Would this work to test if CDI is working?

[bpu699]

I see lots of posts on here where the care either won't start, or dies. The CDI is always a possible culprit, but no easy way to check.


Has anyone tried to use one of those pocket "live wire sensors" they sell at Home depot? You know the one. Looks like a pen, and if you lean it against an active live wire it lights up. I believe this uses inductance to tell if a line is "live."

Could one just use this while cranking to see if the the coil is getting juice, and if juice is flowing out of the coil?

Easy to do, cheap, mobile. I will try this at home this weekend. Seems like it would be a great way to exclude the coil from the problem list, and it doesn't require hooking up a strobe timing light either.

Thoughts?

Bo

[heimtun]

They make specific "induction" testers for spark plug wires - your local car parts place should carry them. I carry one is each of my old cars - it is an easy first test if car doesn't run.

[bpu699]

Originally Posted by heimtun

They make specific "induction" testers for spark plug wires - your local car parts place should carry them. I carry one is each of my old cars - it is an easy first test if car doesn't run.

I have one of those also. Tough to use on a 930 as it won't fit under the intercooler...

Thought the other tester would be easier? Will test this weekend and post.

Bo

[Jonny H]

I doubt this will work as most contactless mains detectors only work on AC. The CDI box outputs ~360V DC.

if you really must test the CDI output use a scope with a 100V probe to test at the coil primary. Remember, it's 4 times the RMS US mains voltage so be careful! DO NOT USE A MULTIMETER. My colleague spanked himself last week on a CDI box despite a first class degree in electronics and 20 years experience! It hurts (apparently) but I'm not about to try it!

Anyhow, this is hardly worth doing as ignition coils are easy to test with power off and wires disconnected. Should have less than 1 ohm on the primary (+ and - connections) and possibly 10000 ohms from - to the HT terminal. Open circuit and it's bad.

If the coil is good and the CDI is making the classic high pitched whine but you have no spark, the only external thing left to check is the distributor signal. On an early car with Kettering points this is a 12v signal, going to ground when the points are closed. On a later car with a VR (star wheel)' this is a sine wave (sort of) of up to 50v pk-pk (increases with RPM) crossing zero volts as the trigger. Both tests will require an assistant to crank the engine whilst scoping the signals.

Summary:

No whine. Check 12VDC to CDI. If that's ok the CDI box 360v supply has failed. Replace CDI.

Whine, no spark. Test coil. If coil ok, then test dizzy input. If ok, then trigger circuit or output circuit has failed, replace CDI.