Coil test for 951 ignition coil

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Coil test for 951 ignition coil

Postby mvz944turbo on Mon Oct 15, 2012 6:55 pm

Does anyone have the factory primary and secondary resistance for a Bosch 951 coil?
Mike Van Zandt
89 951 racecar
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Re: Coil test for 951 ignition coil

Postby pdy on Thu Oct 18, 2012 6:12 pm

Hi Mike:

No replies? Hmmmm...

I know that coils vary from model to model somewhat, but you should be able to determine if the coil is open or shorted.

On my two 911s, I measure about 0.8 Ω on the primary, and 4.6 kΩ on the secondary (Isolated from the car's electrical system).
If the primary is somewhere around ½ Ω to 3 Ω, and the secondary is anything from 1 kΩ to, say, 20 kΩ, at least you can likely
determine that it's not shorted or opened.

Now, there could be insulation breakdown that will cause it to not work (or not work well - insufficiently high spark Joltage).
Unless you have a Hi-Pot tester, that's going to be difficult to determine. There is a reasonably good substitute test you can perform;

BE VERY CAREFUL - THERE IS HIGH JOLTAGE INVOLVED!
1. Connect the primary switched terminal to ground (on the car, or a 12V battery's negative post).
2. Connect the primary hot terminal (31?) to a length of wire with a bare end.
3. Attach the HT wire that goes from the coil to the Distributor to the coil secondary.
4. Place the other end of the HT wire so that its metal part is about 2mm from the ground used in step 1.
5. STAY AWAY FROM THE HT WIRE!
6. Quickly brush the bare end of the wire in #2 across the 12V battery plus terminal. Do Not let the bare end stay in contact with the 12V!
7. If the coil is working okay, you should see little sparks from the HT wire to ground (across that 2mm air gap).

Again, be very careful ofthe High Joltage on the secondary side! :shock:

This does not totally simulate the ignition process, but I have used it many times to see if a coil is okay.

HTH
Paul D. Young http://www.deadpetsracing.com/
PCA Nat'l Club Race Advocate
Now: '78 911SC, '79 911SC, '04 955-S, '06 997-S, '88 924-S, '16 92A
Past: '74 914, '78 911SC, '78 928, '80 924-S, '82 931, '86 930, '03 996-4S
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