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1980 454 tach wiring problem

W

WesternGate

New member
Hey all, I have a 1980 454 with thunderbolt ignition. I just replaced the original gauge cluster, all but the amp meter were not functioning, with new faria gauges. I'm having trouble getting the tach to register. With the key on, the tach needle jumps slightly but while the motor is on it doesn't move. On the wiring harness the brown wire is designated as the tach signal. I cant find much in terms of wiring diagrams online. Any input is greatly appreciated!
Thanks!
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Well, I APPEARS to be wired correctly. I've replaced a number of tachs over the years and have found that the cylinder selector switches to be "touchy" to set. Two issues...the "detents" aren't all that definite on some tachs and on some tachs it is not all that clear which end of the pointer is the "arrow" end. In the latter case, you can end up with the switch 180 degrees out of proper setting.

Did the tach you removed work with the wiring in the dash???
 
Thanks for the reply. The original tach did not work. Out of all the gauges, only the amp meter worked. All the connections were pretty corroded and weathered. I set the new tach to #3 (8cyl) per the instructions but I didnt mess with it much when the motor was running.
 
I'd suggest verifying continuity of your 'brown tach wire' at the gauge back to the gauge.....without a good signal coming from the coil, no tach has a chance....
 
makomark said:
I'd suggest verifying continuity of your 'brown tach wire' at the gauge back to the gauge.....without a good signal coming from the coil, no tach has a chance....
Click to expand...

Continuity of the brown wire between the gauge and the ignition module/ switch box is good. I'm unsure of how to test the module itself though. And I'm assuming that the ignition coil itself is within the housing that the ignition module is attached to? Again I'm having trouble finding schematics of this system... The directions for the new tach say to attach to the "negative side of the coil". I'm unsure what the module links together...
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what you have is a mid-1970's old thunderbolt ignition.....it may be in a 1980 year hull but the engine was built well before 1980....

the factory data for the original thunderbolt ignition shows the tach connection to the switchbox....you can try moving the tach connection to the negative side of the coil....

module test...good question....there are really only two essential functions - make spark and advance the timing as the rpm increases....it may have a separate tach driver in the amplifier...cant tell you as I haven't seen one of those in a few decades....
 
Unless I`m wrong the purple and grey from the module go directly to the coils pos and neg , Tach wire is usually grey connected to the neg side
 
Fired the boat up today and switched around the pole selector on the back of the tach and nothing changed. Still set for an 8 cyl (#3 position) there’s a purple wire that’s connected to a yellow alternator wire that used to run to an ignition light on the old dash. That wire is currently disconnected. Any chance that’s affecting anything?
 
makomark said:
I doubt it....did you try moving the tach wire from the module to the - side of the coil ??
Click to expand...

Am I looking at this correctly? The coil wire comes out of the box, the positive lead connects at the green terminal on the module and the negative side is grounded to the far left? If so, then yes I moved the brown signal wire to the grounded coil wire with no change in the tach.

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maybe this will help a bit....

View attachment original thunderbolt ignition system wiring.pdf

I'm thinking your new tach won't work on the original thunderbolt ignition system...the service manual indicates 'special equipment' required to test that system....and its tachometer.

Think your best bet is to contact the tech support dept of whoever's instruments you bought and see what they say about a different tach or a mod to the one you have....
 
here's a note from Teleflex's reference material on tachometers....:

[h=3]Tachometers (Inboard & Stern Drive 4-Stroke Engines- Standard Ignition)[/h]
  1. Tachometer will not operate on a MerCruiser with Thunderbolt I (early 1970's era, but will operate on subsequent models) or an IndMar LT- 1 electronic ignition. Tachometer will operate with other OEM electronic engine ignition systems. Teleflex has not tested aftermarket systems and cannot guarantee their compatibility.
 
o2batsea said:
Is it possible that you fried the tach?
Click to expand...

I don’t think so. All the other gauges that are in the same circuit work fine. I think makomark is right about the tach not being compatible with the old ignition system. Im going to call Faria this week and see what they think.
 
In my experience,, Faria has always gone above and beyond when it comes to customer service and their products are far superior to others.
 
Hopefully, they will offer a path forward......if not, here's a few more options:

* see if they can repair you old tach
* Contact MSD (Holley) and see if they have a converter that will work
* get a custom made converter

From what I can see, the '-' coil post is grounded all the time and the spark is initiated, from the switchbox, with a pulse thru the "+" post....so a 'normal' tach won't work. To complicate using a new tach, the switchbox likely dumps well over 400VDC when it fires....
 
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