Dancing tach

[Paul Gordon]

The tach in my '71 seems acting weird. The pointer will read the RPM's then bounces at random a few time then back to a normal number. Could the gear or cable be worn out or starting to go or ???

Thanks Paul Gordon

[Joe L.]

Re: Dancing tach

Paul----

It's hard to diagnose a problem like this from afar, but I'd guess that the problem is with the tachometer head, itself. Although Corvettes through 1974 use a mechanical tachometer, it is not a "direct drive" type. There is a "dampening" mechanism in the tachometer head and when this breaks down, erratic indications sometimes commence. There could also be other problems with the tach head. I'd have it serviced by a qualified technician. All this is not to say that the problem could not be in the cable or drive gears. I'd inspect the drive gear and replace at least the inner cable as part of the tach servicing operation.

[Joe L.]

Re: Dancing tach

Paul----

It's hard to diagnose a problem like this from afar, but I'd guess that the problem is with the tachometer head, itself. Although Corvettes through 1974 use a mechanical tachometer, it is not a "direct drive" type. There is a "dampening" mechanism in the tachometer head and when this breaks down, erratic indications sometimes commence. There could also be other problems with the tach head. I'd have it serviced by a qualified technician. All this is not to say that the problem could not be in the cable or drive gears. I'd inspect the drive gear and replace at least the inner cable as part of the tach servicing operation.

[Ed Jennings]

Re: Dancing tach

It would be worthwhile to see that the tach cable is properly lubricated. There is a special lube for speedometer/tach cables, which is graphite based. Only the lower third to half of the cable should be lubed. The gears should be checked also. It's probably in the tach head, but these things are quick and easy (compared to removing the tach) and should be eliminated first.

[Ed Jennings]

Re: Dancing tach

It would be worthwhile to see that the tach cable is properly lubricated. There is a special lube for speedometer/tach cables, which is graphite based. Only the lower third to half of the cable should be lubed. The gears should be checked also. It's probably in the tach head, but these things are quick and easy (compared to removing the tach) and should be eliminated first.

[Patrick H.]

How about a Dancing Temperature Gauge?

Joe,

My temp gauge recently started to dance on occasion as well. I have tried several times to improve the connection of the wire to the sender as this is always a weak spot in the connection, and at this point it really hasn't seemed to help much.

As I go down the road, at times the needle will bounce around, and even "violently" so, and then it may settle down for a while with just a slight twitch on occasion. Other times it will be rock steady.

Does the temp gauge work in the same manner as the speedo head, or should I continue to look at the sender connection?

Patrick

[Patrick H.]

How about a Dancing Temperature Gauge?

Joe,

My temp gauge recently started to dance on occasion as well. I have tried several times to improve the connection of the wire to the sender as this is always a weak spot in the connection, and at this point it really hasn't seemed to help much.

As I go down the road, at times the needle will bounce around, and even "violently" so, and then it may settle down for a while with just a slight twitch on occasion. Other times it will be rock steady.

Does the temp gauge work in the same manner as the speedo head, or should I continue to look at the sender connection?

Patrick

[Jack H.]

Re: Dancing tach

Start with the easy stuff. Undo tach cable and fill with cable lube. Some is graphite based other brands are petroleum or silicon based.

Now, inspect the drive cable head at the distributor. Often it winds up being worn (squared end chewed) from prior owner mis-insertion. If it's badly chewed, that could be a 'slip' factor coupled with a dry inner cable situation.

You can buy just the cable's internal, cut to match length and re-stuff. This preserves the original cad plated coupler ring at the dist end (replacement AC cables are gold dichromate and cost a minor originality deduction).

Last, check the orientation of your distributor. If it's inserted properly, the vac advance nipple SB pointing pretty much straight forward when engine's in proper time. If it's off a tooth or two, it'll still work, BUT you're putting an excess bend on the tach drive cable that'll result in internal wear....

Once the 'easy' stuff has been checked and you still see excessive tach bounce/wander, it's time to re-work the tach. This means pulling the main instrument cluster (not a fun job, but step-by-step documented in your Chassis Service Manual) and going to a good speedo shop for repair/re-cal.

Tips: (1) The big job lies in the labor to R&R the tach, consider pulling the speedo assy while everything's open and doing both at the same time. (2) AC-Delco/United Delco are still alive & well around the country and routinely rebuild classic Chevy speedo/tach units and actually HAVE the parts! (3) Some will do the whole job for you on a drive-in/drop-off basis if you're squeamish about turning your own wrench.

[Jack H.]

Re: Dancing tach

Start with the easy stuff. Undo tach cable and fill with cable lube. Some is graphite based other brands are petroleum or silicon based.

Now, inspect the drive cable head at the distributor. Often it winds up being worn (squared end chewed) from prior owner mis-insertion. If it's badly chewed, that could be a 'slip' factor coupled with a dry inner cable situation.

You can buy just the cable's internal, cut to match length and re-stuff. This preserves the original cad plated coupler ring at the dist end (replacement AC cables are gold dichromate and cost a minor originality deduction).

Last, check the orientation of your distributor. If it's inserted properly, the vac advance nipple SB pointing pretty much straight forward when engine's in proper time. If it's off a tooth or two, it'll still work, BUT you're putting an excess bend on the tach drive cable that'll result in internal wear....

Once the 'easy' stuff has been checked and you still see excessive tach bounce/wander, it's time to re-work the tach. This means pulling the main instrument cluster (not a fun job, but step-by-step documented in your Chassis Service Manual) and going to a good speedo shop for repair/re-cal.

Tips: (1) The big job lies in the labor to R&R the tach, consider pulling the speedo assy while everything's open and doing both at the same time. (2) AC-Delco/United Delco are still alive & well around the country and routinely rebuild classic Chevy speedo/tach units and actually HAVE the parts! (3) Some will do the whole job for you on a drive-in/drop-off basis if you're squeamish about turning your own wrench.

[Jack H.]

Re: How about a Dancing Temperature Gauge?

Temp guage does NOT work like speedo/tach. It's all electric instead of mechanical. Reason you see erratic reading is 'somewhere' you've got faulty wiring that's intermittent.

If 'bounce' is in low temp direction, says power supply feed to temp gauge is sporadically opening. If 'bounce' is in high direction (going off-scale hot periodically), says you've got an occassional short to ground that's bypassing the temp sender. Latter could be in the sender itself, at the temp sender sensor wire, anywhere in the wire back to the gauge as well as in the gauge winding itself.

Since originals used now discontinued 'pinhead' transducers, many have been replaced with the 'buttonhead' temp sender. Means someone along the way replaced sender with now current GM service spare (buttonhead) then switched out original cylindrical slip-on connector for a slide on connector and this could be a problem source (bad crimp/splice).

Last, this should be a no-brainer, but it's often overlooked. Both temp sender and TCS/CEC thermal switch on Sharks are mounted in the cylinder heads on the side of the engine vs. earlier cars where temp sender was mounted on intake mani. If valve cover/gasket isn't making a tight seal, oil can leak and run down the sides of the cylinder head AND give you a nice 'coating' over these transducers resulting in erratic readings....

[Jack H.]

Re: How about a Dancing Temperature Gauge?

Temp guage does NOT work like speedo/tach. It's all electric instead of mechanical. Reason you see erratic reading is 'somewhere' you've got faulty wiring that's intermittent.

If 'bounce' is in low temp direction, says power supply feed to temp gauge is sporadically opening. If 'bounce' is in high direction (going off-scale hot periodically), says you've got an occassional short to ground that's bypassing the temp sender. Latter could be in the sender itself, at the temp sender sensor wire, anywhere in the wire back to the gauge as well as in the gauge winding itself.

Since originals used now discontinued 'pinhead' transducers, many have been replaced with the 'buttonhead' temp sender. Means someone along the way replaced sender with now current GM service spare (buttonhead) then switched out original cylindrical slip-on connector for a slide on connector and this could be a problem source (bad crimp/splice).

Last, this should be a no-brainer, but it's often overlooked. Both temp sender and TCS/CEC thermal switch on Sharks are mounted in the cylinder heads on the side of the engine vs. earlier cars where temp sender was mounted on intake mani. If valve cover/gasket isn't making a tight seal, oil can leak and run down the sides of the cylinder head AND give you a nice 'coating' over these transducers resulting in erratic readings....