Another Tach question, sorry!

305chevy78

Member
Oct 8, 2011
42
Antwerp, Belgium
i've been reading a lot on tach issues. I have not seen, so i will ask if there is a sensor somewhere which can go bad so the tach has no reading when i drive. I got a haynes manual but don't find much info on the tach and its issues. I have read everything about the wire from the distributor and that you possibly can bypass/ cut out the RFI lil round box by the firewall (i dont know the exact name). Stupid maybe but can someone tell me how to measure if the is electricity flowing past that lil box, i'm not an electrician. so to summarise i wonder if there is a sensor somewhere (maybe by the gearbox?) and how can i measure if i have power coming from the distributor. Actually, if you turn the ignition key to the point where the "fasten seatbelt" light comes on, an the tach is working, getting power, would you see the needle move/ drop to 0? Thank you - Dieter
 

77RS

Veteran Member
Lifetime Gold Member
Oct 18, 2006
2,944
Asheboro, NC
The tach signal is sent when the coil fires. Basically, if the car is running, the tach signal is present. The tach signal is sent from the neg side of the coil on both HEI distributors, and "points" distributor systems.

On 78-81 models, the tach should return to 0 when power is ON (engine not running). If your tach isn't doing this, then it could be that it isn't receiving voltage, or that it has a bad connection to ground. That is the most common problem with a non-working tach. When the tach functions (but reads incorrectly), there is usually a problem within the tachometer circuit or meter. The power/ground issues are a fairly simple repair for the DIY'er. If there is a problem within the tachometer itself, it will most likely need to be professionally repaired. If I can be of help with this, just let me know.
Thanks!
Daniel
 

305chevy78

Member
Oct 8, 2011
42
Antwerp, Belgium
hey, daniel thank you. i've had a few issues with bad grounds so far, this could be another one, where can i see the ground, where can i check it? Other bad grounds i had is with the blinkers, appreciate your help! i know the tach was working and suddenly stopped from one time to the next (the car wasn't in my posession at that time), thank you again!
 

305chevy78

Member
Oct 8, 2011
42
Antwerp, Belgium
daniel, thanks again for your help. twisted metal told me the ground is on the printed circuit behind the cluster. i found a ring on the top screw was not all the way fastened and when i turned the power on, the needle drop to 0 so i'm sure it will work now. Anothere member posted a picof the backside of the cluster and indicated the ground so i followed it around. thank you again!
 

305chevy78

Member
Oct 8, 2011
42
Antwerp, Belgium
hi Daniel, i'm back already:whine:
When i turn the ignition the tach drops to 0, goods news i thought. i crank the car and the i think it moves or it seems to float between 0 and 5, once the engine is running, the tach does not moves, just drove round the block . Don't know what to do anymore, any guidance you can give me? thanks Dieter

For anyone out there who may need it, i bypassed the RFI and now the tach work. Got buzzing in my radio now but i figure i can buy a new RFI. its a capacitator of sorts i guess?
 
Last edited:

77RS

Veteran Member
Lifetime Gold Member
Oct 18, 2006
2,944
Asheboro, NC
Glad that there are some great guys on here that were able to help you with locating the ground! :) That's what this community is all about!
 




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