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Tach Issues - Bad Ground?

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72FordGTS View Drop Down
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    Posted: 02-September-2017 at 1:38PM
I had a Rocketman Tach conversion done this spring.  It's worked perfectly so far, until tonight.  I was driving in the evening and used my lights (probably the first time this year as most of the time I drive during the day), and the tach start bouncing around a bit and reading way too high.
 
When I shut the lights off, it seemed to go back to normal.  I am thinking it has to do with a bad ground somehow associated to the light switch.  I also notice the lights produce some electrical noise on my radio.  Does anyone have any idea which ground maybe causing the issue?  I used a ground wire from the instrument cluster for my tach.
 
Anyone have any ideas? 


Edited by 72FordGTS - 02-September-2017 at 1:47PM
Vince

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Post Options Post Options   Thanks (0) Thanks(0)   Quote californiajohnny Quote  Post ReplyReply Direct Link To This Post Posted: 02-September-2017 at 4:18PM
hmm strange. it only does this with the head lights on? idk i'd guess maybe an under hood ground???
JOHN
74 GRAN TORINO S&H CLONE
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90 S10 BLAZER 4X4 LIFTED
77 CELICA CUSTOM
75 V8 MONZA SUPERCHARGED
79 COURIER VERT. SLAMMED
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72FordGTS Quote  Post ReplyReply Direct Link To This Post Posted: 03-September-2017 at 2:36PM

My theory is that somewhere in the headlight circuit there is a bad ground which is causing the issue I see with tach.  Since I used a ground from the instrument cluster, I am sure I could just run a dedicated ground for my tach to fix the tach issue.  But I'd like to find the bigger issue.

Does anyone know where the headlight harness ground wire connects to the body on these cars? 
Vince

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Post Options Post Options   Thanks (0) Thanks(0)   Quote californiajohnny Quote  Post ReplyReply Direct Link To This Post Posted: 03-September-2017 at 5:21PM
small wire on each side attached to the core support by the head lights
JOHN
74 GRAN TORINO S&H CLONE
74 VETTE CUSTOM
90 S10 BLAZER 4X4 LIFTED
77 CELICA CUSTOM
75 V8 MONZA SUPERCHARGED
79 COURIER VERT. SLAMMED
75 VEGA V6 5 SPD
70 CHEV C10 P/U
68 MUSTANG FB CONVERSION
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