digital meter
Thread Starter
Racer
Joined: Oct 2015
Posts: 414
Likes: 407
From: Meadville Pennsylvania
digital meter
My actron digital meter will not read RPM on my 75 with HEI distributor, it flashes wildly and will not settle down. I have tried the tach wire both in front of and after the capacitor and the coil ground.In all instances my meter negative was tried on the carb, engine ground and alternator ground. It works great on my 70 Dodge A body with 340 engine, electronic distributor and external coil using coil minus and carb ground. Are these new digitals not capable of functioning on the HEI units? My original dash tach works but I suspect it of being 200 to 400rpm high and I just want to verify that discrepancy and have an accurate tuning tool.
Thanks for any help, Jerry
Thanks for any help, Jerry
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
On the engine, under the driver side of the HEI should be a 'tach filter' it's off the tach output line/switch point of the black driver module on bottom of the HEI unit.....so place the test probe to the other end of that tach filter, and it's same spot as your OEM tach on the dash.....
There are two capacitors in the circuit, one inside on the dizzy base near the black driver module, and another in the tach filter.....both of which can fail, and cause who knows what problems.....
There are two capacitors in the circuit, one inside on the dizzy base near the black driver module, and another in the tach filter.....both of which can fail, and cause who knows what problems.....
Thread Starter
Racer
Joined: Oct 2015
Posts: 414
Likes: 407
From: Meadville Pennsylvania
On the engine, under the driver side of the HEI should be a 'tach filter' it's off the tach output line/switch point of the black driver module on bottom of the HEI unit.....so place the test probe to the other end of that tach filter, and it's same spot as your OEM tach on the dash.....
There are two capacitors in the circuit, one inside on the dizzy base near the black driver module, and another in the tach filter.....both of which can fail, and cause who knows what problems.....
There are two capacitors in the circuit, one inside on the dizzy base near the black driver module, and another in the tach filter.....both of which can fail, and cause who knows what problems.....
Thanks, Jerry
Race Director
Joined: May 2006
Posts: 18,662
Likes: 2,563
have you tried plugging the wire coming from the tachometer directly to the distributor to see if it effects the factory tach reading?
http://repairs.willcoxcorvette.com/w...stallation.pdf
http://repairs.willcoxcorvette.com/w...stallation.pdf
Thread Starter
Racer
Joined: Oct 2015
Posts: 414
Likes: 407
From: Meadville Pennsylvania
have you tried plugging the wire coming from the tachometer directly to the distributor to see if it effects the factory tach reading?
http://repairs.willcoxcorvette.com/w...stallation.pdf
http://repairs.willcoxcorvette.com/w...stallation.pdf
Thanks for the advise, Jerry
Thread Starter
Racer
Joined: Oct 2015
Posts: 414
Likes: 407
From: Meadville Pennsylvania
Finally got back to the Vette today and the RPM problems with my meter. I followed a suggestion from a member of the MOPAR forum and constructed a Faraday shield to filter out the unwanted electrical pulses from the distributor. I constructed a Faraday shield from a pair of 2 1/2 ft lengths of 4 conductor telephone wire and grounded the shield wires to the engine. It worked beautifully and I was able to set my Idle with a meter that read correctly. Thank you 67 Dart for helping me out with a GM problem. Thanks to everyone on this forum for their suggestions and for all the help on other problems I have had.
Jerry
Jerry
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
Glad it worked out for you. Your premise that a failed tach filter would either show an open circuit or a grounded circuit was faulty. The most common result would be that the filter just would not do anything...which would leave the distributor signal "dirty", i.e. excessive pulses for what rpm the ignition should be producing. Installing a working tach filter might have fixed your problem as well. But, you have a solution and you don't really need a tach filter for your upgraded tach.
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Thread Starter
Racer
Joined: Oct 2015
Posts: 414
Likes: 407
From: Meadville Pennsylvania
Glad it worked out for you. Your premise that a failed tach filter would either show an open circuit or a grounded circuit was faulty. The most common result would be that the filter just would not do anything...which would leave the distributor signal "dirty", i.e. excessive pulses for what rpm the ignition should be producing. Installing a working tach filter might have fixed your problem as well. But, you have a solution and you don't really need a tach filter for your upgraded tach.
Thanks for your post, Jerry