Mag.compatible Tach?

Hey all! Just wondering what the best tach is for use with a Supermag IV? I have an Autometer Recall tach mounted in the car atm but it seems to have "spikes"when the engine is running and i don't think it reads too accurate! I've asked an auto elec. how to rectify it and they say the autometer pick-up or"signal coverter" is getting stray current from the mag and is causing interferenceThey say the entire wiring from the pick-up to the tach should be wrapped in a braided "steel sheath" and then earthed to the chassis! Does this sound right?? Some other racers say forget the tach altogether and put a staging RPM air-ram on the hat and a RPM activated shift light and your set! Suggestions anyone?......Jason.

 

Jason,

It could be interference, but we do fine without shielding on our PM44.
Make sure your tach is for inductive style pickups and connected correctly according the manual.
Try another pickup if available. We once had one that was defective.
Keep the trigger wire away from your plug wires. 90 degree crossings will not give much interference, but if you wrap them together in parallel lines, that could introduce trouble.
Shielded wire can help. Connect the shield to the chassis at the same point where you connect the tach ground. Isolate the other end of the shield (near the pickup) to avoid stray current loops.
Calibration can be checked with a function generator. A square wave signal will do. Just make sure you start with low amplitude signals so you don't damage the tach. Ours was spot on by the way.

HTH.
Marcel

 

Mag Interference!!

Marcel, thanks so much for your reply! it's good to get some feedback from someone that's experienced a problem first hand,(One of the excellent things about this site!!)Firstly the pickup came with the tach when i bought it new for a S/MAG,so the tach should be fine.I have a front mag drive on this engine so the Ign.leads hang down and run along the side of the block so to speak.All wires from the inductive pick up run along the top chassis rail which is about 12 inchs away from the leads and mag at the closest point.When the engine is started and rolling over at idle,lets say it idles around 1800 rpm and hunts to say 2000, watching the tach without touching the throttle it will "spike" up to say 2500-2600 without doing anything!!I checked out the Don Zig magnetos web site last night and the "Interferance" thing is explained in detail. To be honest i've got somewhere else to be lookin' other than the tach when i have the pedal mashed anyways so maybe the stage RPM "air-ram"and shift light is the way to go... Thanks Jason.

 

Jason, check your pm's.

 

Use a MSD 8132 signal converter, I have one on my SMIV with a regular Auto Meter tach and it works perfectly.

Chris

 

Tach Interference!!

Chris! That's what i was lookin' for!! Thanks, Will do!! Jason.

 

do a search in the archives, tons of post on this subject, general opinion for 100% reliability seeemed be cable drive tach. that's what i did and was very happy with it

 

Tach interference!!

Thanks for that vwsamba! Many people have the same idea about the cable drive tach,i personally have never used one, but those that do swear by them! What make or brand is yours? I've searched a few retailers here in OZ with no luck. I must go through the archives here on ITA and see what i find,thanks again!! Jason.

 

mine is a Moroso 9000rpm tach with mechanical memory, i don't think you can buy them new anymore, check your local classifieds or swapmeets, that's where i got mine. best of luck.....

 

Mechanical Tach!

O.K thanks for the help Guys!!!!