Speedometer lag possible cause

[Richard M.]

Re: Speedometer lag possible cause

Hi Tom, Welcome.

I think the first step would be to run the cable from a drill as it could point you in a specific area.

Historically, the speedometer head is most often the issue. The gears wear out.

If the cable was lubricated using grease, it rides up into the head and causes intermittent operation also.

Rich

[Thomas S.]

Re: Speedometer lag possible cause

Thanks,

i hadn't considered the potential for linear movement of the cable. I'll get underneath, disconnect the cable and see what happens.

Hi Tom, Welcome.

I think the first step would be to run the cable from a drill as it could point you in a specific area.

Historically, the speedometer head is most often the issue. The gears wear out.

If the cable was lubricated using grease, it rides up into the head and causes intermittent operation also.

Rich

[Domenic T.]

Re: Speedometer lag possible cause

Is it possible to have excessive side or end play on a PG output shaft that would cause the drive gear and the driven gear to separate enough to cause speedometer needle lag on acceleration and when kicking into passing gear? Speedometer has been rebuilt and plastic gear, bullet holder, replaced along with the drive cable and housing. Looking for possible sources prior to removal of the cluster for another attempt at speedometer head rebuild. BTW, the position of the steel gear was checked and found to be centered on the shaft and showed no signs of wear or damage.

Al,
Didn't read all the replies but I did fix a laging speedo once that had a cable that was a bit to long and was putting pressure on the speedo. I backed off the attaching nut and it went away. That was not the fix as the cable was to long. I think I found a fibre washer that made the cable tight. just a thought.

Dom

[Al R.]

Re: Speedometer lag possible cause

Everyone, this problem was solved. I'm not sure who resurrected this thread, but it is no longer an issue for me. Thanks to all who originally responded and to those who continue to offer suggestions. Al

[David C.]

Re: Speedometer lag possible cause

Hi,
How was this problem solved?
I have a 73 in which the speedo needle was lagging and then today the speedometer simply quit working entirely.
Can anyone advise a step by step process to diagnose the issue?

Thanks.

[Thomas S.]

Re: Speedometer lag possible cause

In my case the speedometer head was rebuilt. The older speedometer were mechanical with the speedometer cable spining a magnet in the speedometer head. The speedometer magnet would move a metal piece was attached to a clock-type coil spring. The rotating magnet is coupled to the spring magnetically and the spring is attatched to the pointer that you see.

the whole sytem is dependant on low friction with the head and a proper level of magnetism. When the guages are taken apart the are lubed, demagnetized and a bunch of other things. This is not something you can do yourself as it requires delicate eguipment and calibration. I sent my entire instrument to Red Line Guage in North Carolina and the took care of the problem as well as cleaning and repainting all of the other guages.

I was told by the owner of Red Line never to lube a cable and that doing so can cause more problems. I know that bit of advise is likely to unlease a bunch of counter claims, but his reasoning was sound. Your cable could have broken as well. If that's the case you would have to replace it with a new one. On my 63, the only way to do that is from the speedometer end of the cable so the cluster would have to be removed.

Replacment cables (the inside part) can be slight off in length. When I got my guages back and installed a new cable, it was too long and I ended up damaging the just repaired guage.

If you can get to the end of the cable that attaches to the transmission, remove it and put the cable in a drill. Run the drill in reverse and see if you get a speed reading. If you do, then the problem could with the gear in the trans which is typically plastic and specific to the rear end gear ratio. if you don't get a reading on the speedo, it could still be the cable or speedo head.

Unfortunately, there's no easy way out. If it is relatively easy to get to you guages or at least to the back of the speedo to remove the cable that will make things easier because you can the see if the cable is moving with a drill attached, if so then its likely the guage.

Finally, all of the above only applies if you have a mechanical driven speedo. I'm not sure what they used in 73. If it's electric then it's a wholeother ballgame.

[Michael W.]

Re: Speedometer lag possible cause

Hi,
How was this problem solved?
I have a 73 in which the speedo needle was lagging and then today the speedometer simply quit working entirely.
Can anyone advise a step by step process to diagnose the issue?

Thanks.

Does the odometer still operate?

If yes- the speedometer mechanism needs to be serviced.

If no- most likely the cable or drive gear(s) has failed.

[Joe L.]

Re: Speedometer lag possible cause

In my case the speedometer head was rebuilt. The older speedometer were mechanical with the speedometer cable spining a magnet in the speedometer head. The speedometer magnet would move a metal piece was attached to a clock-type coil spring. The rotating magnet is coupled to the spring magnetically and the spring is attatched to the pointer that you see.

the whole sytem is dependant on low friction with the head and a proper level of magnetism. When the guages are taken apart the are lubed, demagnetized and a bunch of other things. This is not something you can do yourself as it requires delicate eguipment and calibration. I sent my entire instrument to Red Line Guage in North Carolina and the took care of the problem as well as cleaning and repainting all of the other guages.

I was told by the owner of Red Line never to lube a cable and that doing so can cause more problems. I know that bit of advise is likely to unlease a bunch of counter claims, but his reasoning was sound. Your cable could have broken as well. If that's the case you would have to replace it with a new one. On my 63, the only way to do that is from the speedometer end of the cable so the cluster would have to be removed.

Replacment cables (the inside part) can be slight off in length. When I got my guages back and installed a new cable, it was too long and I ended up damaging the just repaired guage.

If you can get to the end of the cable that attaches to the transmission, remove it and put the cable in a drill. Run the drill in reverse and see if you get a speed reading. If you do, then the problem could with the gear in the trans which is typically plastic and specific to the rear end gear ratio. if you don't get a reading on the speedo, it could still be the cable or speedo head.

Unfortunately, there's no easy way out. If it is relatively easy to get to you guages or at least to the back of the speedo to remove the cable that will make things easier because you can the see if the cable is moving with a drill attached, if so then its likely the guage.

Finally, all of the above only applies if you have a mechanical driven speedo. I'm not sure what they used in 73. If it's electric then it's a wholeother ballgame.

Thomas------


All 1953-82 Corvettes used mechanical speedometers. All tachometers were mechanical through 1974 and used a magnetic drive similar to the speedometers. 1975 and later tachometers were electric.

The type speedometer that you described is what's used in all. Rebuilding these can be tricky and, sometimes, problematic----sometimes they come out good, sometimes not. The best plan is a new speedometer mechanism but they're long-since GM-discontinued.

Corvettes would have been better if DIRECT DRIVE type speedometers and tachometers had been used. But, alas, they were too expensive to use in a mass produced vehicle.

[David C.]

Re: Speedometer lag possible cause

Does the odometer still operate?

If yes- the speedometer mechanism needs to be serviced.

If no- most likely the cable or drive gear(s) has failed.

Thanks everyone for your advice.
Michael to answer your question, the odometer was working but stopped along with the speedometer needle yesterday.
I suppose I will try the approach of attaching the transmission end of the cable to a drill to help isolate the problem.

It's been one of those weekends -- a couple problems popping up out of nowhere, including now the cowl hood air induction door no longer operates. One of the challenges of owning a classic car, I guess. I can't help but wonder if the heat and humidity here in So. Florida contributes, especially to electrical issues.