look normal for points with <1000 miles on them?

Tue Dec 03, 2019 6:19 pm

Glad to hear that made sense, now you know what your next project is :D

Don't forget the grounds for everything too, they will be just as funky, electrons just want to go home (like ET), if the ground connections to the body are weak it doesn't matter how clean the positive terminals are, there's still a interruption in the circuit.

Don't skimp on the terminals or tool, those plastic covered ones you squish with pliers just aren't good enough, non insulated double crimp terminals and the dedicated ratcheting crimper are the best solution. If you don't like exposed terminals you can add some heat shrink tube over them after crimping.

Tue Dec 03, 2019 6:56 pm

virusdoc wrote: Resistance of coil (terminals 1 to 15): 3.6 ohms
From distributor conductor (center) to either terminals 1 or 15: 9.2 k ohms.
This is within the acceptable range suggesting your coil may still be okay.

As mention by busdaddy, you need to clean up your wires and connections between the battery and the ignition coil. Lots of points where resistance can reduce the current that makes it to the ignition coil.
Battery terminals
White junction block below rear seat.
Red #30 wire(s) at fuse box INPUT.
Ignition switch plug wires.
Black #15 wires at fuse box INPUT.
Black wires at #15 terminal of ignition coil.
The current has to travel that entire path before it reaches the ignition coil. Clean all those points and you should see a measurable voltage increase at the ignition coil.
For your coil voltage measurement, remove the #15 black wire from the coil that comes from the ignition switch. It will be the only black wire on the #15 terminal of the ignition coil that when removed from the coil has 12v at the end of the wire.
The voltage difference between the end of this wire and the battery is voltage loss due to resistance. Test the voltage at each of the points above and you can see which segment of the circuit is causing the greatest voltage loss. To get the most consistent voltage measurement you want a common ground. Unfortunately, after decades of rust chassis ground around the car can be up to 1.0v different. Use a battery jumper cable connected to the negative terminal of the battery as your mobile reference ground. Connect your voltmeter COM wire to this for your various tests. This will give you the most accurate measurements from the different points.
You will probably find the ignition switch is causing some of the voltage loss. Not unusual for an original switch or one that is decades old.

Voltage measurements taken on the #1 (-) terminal of the ignition coil will be influenced by the points being opened or closed. But the #15 (+) terminal side voltage measurements should not be influenced by the points (or not as much). It has to do with how much current is making it to that point in the circuit and how many other things are connected at that point pulling current away from that point. Your voltage readings at #15 should be only slightly less than the battery (with clean wire connections).

You pic of your points is not very helpful. Take a pic with the contacts OPEN. Here are points from the gallery with about 2000mi:

You can see the discoloration and raised point. The opposite contact is probably pitted. This is slight burning of the points. These can be cleaned up with a point file so they are flat once more.
New points will have no marks on the contact surface. They will be flat on both contacts for max contact area.

Get your wiring cleaned up so you get high voltage readings at the ignition coil and you may see the quality of the spark improve as well.

Tue Dec 03, 2019 7:02 pm

Plus in the end one should be using a dwell meter to check new points and check used points that are always going to have some pitting. Dwell is everything.

Tue Dec 03, 2019 7:09 pm

virusdoc wrote: Car running:
Battery: 12.5 V
Coil terminal 1/15 voltage: 4.6 V (this seems far too low)
Alternator: 13.7V
Was this the max voltage readings you got even when you raised the rpms?

At 2100rpms you should get a minimum voltage reading of 13.8v at the battery terminals. This is what is needed to recharge the battery.

This is with a fully charged battery (12.6v). If your battery was weak, the voltage readings will be low. Recharge your battery and check the voltage at the battery again. If your charging system cannot produce 13.8v it will eventually drain the battery or at best leave the battery partially charged all the time. Cleaning connections will help to raise the voltage readings here as well.

Tue Dec 03, 2019 7:39 pm

Thank you for all the useful suggestions. If you all weren’t so kind I’d feel a bit like a moron now. I have a PhD in microbiology and I’m accustomed to knowing what I’m doing. But this stuff is all new for me, and it’s humbling to feel like a student again. Nevertheless, I’m enjoying it. Thanks again.

Tue Dec 03, 2019 8:00 pm

Personally learned how to work on these old VWs before the internet well over 3 decades ago. Had some good VW folks that took the time to help a youngster back then. Still learning even now!

Think we can speak for most folks here, just passing it on...

Tue Dec 03, 2019 10:03 pm

More on dwell here:

https://www.thesamba.com/vw/forum/viewtopic.php?t=716571&highlight=setting+dwell

Fri Dec 06, 2019 3:05 am

Picture of your coil please, the oiless ones will definentely not withstand any abuse. May haps you burned some insulation in the coil allowing voltage loss across your primary and secondary making the coil defective, but functional.

Fri Dec 06, 2019 5:54 am

The coil and points are cheap and easy to replace.

Fri Dec 06, 2019 10:32 am

on one of my vws I added a relay to cure ignition circuit voltage losses. relay is near the coil, fed from the alternator positive. I got about as much voltage on the coil possible now.

For the biologist look up youtube for tutorials on points ignition...

https://youtu.be/LT4m2LLKwnE

You learned some tough subjects, learning ignition systems on old Bug will be easy for you!

Bug On, With Spark!

ps do you have info on fabrication of a Ag/AgCl electrode? you can pm me on that.

Fri Dec 06, 2019 10:35 am

Just adding to the ideas. If it is a spark issue, meaning you have fuel, air and compression. Also, seeing those not to bad looking points, my thought was, what does the rotor look like and has it been burned through?

Worth a look, if you are chasing spark! Also, be sure it spins when cranked. Seen a couple hold still!