Crane optical sensor with MSD

[jameshtaylor]

Hi Steve,
THink I recall you speaking favorably about use of the Crane optical sensor (without the xr700 amplifier) with an MSD 6AL on the early (74) cars. I tried to do this once, fouled something up, and ended up adding the amplifier, which worked well. I'm now trying to make an A/F meter work with my setup and can't get an rpm reading to the A/F software. Most of the possible problems have been eliminated, and I'm left thinking the combination of the crane amplifier and the MSD may be a problem. Crane's literature suggests the MSD/optical sensor combination, so I'm thinking of plunging in. Is my memory that you like the combination correct? And do you use a ballast resistor as they suggest?
Thanks, Jim Taylor

[Steve]

Jim,

So,...to be clear, you are trying to add an AFR meter to your car?

Generally speaking, ignition systems have no effect on AFR systems provided that proper wiring practices are followed. MSD's do generate a lot of EMI so this means that the ignition wires must be suppression ones and all AFR wiring must be properly routed and distanced to minimize interference. Further, the MSD case should be grounded.

Oxysensor voltages are quite small and easily overwhelmed by ignition interference so its imperative that all connectors have continuity from the sensor to the display and routed away from the ignition system.

I would suggest reviewing your ignition installation and grounding practices as well as confirming continuity in your AFR meter as places to start.

[jameshtaylor]

Yes, trying to add A/F ratio meter. I have a fine A/F signal, but I'm getting only a bit of data about rpm... something is getting thru but is more like noise and unreadable.
Connections and grounds are good.
The MSD is grounded as prescribed by them, though the case itself isn't. Do you really mean an extra wire from the case to ground?
And the line from the O2 sensor leaves the header a good 2' from the MSD box then heads thru the tunnel as usual.
Thanks again, Jim

IMHO, you need a case ground with MSD's due to the RFI they generate.

It sounds like either an improperly shielded sensor cable and possible interference from the plug wires. Remember, continuity is very critical when dealing with such small voltages so check those connectors. You may need a scope to troubleshoot this and make certain everything is working

[jameshtaylor]

An update. Redid all connections incl grounds, tossed the Crane box and optical "points" and installed a Pertronix. Something I did got the AFR meter working perfectly! Thanks.