Current Draw

[jpetillo]

A few questions on current draw...

Does anyone knows the current draw from the coil? I saw for an MSD coil a quote suggesting that the ignition will draw 1 amp per 2 thousand RPM for a 4 cylinder. That would suggest 3.5 - 4 A max for us. Would this be the right ballpark number for our Bosch coils?

Also, what's the current draw for the starter solenoid (from the starter button) for the VW 12 V & 6 V starters?

Thanks, John

[SR Racing]

Does anyone knows the current draw from the coil? I saw for an MSD coil a quote suggesting that the ignition will draw 1 amp per 2 thousand RPM for a 4 cylinder. That would suggest 3.5 - 4 A max for us. Would this be the right ballpark number for our Bosch coils?

An average of that would be close, I would think. It will depend a bit upon dwell. The RPM will have little effect.

Also, what's the current draw for the starter solenoid (from the starter button) for the VW 12 V & 6 V starters?

Probably close 5-10 range. (solenoid only)

I can measure these tommorow if you really want to be close.

I assume you are trying to determine the wire gauge that you should use?
For function, the bigger the better. For practical use 12 to 14 is fine for those circuits.

The 12V line that runs from your kill switch to the dash should be 12. That circuit will typically power the ignition, solenoid, gauges, and rain lamp.

Wires from the gauge sensors (OP, Oil temp, etc.) can be 16 to 14. Same as the coil to tach wire and switch to rain lamp. All above should be standed wires and be "Automotive" wire, sometimes referred to as "primary wire".

Crimp well to spade (or slip on) connecters, (solder if you want) and heat shrink the ends. After things are screwed down put a dab of RTV on each screw/connector to prevent loosening and corrosion.

[jpetillo]

Jim,

Thanks for the numbers and recommendations. I was concerned for both wire size and connectors. Also thanks for the offer to measure the currents, but I think what you mentioned is in line with what I thought, so I'm all set.

John

[fvracer27]

John use large heavy wire so we can slow you down a bit

[jpetillo]

Mark, I'll forward your recommendation to the engineer. I'll let you know what she comes back with.

[racing stuff]

If you are going to apply a RTV to the screw or connection for your wire, do not use a type that is corrosive, as most typically are.
Open the tube and smell it, and it has a "vinegar" smell to it, than that is the one you do not want to use.
The formula/sport-racer wiring kits we sell, primarily use 16 gauge wire, but with one length (red) of 14 gauge for the main power distribution (not starter to battery or main ground).

[FVartist]

Rather than RTV, use dielectric grease on the connection and seal with shrink tubing

[brian]

Is there a simple way of bench testing a coil? I have asked this before but can't recall the answer. I have several used coils and would like to know if I should keep them.

[BLS]

Brian, from memory, ohm check between pos and neg terminals should be 3-5 ohms (need a good ohmeter for this) and between the center spark lead around 9K-10K ohms with a higher reading being worse than a lower reading.

A nice "blue" pop between the coil and lead held about a quarter inch from ground while cranking would indicate a decent coil i think.

I *think* this is about right...

Regards,
Barry

[FV80]

Barry's response is about right IIRC- HOWEVER, it does not address HEAT or VIBRATION. Either of which can cause real problems. You can use a heat gun to raise it up to around 125 degrees and you can BANG it on the floor with the leads attached to see if it changes the reading. For the secondary (the coil wire end), compare it to a known good coil rather than take our 'guesses' .

Steve, FV80

[SR Racing]

Barry's and Steve's are right.

Another dynamic issue that comes into play though: These measurements are fine for statically testing the coil, and if they are right, the coil will probably fire a plug. However, as compression goes up in the cylinder it requires more voltage to fire the plug.
When the plug fires at idle for example, it may do so at 8,000 volts or so. When the plug fires the coil, output is effectively shorted and that is all it produces. However, at higher compression the plug may not fire until say 15,000 volts . A coil that has been over heated, or just internally damaged from vibration etc. can arc internally when the cylinder pressures go up. So at the highest points of compression where you may require 16,000 to avalanche the plug, the coil arcs internally and you lose spark at the plug.

We built a distributor tester that actually spins the distributor with a RPM adjustable motor, a coil under test, and an adjustable spark gap to test the available voltage. Coils that seem to measure fine with the ohmeter may often fail. (and some that don't measure so well, might be fine.)

For $30 (Coil), if you are in doubt, junk it. Much better than wasting a session on track at $100+.

BTW, all the above applies to wires and the cap, etc. ie. The engine starts and idles fine. But when the cylinder pressures go up and coil voltage reaches higher levels, you can fire though a coil wire insulation to ground or through tiny cracks or carbon paths in the dist. cap. So most all of these failures won't show up until you are at higher RPM and load.

[SR Racing]

If you are going to apply a RTV to the screw or connection for your wire, do not use a type that is corrosive, as most typically are.
Open the tube and smell it, and it has a "vinegar" smell to it, than that is the one you do not want to use.

The smell is acetic acid that is expelled during curing (or even worse when exposed to water before curing.) This has always been an issue in the electronics industry (PC Boards etc.) , but after cure is not really an issue in our use. You can certainly use an alcohol (or other) based material, but you will not find a problem using any standard RTV on our connectors. After cure, standard RTV is ALMOST inert.

[racing stuff]

I still would not use an acetoxy type silicone on electrical components.