Refurbishing Bosch black coils

Refurbishing Bosch black coils

We receive a lot of emails about properly refurbishing the original Bosch Black Coil. To sum it up, the hardest part is finding coils that are worthy of this task in the first place. Two key questions we often answer. All our coils are properly tested before they leave our workshop.

What is the proper resistance for these original coils, both 6v and 12v?


For an original VW 6v coil, your resistance in ohm’s should be (not less than) 1.6 ohms on both the short 3 1/2 inch coil and 4 inch coil. Voltage should read with coil installed and the key turned on in the ignition at 4.8 volts minimum or a little more for the 6v coil.

For the 12v coil 4 inches in length, the resistance in ohms doubles to 3.2. Naturally resistance doubles from 6v to 12v. However, with the 3 1/2 inch 12v shorter coil your minimum resistance will read 4.2 ohms minimum resistance. The shorter 3 1/2 inch 12v coil resistance reads 4.2 ohms resistance do to additional internal coil windings. This gives the shorter original VW 12v coil a jump in resistance from the longer version. The 12 v coil installed and key turned on in the ignition should read 9.6v minimum or greater.

Finally, for the 6v and 12v VW coils if both resistance (ohms) and voltage checks out to factory specs you are good to go!

What type of life expectancy do the restored German and NOS coils have?

The life expectancy of a 6v or 12v coil varies. Coils basically have no moving parts other than electrical current. The only way a VW coil will go bad is a sudden excessive surge of electricity that could fry the coil windings. Another way would be crossing the positive and negative wires to the coil. This will fry the wire loom and cause a break in the interior loop of the coil.

As always, please contact us if you have any questions about our restoration process.

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Photography: Timm Eubanks

Eric Shoemaker
Eric Shoemaker


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