Need accurate test for tach drive system.....
Thread Starter
Racer
Joined: Feb 2002
Posts: 308
Likes: 0
From: Greenfield OH
Need accurate test for tach drive system.....
I know it is an open ended question but here's the deal. According to the previous owner, the tach was working until "a few months ago". It has a Mallory distributor with what appears to be the correct tach drive outlet. The cable is attached to the distributor as well as the back of the tachometer.
What is the easiest way to see what is the culprit in the system? Is it as simple as disconnecting the cable from the distributor and spinning it to see if the tachometer responds? Can I disconnect the cable from the back of the tach and spin the input shaft and check for a response? What method(s) will give me accurate results? Thanks!!
Weasle
What is the easiest way to see what is the culprit in the system? Is it as simple as disconnecting the cable from the distributor and spinning it to see if the tachometer responds? Can I disconnect the cable from the back of the tach and spin the input shaft and check for a response? What method(s) will give me accurate results? Thanks!!
Weasle
Burning Brakes
Joined: Nov 2000
Posts: 948
Likes: 0
From: Johnston Iowa
Re: Need accurate test for tach drive system..... (WeasleVette454)
It is alot easier to get the cable off the dist and try to spin it I would imagine.
Trying to get it off the back of the tach might be tuff.
[Modified by James, 4:03 PM 3/5/2002]
Trying to get it off the back of the tach might be tuff.
[Modified by James, 4:03 PM 3/5/2002]
Team Owner
Joined: Jun 2001
Posts: 43,136
Likes: 155
From: Thomson Georgia
Re: Need accurate test for tach drive system..... (WeasleVette454)
Yeah disconnect it and spin the cable the tach should move......If not u need a new cable.......if it does spin....U probably need a new cross gear or main shaft in the distributor
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
Re: Need accurate test for tach drive system..... (WeasleVette454)
Weasle:
If I recall correctly (y'all pounce if I'm mistaken), distributor mainshaft runs at half speed of crankshaft. I'm certain that tach output coupling runs at 1:1 ratio to distributor mainshaft. So, if engine is running at 1000 rpm then distributor is turning at 500 rpm and tach output coupling is turning at 500 rpm while tach will register at 1000 rpm. So, tach registers at double its input rpm. To help diagnose the cable and/or tach, first try this: Disconnect tach cable from distributor coupling, carefully-gently chuck cable end into REVERSIBLE drill motor, spin drill in reverse (facing chuck it should turn clockwise). Many drills'll turn about max 600 rpm, so your tach should register about 1200 rpm (or twice the speed of your drill). When driven by drill ... If your tach needle doesn't move or if it bounces ... then you know you've got a problem with cable and/or tach ... but you may also have a problem in the distributor too. Distributor's tach gears are difficult at best to see into when on engine. To properly inspect the distributor, I suggest you pull it from motor, clean out the grease&gunk from tach gears and closely inspect on benchtop. In addition to stripped crossgear, the gear cut into the mainshaft can be chewed up/flat-spotted. Hope this helps.
Jack
If I recall correctly (y'all pounce if I'm mistaken), distributor mainshaft runs at half speed of crankshaft. I'm certain that tach output coupling runs at 1:1 ratio to distributor mainshaft. So, if engine is running at 1000 rpm then distributor is turning at 500 rpm and tach output coupling is turning at 500 rpm while tach will register at 1000 rpm. So, tach registers at double its input rpm. To help diagnose the cable and/or tach, first try this: Disconnect tach cable from distributor coupling, carefully-gently chuck cable end into REVERSIBLE drill motor, spin drill in reverse (facing chuck it should turn clockwise). Many drills'll turn about max 600 rpm, so your tach should register about 1200 rpm (or twice the speed of your drill). When driven by drill ... If your tach needle doesn't move or if it bounces ... then you know you've got a problem with cable and/or tach ... but you may also have a problem in the distributor too. Distributor's tach gears are difficult at best to see into when on engine. To properly inspect the distributor, I suggest you pull it from motor, clean out the grease&gunk from tach gears and closely inspect on benchtop. In addition to stripped crossgear, the gear cut into the mainshaft can be chewed up/flat-spotted. Hope this helps.
Jack
