Low Coil + Voltage (need a gut check here)

[DougB]

Hello all,

I just wanted to see if you all can give me a gut check on something I encountered this morning. So I was headed out to get the van inspected ('75 Westy, 2.0 FI engine) and while testing the backup lights I noticed they were dim. So that seemed to be due to the ballast resistor I've been running to protect the seemingly ever-so-delicate optical trigger sensor on my Mallory Unilite distributor. I think the resistor must've gone bad because on the other side of it I was only sending 5V to the coil. Interestingly enough the engine still ran fine.

So I bypassed the resistor and then my backup lights perked back up...but a quick check with the voltmeter showed 10.5V at the + coil terminal, and I thought it was supposed to be 12V. I can also check with a different voltmeter just to verify, but any thoughts on what could cause a small voltage drop like that?

Thanks in advance,
- Doug
_________________
'75 Campmobile (tin-top to SpaceRoof)
'73 Fastback
'52 BMW R67/2
'41 Zundapp KS600
'55 Puch SGS250
A very, very understanding wife

[Stuartzickefoose]

Voltage drop through ignition, and anything between the battery->ignition->starter->coil would cause a drop like that.

Why a resistor on the coil? Does the Mallory style require one? I just found this issue causing a bad running issue in a 70 ford f250 where the supply wire had a resistor and the electronic ignition was AFTER the resistor causing a low spark issue. Moved the wire from after to before and bingo had strong spark.
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Stuart Zickefoose

2011 Jetta Sportwagen TDi 6 speed manual

206-841-7324
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[DougB]

Mallory recommends the resistor as a way of evening out voltage spikes that could kill the optical trigger unit. With or without the resistor they seem to have a fixed life, though. I have a good VW dizzy with a brand new vac unit and a Pertronix set in there just waiting to go in...perhaps the next time Colin is in town I'll finally take him up on his offer to help install it.

- Doug
_________________
'75 Campmobile (tin-top to SpaceRoof)
'73 Fastback
'52 BMW R67/2
'41 Zundapp KS600
'55 Puch SGS250
A very, very understanding wife

[kreemoweet]

Under what circumstances did you make those voltage measurements? I.e., was engine running, and at what rpm?

The whole point of a ballast resistor is to reduce the voltage, and therefore the current, thru the coil. Whether a ballast resistor is required or
not, and what value it should have, depends on the particular coil.

Ideally, there should be no voltage drop at all from wire connections or switches, but there will always be some, and it will increase with the amount of current.
An unballasted coil should be receiving full battery voltage (engine not running) or full alternator output (13.5-14 volts) with engine running. Anything less is
caused by wire and connection imperfections and loss thru the ignition switch contacts.
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'67 bug: seized by the authorities
'68 bug: seized by the authorities
'71 kombi: not yet seized by the authorities

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[telford dorr]

At low RPM, the current through the primary of an ignition coil is limited by the primary winding resistance plus any added ballast resistance. At high speed, the current is limited by the primary winding inductance, which is a function of the number of primary winding turns and the nature of the steel in the coil core.

A ballast resistor is typically used with a special low resistance primary coil to act as a low speed current limiter. The advantage of using a ballast resistor in this case is that the heat from the resistor energy loss is radiated from the resistor, not the coil itself. On a stock VW coil, the resistance is all inside the coil. Thus, a stock coil should not use any external resistance, as it will further reduce the coil primary current, and thus the coil output. As I recall, the stock coil has a primary resistance of around 3.5 to 4 ohms, so the worst case coil primary current should be 4 amps ot less. As long as your ignition module can handle that, you should be good with no added resistance.
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[DougB]

Thanks for the input, all!

Thew readings at the coil were taken when the ignition was on but the engine not running, and then . again with the engine running. In both cases (and without the ballast resistor) I was getting 10.5V at the coil + terminal. My charging circuit, as read by a VDO volt gauge, is pretty consistently at 14V.

- Doug
_________________
'75 Campmobile (tin-top to SpaceRoof)
'73 Fastback
'52 BMW R67/2
'41 Zundapp KS600
'55 Puch SGS250
A very, very understanding wife

[Wildthings]

[cpd419]

The voltage to my coil when the engine is running and cold is the same as the output of the alternator, 13.5 volts. After running 5 minutes or so it slowly drops to a little over 10 volts. I haven't long drive tested it just idling about ten minutes. It's a Mexican Bosch blue coil with internal resistor. I'm guessing its right? I'm running points and a 009 dizzy. It runs fine after its warmed up.
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66 beetle

[telford dorr]

The voltage on the black coil wire on coil terminal 15 should ALWAYS be at battery voltage when the key is on. If not, trace it back to the fuse block, ignition switch, fuse block again, headlight switch (depending on the model year), then the starter. Somewhere in this chain you'll find a sudden voltage drop. Remove wires, buff and clean, reinstall (checking terminal tension).
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[Abscate]

[cpd419]

Thanks for the replies. I have a toggle switch to kill the coil and fuel pump (electric) and I'm guessing that could be the problem. The car runs great with points, I'm changing to electronic ignition and I don't know if it's enough voltage.
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66 beetle