Tachometer troubleshooting Anyone?? - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Electrical
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 04-19-2008, 07:30 PM
Registered User
 

Last journal entry: How sweet it is!
Last photo:
Join Date: Jul 2007
Location: canada
Posts: 56
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Tachometer troubleshooting Anyone??

Wondering if anyone here has ever had success trouble shooting a tachometer before?

My stock tach in my 1979 Trans am just stopped working! IT would no longer read anything. The needle just sits at zero.

So i aquired a good used one from a parts car that we "KNOW" was working before we removed it. After installing the replacement, it works "BUT" is completely in-acurate. For instance at idle it bounces from 4000k to 6000k.

Since we know this previous tach was working what else can i check?
The HEI-cap is pretty much brand new. Its a Pro-form rebuild kit with high voltage coil and module. The original tach use to work just fine with this cap.

I have enclosed 2 pics. of the tach. the back connecter has a piglet wire that plugs into the tachometer prongs. with 2 seprate wires leading into the tach-plug-in-prongs can i assume that one of these wires acts as a ground and the other leads to the disributor cap?

I assume that im dealing with a possible wiring issue here but can't imagine how one tach would just die completely, replace it and am now getting wacky readings with the replacement tach?

the wacky reading confirms that the tach is getting some-sort of signal. just NOT correct?

any ideas or suggestions? thanks!
Attached Thumbnails
Click image for larger version

Name:	tach.jpg
Views:	390
Size:	16.0 KB
ID:	29357   Click image for larger version

Name:	back tach.jpg
Views:	482
Size:	14.4 KB
ID:	29358  

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 04-19-2008, 10:50 PM
BORTI's Avatar
BLOWN 357
 
Last photo:
Join Date: Apr 2008
Location: SAN PEDRO
Age: 43
Posts: 126
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Tach Problem

It sounds like a grounding issue.Being that it reads some sort of signal.If there is an aftermarket ignition box you may need a tach adapter.If there is not just check all wires.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 04-19-2008, 11:01 PM
Registered User
 

Last journal entry: How sweet it is!
Last photo:
Join Date: Jul 2007
Location: canada
Posts: 56
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
NO aftermarket ignition box involved. Just a standard HEI distributor that houses an aftermarket coil and module.

NOTHING had changed between the time the tach work and the time it croaked. So a little baffling.

I will look into your suggestion in the morning. thanks
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 04-20-2008, 05:25 AM
Jake_Dragon's Avatar
Member
 

Last journal entry: New Carb
Last photo:
Join Date: Jul 2005
Location: Central Florida
Age: 49
Posts: 772
Wiki Edits: 0

Thanks: 2
Thanked 2 Times in 2 Posts
Do those cars have a tach filter? If its bad it could cause your problems.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 04-21-2008, 08:33 PM
Registered User
 
Join Date: Aug 2006
Location: Alexandria, VA
Posts: 1,756
Wiki Edits: 0

Thanks: 4
Thanked 65 Times in 63 Posts
The stock GM tach wiring on my '75 pickup has three wires: ground, 12 volt power, and tach signal. If yours has only 2 wires, it must be grounded through the case/frame. I would verify that this ground point is connected to the instrument panel wiring, and that you have 12 volts on the other terminal. On my tach the ground is black, power is red/pink, and the input signal is brown.

Most GM tachs used a tach filter. You can buy them off the net for about $50, or make your own for about $5-10. Lack of a filter may screw up the readings, but usually won't make the tach fail.

There is also a tach circuit board on the back of the tach. It counts the ignition pulses and converts them to an analog signal to drive the needle. These boards are soldered or jumpered different based on whether the car is a 6 or a V8.

Some of the circuit boards develop breaks in the solder traces, which causes erratic readings. My tach sticks at about 3750 on a cold start, until the truck is warmed up. I think this is due to a bad solder trace that finally warms up and makes contact.

Bruce
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 09-26-2009, 01:25 PM
Registered User
 
Join Date: Sep 2009
Location: Florida
Posts: 4
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Electrical Issues - Tachometer

It looks like about 100 days since the last post - but I'm having a similar tach issue.

At idle the tach shows the 600 - 750 rpm it should... for about 5 minutes, then the needle starts bouncing around 3000 - 4000. If I turn on all the accessories it may calm back down, but may not either. At 70mph / 2300 rpm it also seem to even out.

I have a 1967 Mustang with a 351W out of a 69.
I replaced the points distributor with a Accel breakerless (71204e) about 4 years ago. I am running a matching Accel Coil. Accel 10.8mm wires. 100 amp single wire alternator.

First the module in the distributor burned out. I replaced that, but noticed that the coil was getting extremely hot. That is when I first noticed the Tach Issue.

As the coil has never run that hot before, I figured that maybe the coil had caused the module to burn, so I replaced the coil with another matching Accel coil. This did nothing to alleviate the tach issue. I tested the setup with a new AutoMeter tach - the new tach does exactly as the old did.

I have a new Painless wiring harness throughout, but checked the wiring anyway. I ran a new ground wire from the distributor to the Block eliminating connectors and shortening the length and increasing the size of the wire, as the one on it looked like it got a bit hot at one time.

I checked all grounds from block to body (I have 2 something like 2 gauge grounding cables).

I checked the ground to the tach and switched it to another ground just to make sure, but this made no difference.

I pulled the alternator and had it bench tested under load for about 10 minutes, but the alternator shop swears that the alternator has no issues.

So now I am at a loss - I mean that is really all there is on a 67: Alternator, coil, distributor/module.. and of course the Tach strung together with some high dollar wire...

Any help would be appreciated...

Thank You

Lee
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 10-19-2009, 10:13 AM
Registered User
 
Join Date: Oct 2009
Location: Texas
Posts: 1
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Tach prob

I found your post through a search, but am havuibng the same problem with a 67 as well. Any luck on a solution? What have you tried? Maybe we need to run a larger wire for the tach signal. I too run a mallory distributor with pointless ignition.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 10-20-2009, 12:20 AM
Registered User
 
Join Date: Oct 2009
Location: Massachusetts
Posts: 11
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by lfmgtc59
It looks like about 100 days since the last post - but I'm having a similar tach issue.

At idle the tach shows the 600 - 750 rpm it should... for about 5 minutes, then the needle starts bouncing around 3000 - 4000. If I turn on all the accessories it may calm back down, but may not either. At 70mph / 2300 rpm it also seem to even out.

I have a 1967 Mustang with a 351W out of a 69.
I replaced the points distributor with a Accel breakerless (71204e) about 4 years ago. I am running a matching Accel Coil. Accel 10.8mm wires. 100 amp single wire alternator.

First the module in the distributor burned out. I replaced that, but noticed that the coil was getting extremely hot. That is when I first noticed the Tach Issue.

As the coil has never run that hot before, I figured that maybe the coil had caused the module to burn, so I replaced the coil with another matching Accel coil. This did nothing to alleviate the tach issue. I tested the setup with a new AutoMeter tach - the new tach does exactly as the old did.

I have a new Painless wiring harness throughout, but checked the wiring anyway. I ran a new ground wire from the distributor to the Block eliminating connectors and shortening the length and increasing the size of the wire, as the one on it looked like it got a bit hot at one time.

I checked all grounds from block to body (I have 2 something like 2 gauge grounding cables).

I checked the ground to the tach and switched it to another ground just to make sure, but this made no difference.

I pulled the alternator and had it bench tested under load for about 10 minutes, but the alternator shop swears that the alternator has no issues.

So now I am at a loss - I mean that is really all there is on a 67: Alternator, coil, distributor/module.. and of course the Tach strung together with some high dollar wire...

Any help would be appreciated...

Thank You

Lee
The old cars used to have a resister in line with the coil or a resister wire going to the coil from the key switch. It droped the voltage when running so as not to over heat the coil. For starting a wire came from the starter selinode marked R (hot in starmode) and went to the + on the coil after you let go of the key, power went through the resister wire to the coil at around 7+ volts . Obviously this could mess up your tach.
Reply With Quote Quick reply to this message

Recent Electrical posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
If you do not want to register, fill this field only and the name will be used as user name for your post.
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:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

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


Similar Threads
Thread Thread Starter Forum Replies Last Post
Correct Tachometer hookup newdad1 Engine 3 05-30-2007 09:14 PM
Sun Tachometer Darryl Dudley Electrical 1 11-25-2006 10:50 PM
tachometer help Turbo Terror Electrical 7 02-07-2005 05:34 PM
tachometer manufacturer johnb1 Electrical 2 04-11-2003 07:00 AM
Tachometer wiring fred56 Electrical 5 03-13-2003 06:27 AM


All times are GMT -6. The time now is 06:43 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.