71 Tach drive

[Richard A.]

My tachometer on my 71 is not working. When I took off the old tach cable from the distributor it was shredded inside. I installed the new cable under the dash, used a drill to make sure the tach was working which it was, installed the new cable back into the distributor, started the engine and still no tach movement in the dash. I assume I need a new tach drive gear in the distributor. What is the procedure for installing a new tach drive gear into the distributor, if this is what I need? From pictures, it looks as though it is simply threaded into the distributor.

Thanks, Rick

[Harmon C.]

Re: 71 Tach drive

The housing is threaded and you need a tool to span to both sides of the slot to remove it if it is real tight. I made a tool out of sheet metal. The housing that holds the gear wears so I would replace every thing and a new main shaft for the distributor will be needed if it is worn.
A new gear may solve the problem but I would replace every thing and be done for a long time.

[Jack H.]

Re: 71 Tach drive

Double check your cable installation before you tear into the distributor! You may well have worn out the distributor's drive gear, but there's a small chance the tach malfunction is simply a 'cockpit' problem.

The drive cable has a square shaped end that must make a TIGHT fit into the distributor. It's possible that when you installed the drive cable, you were in a hurry and didn't fully insert the cable into the distributor's keyway and FULLY torque the cable's ring down.

If so, the drive cable may be slipping. So, I'd re-check the cable to distributor connection just to insure you don't have a hook-up, cockpit problem....

[Joe L.]

Re: 71 Tach drive

My tachometer on my 71 is not working. When I took off the old tach cable from the distributor it was shredded inside. I installed the new cable under the dash, used a drill to make sure the tach was working which it was, installed the new cable back into the distributor, started the engine and still no tach movement in the dash. I assume I need a new tach drive gear in the distributor. What is the procedure for installing a new tach drive gear into the distributor, if this is what I need? From pictures, it looks as though it is simply threaded into the distributor.

Thanks, Rick

Rick-----


If you remove the CROSS GEAR and find that it has a "chunk" out of it OR if it appears otherwise worn, then you MUST also replace the distributor mainshaft which has, as an integral part, the gear the cross shaft articulates with.

[Alan S.]

Re: 71 Tach drive

Hi Richard,
Which direction should the drill spin to test the cable and tach unit? Thanks!
Regards,
Alan

[Joe L.]

Re: 71 Tach drive

Hi Richard,
Which direction should the drill spin to test the cable and tach unit? Thanks!
Regards,
Alan

Alan-----

The distributor drives the cable in a CLOCKWISE direction. So, the drill should turn in the same direction.

[John H.]

Re: 71 Tach drive

The mainshaft rotates clockwise (as viewed from the top), which rotates the tach drive cross gear clockwise as you look into the tach drive cavity in the distributor; however, if you're looking at the distributor end of the tach cable, the cross gear turns the cable counter-clockwise, so you want the drill to operate in reverse.

[William C.]

Re: 71 Tach drive

If the crossgear is broken, in addition to replacing it and the distributor mainshaft you will 98% of the time have to repair the housing, as housing wear generaly is the cause of the gear misalignment that causes the gear to break in the first place. I believe 71 was the last year the distributor did not have the hole in the housing and the thrust bushing seated into the hole inside to reduce the case wear, but I'm not certain.

[Terry M.]

Re: 71 Tach drive

The hole in the housing began mid-1970 production (the s/n change point in in the 1970-72 TIM&JG) -- and some of us judge for either the presence or absence of it depending on the build of the car.

[Joe L.]

Re: 71 Tach drive

The mainshaft rotates clockwise (as viewed from the top), which rotates the tach drive cross gear clockwise as you look into the tach drive cavity in the distributor; however, if you're looking at the distributor end of the tach cable, the cross gear turns the cable counter-clockwise, so you want the drill to operate in reverse.

John-----


Yes, you're correct. Operating the drill in forward would only work if you were trying to drive the gear system in the distributor. VERY, VERY few people ever want to do that; maybe no one! To drive the tachometer for test purposes, the drill does need to be in reverse.