CDI Troubleshooting
 

I suspect my CDI unit in my '78 930 has gone bad, it "sings" at key on put there appears to be no spark. Is there a way to tell a good unit from a bad one without swaping it out with a known good unit?

Thanks

Eric

I can give you the info from my haynes manual for the cds. Say's to check for voltage at terminal 4 at the coil ( the ht terminal). Remove the coil to distributor ht lead at the distributor and support it 10 mm from a ground on crankcase.Do not ground out or hold too far from ground or damage to coil will occur inside the coil.If no spark occurs then remove the ignition coil connections connections and connect an ohm meter between the coil terminals 1 and 15 ( primary ) ; there should be a resistance of 0.4 to 0.6 ohms. Repeat the check between terminals 1 and 4; there should be a resistance of 650 to 790 ohms.To check the current draw by the cds unit, connect a ammeter into the the No. 15 wire from the ignition switch, then disconnect wire no. 1 at the distributor.With the ignition on, There should be a current of 1.0 to 1.9 amps. If outside this range a replacement cds unit should be obtained.
Thats all the says about the cds unit.so I hope this will help you out. Check you rev limiter in the distributor for a short also. could be the culprit.let us know what you find. good luck;)

I don't think the same exact tests as the early 3-pin CDI unit should be applied to the magnetic pickup, 6-pin version of the CDI-unit.

The two pins attached to the green shielded cable should be tested at the CDI-cinnector for a reading of 600 Ohms +/- 10% ... the green cables age and crack -- going 'open' so the CDI-unit doesn't get the trigger signal it needs, and are a known trouble spot on post-77 ignition systems.

The reading across the 2 pins at the that go to the distributor (pick up) is 575 ohms, new wire to the distributor, so that checks out. is there any thing that can be checked at or in the CDI box ? Transistors? Diodes?

Thanks

Eric

Eric,

Sounds like the green cable is OK. You probably should pop the distributor cap and look for any kind of problem with the 'star' magnetic reluctor ... looseness or wobble on the shaft, proper operation of the centrifugal advance mechanism, etc. It wouldn't hurt to remove your distributor, disassemble parly, clean and lubricate with engine oil. Loose, worn bushings could cause excessive clearnce between the pickup coil and reluctor and reduce the signal Voltage to the CDI-unit ... which is very low -- less than 1.0 Volt when operating normally!

First, check the coil windings ... 0.4 to 0.6 Ohms from terminals A to B (or terminal 1 -- if an older coil), and 650 to 790 Ohms from terminal B (or terminal 1) to terminal 4. Inspect the coil for any signs of oil leakage or overheating.

Be sure to clean all grounds at the wiring harness ... at the coil, CDI-unit, tranasaxle to body ground strap, etc.

If you have access to an oscilloscope, you could check for the presence of 400 to 460 Volt pulses accross terminals A nd B at the coil while cranking the engine ... their presence would rule out CDI-unit entirely as a problem. If you don't find the output pulses, disconnect the 6-pin connector at the CDI-unit, and check at terminals 7 and 31d (from the green cable) for a low-level pulse waveform while cranking.

Thanks for the tips I'll give it once over tommorow AM.

After reading warrens reply I guess I should have looked closer at the book.Good thing that guys like warren and John are around to watch over us dummies or we would all be WALKING. I will be interested in your findings because I have the same ignition on my car.The haynes book has little info for the turbo system, but I have found that most is applicable to the basic maintainance,Thanks for the correction,Warren,I would hate to tell someone to burn up their cds Unit.:o