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Old 06-12-2002, 04:06 PM
Posts: n/a
1973 SL 450 Ign. Coil Resistance

I am trying to find the reason for a backfire and missing on this car before it warms up. I checked timing with a light and the firing seems to be skipping hits randomly. The higher the RPM the more misses. I checked the coil primary resistance and it is only .5 ohm. The secondary is around 9.6K ohms.

Can anyone tell me if this is bad? The parts store in town says the book primary resistance is supposed to be about 1 ohm.

The points and rotor look fine. The dist. cap is clean and I just removed some minor carbon deposits from the contacts. Plug wires look ok and ohm out around 5 K ohms a piece.

Am I looking at the right area???

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Old 06-12-2002, 04:23 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
I don't think so. The TDM shows primary resistance foir the coil 0 221 122 001 at 0.38 - 0.43 ohm. No figure for secondary winding.

The usual problem on D-jet cars that backfire when cold is a combination of too little timing and/or mixture too lean. Check your timing with the vacuum off the distributor. The spec 5 deg ATDC is for the vacuum on the distributor AND working. By removing the line the vacuum should increase at least 10 deg. So, if you take the line off it should be at 5 deg BTDC minimum to be in spec. I prefer to set those cars to zero to 3 deg BTDC with vacuum and if the vacuum don't work I will add ten degrees to that. So with the line off I am advising 10-13 deg BTDC.

This should give better than 15in vacuum at idle. Both timing and fuel mixture should be increased till you get maximum idle vacuum. Set while warm.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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