72 intermittent tach

[Erich M.]

My tach becomes erratic on hot days when idling in traffic and engine temperature starts to climb. Needle either bounces or drops to zero. As soon as engine temperature drops or running car on cool days ( 80 degrees or less) tach works fine. Any thoughts on what the problem may be? The tach has been behaving this way for a year now. When car is started from cold state and driven at average temperatures of less than 80 deg, tach works fine. Oringinal Owner, 72 LT1

[Patrick H.]

Re: 72 intermittent tach

I would first suspect that the problem is at the tachometer, especially if you've not ever serviced it.

I would remove the cable from the tach and check the end; be sure it is square. Now remove the bronze adapter and see how bad the cross gear looks, and how old the grease may be. I would not be surprised to see broken gear teeth and grease well beyond its intended lifespan. If you see broken teeth, or see nothing wrong, check the distributor mainshaft (will probably have to remove distributor to do this) and look for a broken/worn gear on the shaft.

[Erich M.]

Re: 72 intermittent tach

Patrick, thank you for your response. When you say "remove the cable from the tach and check the end", I assume you are referring to the distributor end. All of your suspicions seem to be aimed at the distributor end. Is that correct? If so, and there is a gear tooth problem somewhere on that end, why would I be getting a temperature effect. Wouldn't a broken gear show up in the tach behavior at all temperatures?Original Owner, 72 LT1 Coupe Bowtie

[Patrick H.]

Re: 72 intermittent tach

Patrick, thank you for your response. When you say "remove the cable from the tach and check the end", I assume you are referring to the distributor end. All of your suspicions seem to be aimed at the distributor end. Is that correct? If so, and there is a gear tooth problem somewhere on that end, why would I be getting a temperature effect. Wouldn't a broken gear show up in the tach behavior at all temperatures?Original Owner, 72 LT1 Coupe Bowtie

Yes, I would look first at the distributor end. Temperature affects grease thickness - if there is any left. In addition, checking out the distributor end is FAR easier than pulling the tach. Make sure the easy stuff is verified first, and leave the tach for last.

[Alan S.]

[Edward J.]

Re: 72 intermittent tach

Erich, I would add to make sure the cable is screwed in completely to the dist. end and also clipped in all the way to Tachometer.

[Jim T.]

Re: 72 intermittent tach

My tach becomes erratic on hot days when idling in traffic and engine temperature starts to climb. Needle either bounces or drops to zero. As soon as engine temperature drops or running car on cool days ( 80 degrees or less) tach works fine. Any thoughts on what the problem may be? The tach has been behaving this way for a year now. When car is started from cold state and driven at average temperatures of less than 80 deg, tach works fine. Oringinal Owner, 72 LT1

Eric being the original owner on your 72 LT-1, if your tach crossshaft gear has never been serviced with grease and is still a good working system, you are very fortunate. Maintenance on this is not in the GM service manual or my owner manuals for my 68 and 70.