<VV> Dwell/Tachs

[Padgett]

Being somewhat of a bug on instrumentation when I found that my aulden 
dwell tach was not working properly, I looked into modern replacements. 
Since I have several Actron instruments, the CP-7677 seemed appropriate 
since in addition to being a dwell tach, it has a host of other DVM-type 
functions.

Unfortunately there were some limitations and interesting discoveries: 
First the "low battery" indication is just a flag and even though the first 
9V battery I grabbed was only putting out 5v, the display was nice and 
bright, just rong (have advised then that making the low battery warning 
blink might be a Real Good Idea since most DVMs will fade the display 
before they become inaccurate).

The next issue was more serious: when connected to the Corvair coil, the 
display went bananas, blinking just about all the characters that could 
blink and never staying anywhere long enough to read. Called Actron and 
found that this is a known issue and initially they recommended a CP-7605 
analog meter (bought on of those and works well but is limited to RPM 
(0-2500 for 6 cyl) Volts, and Dwell. Very simple and seems reasonably rugged.

But back at the CP-7677. Rather than return since it has a number of 
features I wanted for my computer cars, I tried some experiments: coax 
cable didn't work. Wrapping the meter in tinfoil and holding behind the 
bumper helped but was erratic. Finally constructed some new leads from 
shielded twisted pair  with about 3" bare for the gator clips and that 
worked. Now have a stable display readable to +/- 10 rpm.

Have suggested to Actron that a meter advertised to read dwell and rpm 
should be able to even in a noisy environment and they seemed receptive. 
Time will tell.

However unless you have a real need for a DVM and are able to construct 
your own shielded test leads, I would suggest the less expensive CP-7605 
might be a better choice than the CP-7677

http://www.actron.com/product_category.php?id=252

Anyone have experience with other similar test equipment ?

Padgett