other My speedo's out of control

[peterschop]

So I was driving back from Vintage Fest today on Chicago side streets in slow traffic when my speedometer decided it wanted to take off. I was only doing about 25mph when I looked down and saw it hovering from around 120mph, going past the 630 mark and all the way to the peg. My fellow 02 passenger said he had seen this before and It could break the peg off if I continue to drive it like that so I disconnected the speedo cable. He said something about magnets in the speedometer had come loose? After I disconnected the cable, it continued to read 45mph, slowly going down to 25 by the time I got home with the cable disconnected. 

 

A little background on this speedometer. After I replaced the automatic transmission with a G245, the speedo always read high. When I checked it with a gps app I found it reads about 20mph high at 45 and the level of error increases the faster I drive. What's going and can this be fixed? This is a 1974 with a 3.64 rear end.

[Mike Self]

Had the same thing happen to my '73's speedo some years ago...and was able to repair it.  I did a Roundel column on the repair procedures--drop me an IM and I'll send you a copy.

 

mike

[FunkyLaneO]

Here is a little video I made for another FAQ member a while back on how to fix the problem you are having. https://www.youtube.com/watch?v=oh4C5a1pSbI

[peterschop]

Thanks for the article Mike and great video Funky. This could also explain why my speedo has been reading fast.

[2fysh]

Pete I wouldn't lie to you.  I knew Mike would jump in here with the article.  He sent me the same one.  It was cool riding down W. Division with Pete going 120 MPH!!!!

 

Thanks again for picking me up.  Loved the show.  Sorry I had to get back.  Lucky to report that Wifey had a great day and wasn't to worried that we were gone so long.  It was, in fact, her birthday. 

 

And I meant it would break the needle, not the peg.  My bad for the wrong info.

Edited September 20, 2016 by 2fysh

[peterschop]

8 minutes ago, 2fysh said:

Pete I wouldn't lie to you.  I knew Mike would jump in here with the article.  He sent me the same one.  It was cool riding down W. Division with Pete going 120 MPH!!!!

 

 

Jason, it was fun. Even though my speedo said 120 mph, it felt more like 20 mph with that disoriented driver in front of us. I am glad to hear everything's good between you and your wife. I mean after bringing her to Chicago for her birthday and then taking off to go to Vintage Fest and then stopping for more 02 parts for my car on the way back. I hope you were able to make the show on time.

 

As for the speedometer, I replaced it with a spare of unknown condition. After a drive, I found that the spare works but the odometer doesn't. I will repair the one I took out, swap and then repair the spare.

[peterschop]

Yesterday I repaired the wacko speedo. The fix was quite easy with the guidance Mike and FunkyLaneO. After the gauge was out of the cluster, it only took about five minutes to fix. I only needed a 22mm wrench to remove the nut and a small screwdriver for two screws. I did not take the odometer drive off since that already worked. What had happened was that the magnetic disc slid forward on the shaft and was contacting the receiver sending the needle to it's limits. All I did was push the disc back flush with the end of the shaft with my fingers. I did not take it all the way off and use red loctite as in the video. If it happens again, I will locktite it.

 

Now that my "magnetic eddy currents" are back in order, and the gauge is back where it belongs, the speedometer works the way it should. I checked the speed for accuracy with a gps app and it appears to be about 3 MPH too fast around 45 to 60. The rear of the speedometer was marked with W=1.30. My spare speedo seemed to be right on the money as far as speed. It was marked  W=1.28.

 

 

 

Back cover removed after taking off 22mm nut

 

On the left is the drive with the magnetic disc that has slid forward on the shaft. You can see on the right where the disc has worn into the receiver.