Looking for help on tachometer

[Voltronics]

Hey y’all,

 

looking for some input. Bought a r688st 1985 has a cable driven tach. Cable was really worn and old. So I replaced it used same ends as the old one and tach still doesn’t move with engine on. Could the tach itself be bad. I noticed in the back with the tach cable there are adapters but the old cable had none when I took it off am I missing something?

[Joey Mack]

Is the cable reaching into the tach far enough ?

[Voltronics]

2 hours ago, Joey Mack said:

Is the cable reaching into the tach far enough ?

I would assume so it hits both stops when screwing it in. When I had it in the engine side u could spin the cable free which doesn’t seem right to me. 

[Joey Mack]

Isnt there an insert for the end if the cable at the drive end?   Im forgetting things lately.

[Voltronics]

2 hours ago, Joey Mack said:

Is the cable reaching into the tach far enough ?

To clarify the gauge didn’t work when I bought it, the cable was in rough shape so I bought same length cable and put the same ends on as previous which means didn’t use any adapters and gauge still doesn’t work.

thanks

[Voltronics]

Just now, Joey Mack said:

Isnt there an insert for the end if the cable at the drive end?   Im forgetting things lately.

That’s what I’m wondering I just took the old cable out and put the new one in didn’t know if I had to use an adapter on drive end as the old one didn’t have one.

[mechohaulic]

1 hour ago, Joey Mack said:

Isnt there an insert for the end if the cable at the drive end?   Im forgetting things lately.

there should be as you said an end on the drive side which "plugs in to" the cable housing, most are 1 +- long having 1/8th in shaft with flat spot which goes in to tach drive. opposite end round barrel having internal square  for cable to fit in to. no cable end =start there, with all safety concerns met. remove cable from eng drive side start eng see if the center is spinning. with end installed on cable drive side spin by hand = old days a second person watched the tach in cab. today guess a person puts their phone on steering wheel while in video mode ?? have seen many cables missing drive fitting. rare occasion the drive gear under the two bolt housing failed .

[Voltronics]

On 8/14/2025 at 8:52 AM, mechohaulic said:

there should be as you said an end on the drive side which "plugs in to" the cable housing, most are 1 +- long having 1/8th in shaft with flat spot which goes in to tach drive. opposite end round barrel having internal square  for cable to fit in to. no cable end =start there, with all safety concerns met. remove cable from eng drive side start eng see if the center is spinning. with end installed on cable drive side spin by hand = old days a second person watched the tach in cab. today guess a person puts their phone on steering wheel while in video mode ?? have seen many cables missing drive fitting. rare occasion the drive gear under the two bolt housing failed .

Thank you, I will start here

[mechohaulic]

11 hours ago, Voltronics said:

Thank you, I will start here

and don't forget updates are ALWAYS APPRECIATED . we all like expanding OUR knowledge ;;;young and OLD.

[Joey Mack]

And Please remember to acknowledge the one that solved it..  Its a simple curtousy.  

Edited 10 hours ago by Joey Mack
More content needed

[Voltronics]

12 hours ago, mechohaulic said:

and don't forget updates are ALWAYS APPRECIATED . we all like expanding OUR knowledge ;;;young and OLD.

Well got the adapter piece in but rhe rpm gauge needle on wiggles slightly doesn’t actually move either rpm of engine idk if the needle went all the way around at one point and is stuck now or what

Edited 10 hours ago by Voltronics

[mechohaulic]

12 hours ago, Voltronics said:

Well got the adapter piece in but rhe rpm gauge needle on wiggles slightly doesn’t actually move either rpm of engine idk if the needle went all the way around at one point and is stuck now or what

is the adapter a new or old one, old one may be rounded out inside cable end. new inside cable should have a crimped square fitting on it at tach end? easy check would be take cable off tach end ; start engine see if inner cable spinning. if so safely hold the inner cable slightly, can't hold to a stop == everything ok to that point. square input end in tach itself may be rounded instead of square. catches enough to move gauge ;not square enough to give accurate movement.

[Voltronics]

3 minutes ago, mechohaulic said:

is the adapter a new or old one, old one may be rounded out inside cable end. new inside cable should have a crimped square fitting on it at tach end? easy check would be take cable off tach end ; start engine see if inner cable spinning. if so safely hold the inner cable slightly, can't hold to a stop == everything ok to that point. square input end in tach itself may be rounded instead of square. catches enough to move gauge ;not square enough to give accurate movement.

I have brand new cables and ends and adapter the tach needle bounces ever so slightly but won’t move almost like the gauge has gone around once and stuck now but I will take off the cable on gauge side and see if it’s spinning,

thank you

[mechohaulic]

with all the cable parts being new ; I'd go with the tach it self the issue. when cable disconnected spin the tach it  self by hand. the brass outer end will move the gauge . if easy access completely remove gauge to check. for a tach to do a complete spin around will take some .

[Geoff Weeks]

With cable drives, the end has to protrude far enough to engage with the corresponding female end in the gear and tach. They also have to have ends of the correct size (this is where adaptors sometimes come in). Too short or too small and you might just get a little bounce. 

Pull the core (if possible) and stick into the tach and spin it to see if the tach moves, and that it fits the female in the tach.

 You are there, we are not, so you are going to have to make the assessment. 

 If that looks good, attach to the engine and run it, make sure it is turning the cable.