Tachometer - Impala Tech
Electrical & Wiring Troubleshooting electrical problems

 
LinkBack Thread Tools
post #1 of 11 (permalink) Old 12-01-2013, 10:43 AM Thread Starter
Senior Member
 
Join Date: Aug 2009
Location: Lake of the Ozarks
Posts: 109
 
Tachometer

I have a 1964 Impala with a factory tach. The car car has been converted to a HEI ignition system. is it ok to re-attach the tach or will it blow?
dbuchholtz is offline  
Sponsored Links
Advertisement
 
post #2 of 11 (permalink) Old 12-01-2013, 11:45 AM
Moderator
 
Join Date: Jan 2006
Posts: 12,381
 
There should be a Tach terminal on your HEI cap. Plug the tach into that terminal and it should be the same signal that it got from the points distributor.

Big Dave
Big Dave is offline  
post #3 of 11 (permalink) Old 12-01-2013, 02:00 PM
Senior Member
 
Join Date: Apr 2013
Location: Canada
Posts: 325
 
You will get different answers. I've spoken to guys who say an HEI killed their factory tach, others say they've run an HEI for years with no issue.
Darth is offline  
 
post #4 of 11 (permalink) Old 12-01-2013, 02:57 PM Thread Starter
Senior Member
 
Join Date: Aug 2009
Location: Lake of the Ozarks
Posts: 109
 
Guess there is only one way to find out. Plug it in and if it blows....open up the catalog, get out the credit card and buy another one.
dbuchholtz is offline  
post #5 of 11 (permalink) Old 12-01-2013, 03:59 PM
Moderator
 
Join Date: Jan 2006
Posts: 12,381
 
The factory tach can not be killed by the HEI as it is designed to drive a factory tach IF and ONLY if you wire it up correctly. In digital devices polarity is as deadly as over voltage. Which brings up the next installation tip.

Any time you are working on any electrical circuit on your car un-bolt and disconnect the positive battery terminal. This gives you time to double check your circuit as electricity works at the speed of light. If you got a woopsie in your circuitry it is far cheaper to find it before you plug it back in.

I have also read on line where someone has plugged in their tach to three different HEI distributors and their tach doesn't work. I would at that point assume that the results will be the same for the fourth HEI, but no they tried a fourth distributor anyway and the tach still didn't work. This is an example of one of natures laws or in the words of John Cleese as spoken in the world famous parrot sketch "that is a dead Parrot!"

Once blown a tachometer will not work again until repaired. If it worked before the HEI was installed with your old points distributor and you plugged it into the tach terminal of your HEI and it stopped working I would suspect while plugging it in you also contacted the power supply wire for the HEI and fried your tach doing so. That wouldn't have happened if the battery had been disconnected and an inspection mirror used to see if the female crimp on terminal you used on the wire is contacting any of the adjacent terminals because it is too wide. It is better to discover this before the power is supplied as you can fry the HEI module as well as your tach.

Big Dave
Big Dave is offline  
post #6 of 11 (permalink) Old 12-01-2013, 05:44 PM
Senior Member
 
Join Date: Apr 2013
Location: Canada
Posts: 325
 
As I said, you will get different answers!
Darth is offline  
post #7 of 11 (permalink) Old 12-01-2013, 05:48 PM
Senior Member
 
Join Date: Apr 2013
Location: Canada
Posts: 325
 
I saw this on another forum, written from someone at American Auto Wire who make repro harnesses for our old cars-

"Most Original OE analog tachs WILL NOT work with an HEI distributor, period. You need a tach filter in series between the tach wire and the distributor. We field at least 10 calls like this every month. Occasionally, and I mean occasionally, we hear about this one or that one working, but more often than not, they simply do not work at all, or read way high or way low (I have never had one work personally). The waves that the points emit are different then the pulse that the HEI puts out. That is why EVERY single big cap style HEI equipped Chevy car had a tach filter on them (other GM's also. My 1977 Pontiac Can Am had one as well). Be it Corvette, Chevelle, Nova, or Camaro (Heck, even the VEGA and Chevette), they all used them from the factory. I just went thru this with a 70 Monte Carlo and a 1966 BB Impala. Both were new harness installs with factory in-dash tachs and both were using a big cap HEI distributor. Neither would not work at all. Put an NOS 74-79 style tach filter in series between the tach and the brown lead on both cars, and viola, the tachs worked perfectly!"
Darth is offline  
post #8 of 11 (permalink) Old 12-01-2013, 06:07 PM
Moderator
 
Join Date: Jan 2006
Posts: 12,381
 
It still won't blow the tach up. The filter is an LC circuit and is still sold today across your local Chevy Service Parts counter.

A pulse of electricity is a pulse, whether it be a square wave or a spike of energy that needs a RMS calculation to determine it's true potential. If you stay within the parameters of voltage and amperage and retain polarity the tach will survive being connected.

The filter was sold not for the tach acting, up but for your FM radio getting static in the lower bands. If you listened to the NPR and Church radio frequencies. It is called a radio filter in the parts book.

Big Dave
Big Dave is offline  
post #9 of 11 (permalink) Old 12-02-2013, 06:27 PM
Moderator
 
Join Date: Jan 2007
Location: Lewisburg, Ohio
Posts: 2,909
 
I had to replace the tach filter on my 88 Monte Carlo SS . The local GM dealer had it.
dadstoy is offline  
post #10 of 11 (permalink) Old 12-05-2013, 10:13 AM
Senior Member
 
Join Date: Jul 2008
Location: Michigan
Posts: 155
 
As I understand it, you only need the filter if you change from a points to a HEI distributor. Why would you need a filter on an '88? By then weren't they all HEI?
sz0k30 is offline  
post #11 of 11 (permalink) Old 12-09-2013, 07:14 AM
Moderator
 
Join Date: Jan 2007
Location: Lewisburg, Ohio
Posts: 2,909
 
Yes it was a HEI but it was the small one with a external coil. Filter was mounted on the firewall.
dadstoy is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Impala Tech forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome