Speedo is "bouncing"
Thread Starter
Le Mans Master
Joined: Apr 2012
Posts: 6,956
Likes: 4,811
From: Camden SC
Speedo is "bouncing"
Hey guys,
New owner of a 1973 coupe here. I have a fast question. When driving down the road, the needles on both my speedometer and tachometer are "bouncing". For example, if I am trying to hold a 40mph speed, the needle is constantly bouncing between 35mph & 45mph. Tach is doing the same. Any suggestions?
Thanks
New owner of a 1973 coupe here. I have a fast question. When driving down the road, the needles on both my speedometer and tachometer are "bouncing". For example, if I am trying to hold a 40mph speed, the needle is constantly bouncing between 35mph & 45mph. Tach is doing the same. Any suggestions?
Thanks
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,137
Likes: 4,247
From: US-PNW
This could also be a symptom of too much lube that has worked itself into the gauge heads where it's interfering with the magnets movements. If this is the case, there's no fix short of pulling the dash and cleaning out the excess.
Cable lube is graphite in a liquid carrier that evaporates. The wrong lube is oil and that's what can get into the gauge heads. You can probably find the correct lube at NAPA or the other local auto parts stores. A small tube goes a long way, so it's not something I've had to buy recently.
To see if it's too little lube, address the tachometer cable first since it's the easiest to get to. Disconnect it at the distributor and put about a half dozen drops of the graphite into the end. Once the cable starts turning, the rotating cable will transfer it up the cable. It won't be instantaneous but you should see if it stops the bounce within, I'd guess, 10-15 minutes.
FYI, the cables can't be pulled out from the distributor or transmission ends. There is a cable stop at each gauge.
One last thing, do you have a single long speedometer cable from the firewall to the trans? Some cars had two shorter cables with a connection right where the trans tunnel starts. If you have two short cables, you can disconnect it there and apply lube to the upper one.
Cable lube is graphite in a liquid carrier that evaporates. The wrong lube is oil and that's what can get into the gauge heads. You can probably find the correct lube at NAPA or the other local auto parts stores. A small tube goes a long way, so it's not something I've had to buy recently.
To see if it's too little lube, address the tachometer cable first since it's the easiest to get to. Disconnect it at the distributor and put about a half dozen drops of the graphite into the end. Once the cable starts turning, the rotating cable will transfer it up the cable. It won't be instantaneous but you should see if it stops the bounce within, I'd guess, 10-15 minutes.
FYI, the cables can't be pulled out from the distributor or transmission ends. There is a cable stop at each gauge.
One last thing, do you have a single long speedometer cable from the firewall to the trans? Some cars had two shorter cables with a connection right where the trans tunnel starts. If you have two short cables, you can disconnect it there and apply lube to the upper one.
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,137
Likes: 4,247
From: US-PNW
Do you know if the gauges or driver's dash panel have been restored/replaced? That might have provided the opportunity for someone to lube them before you bought the car.
Le Mans Master
Joined: Nov 2001
Posts: 8,799
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From: LONG ISLAND
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From: Benicia, CA
I have a 73 also with the exact same problem. Coincidence?
The speedo also registered about 10-15% faster than actual mph.
I've replaced the speedo cable and sleeve and made sure to use graphite lube when installing, as well as replacing the driven gear in the transmission. It still bounces some of the time. Other times it's steady (usually after the car gets hot). It reads only about 2-3% too fast now at freeway speeds.
I completely redid the interior a year-and-a-half ago and when the speedo and tach were out I attached a cordless drill to the input and neither of them bounced. I'm thinking that the sharp bend the cable makes right next to the transmission might be the cause. There are 90 degree adapter gears that can be installed there, I just haven't gotten around to getting one.
I haven't messed with the tach cable, yet, but after the engine bay gets hot it will drop to zero and only occasionally come back to life. Not sure if the cable or the gear on the distributor shaft is to blame.
The speedo also registered about 10-15% faster than actual mph.
I've replaced the speedo cable and sleeve and made sure to use graphite lube when installing, as well as replacing the driven gear in the transmission. It still bounces some of the time. Other times it's steady (usually after the car gets hot). It reads only about 2-3% too fast now at freeway speeds.
I completely redid the interior a year-and-a-half ago and when the speedo and tach were out I attached a cordless drill to the input and neither of them bounced. I'm thinking that the sharp bend the cable makes right next to the transmission might be the cause. There are 90 degree adapter gears that can be installed there, I just haven't gotten around to getting one.
I haven't messed with the tach cable, yet, but after the engine bay gets hot it will drop to zero and only occasionally come back to life. Not sure if the cable or the gear on the distributor shaft is to blame.
