Topic: Tachometer question

[mach514]

On my 69 Shelby GT350 when the car is started and is at idle the Tach reads 0 as you raise the rpm it starts to read. It is incorrect by about half. After doing a bit of research it appears that for the Tach to work correctly Ford intended it to have 1.5 ohms of resistance built into the wiring. 
After doing some crawling around in the dash the Tach pigtail under the dash to the gauge feed pigtail in the engine compartment has the resistance wire built into it. I've tested this wire and it does indeed have the correct 1.5 ohms from the dash to the coil.  Where it appears the issue is that the new yellow top coils also have 1.5 ohms  built into them.  This brings my total ohms to 3 causing my Tach to be lazy. What do you guys recommend to correct the issue?  Is there someone out there reproducing the yellow top coils with out the resistance built in them?  Or do I bypass the resistance wire under the dash between the Tach and gauge feed? I need to make sure that whatever I do to the car it  stays visually correct and that I do no permanent harm to my wiring. I'm going to run a temporary wire tomorrow to bypass the resistance wire to test my theory and see if my Tach wakes up at idle.  Any advice is very much appreciated. Thank you,   Dan

[Bob Gaines]

On my 69 Shelby GT350 when the car is started and is at idle the Tach reads 0 as you raise the rpm it starts to read. It is incorrect by about half. After doing a bit of research it appears that for the Tach to work correctly Ford intended it to have 1.5 ohms of resistance built into the wiring. 
After doing some crawling around in the dash the Tach pigtail under the dash to the gauge feed pigtail in the engine compartment has the resistance wire built into it. I've tested this wire and it does indeed have the correct 1.5 ohms from the dash to the coil.  Where it appears the issue is that the new yellow top coils also have 1.5 ohms  built into them.  This brings my total ohms to 3 causing my Tach to be lazy. What do you guys recommend to correct the issue?  Is there someone out there reproducing the yellow top coils with out the resistance built in them?  Or do I bypass the resistance wire under the dash between the Tach and gauge feed? I need to make sure that whatever I do to the car it  stays visually correct and that I do no permanent harm to my wiring. I'm going to run a temporary wire tomorrow to bypass the resistance wire to test my theory and see if my Tach wakes up at idle.  Any advice is very much appreciated. Thank you,   Dan

I suppose one of the easiest things to do would be to ditch the repo coil for one of the many hundreds of thousands of factory Ford yellow top coils that are still to be had from salvage yards,swap meets, ebay etc.  They seldom go bad. 

[67350#1242]

It is normal for original yellow top coils to have 1 to 2 ohms primary resistance so I doubt that would be the cause. If the problem started when changing to the repro coil, of course it would be your primary suspect.  It wont hurt to see what happens when bypassing the pink wire, but don't run it that way too long.   

[mach514]

I dug through my piles of parts and found 2 old yellow top coils, and one reproduction coil. All in decent shape and all putting out 1.5 ohms. I read through the shop manuals and it says they should be right at 1.5 ohms.  I bypassed the resistance wire from the tach to the coil briefly and no change in my tachs readings. So at this point I’m thinking my tach needs to get pulled out and sent out to be rebuilt and recalibrated before it gives up completely.  Does anyone have a recommendation for a tach repair service?  The gauge it self is beautiful it just needs mechanical work not cosmetic.  Thank you for the advice.   Dan.

[midlife]

PM sent re tach refurbishment.

[mach514]

Thank you Midlife, I really appreciate it.