Good Morning,

I have what I will call an intermittently good speedometer.

over the summer it worked well 90% of the time and 10% of the time it indicated between 10 and 15% slow.

for 3rd coast is was rock solid and gave me no problem until I got caught in a small rain shower. During that drive it went from actual speed back down to about 10% slow and has been there since.

to be specific, it indicates correctly up to about 20 MPH before it "boggs down". Then for or the next 20 MPH or so in acceleration it only climbs in indication to 30 MPH vs about 40 MPH (ish) actual speed.

Once it starts increasing again above 30 indicated, it continues to show faster speed at what appears to be linear progress with speed but maintains that 10-15% slow indication.

I performed Wally's annual electrical maintenance procedure over the weekend hoping for a simple ground problem but it had no impact on the speedometer.

I have not yet inspected the remaining ground points and will start there this weekend but am hoping someone will have seen this specific problem before. My forum searches didn't turn up a threat that discussed this issue.

So in summary - cruise control works, speedo works, it just boggs down at 20 indicated and when it picks up again its behind the actual speed and never catches up

look forward to your ideas.

thanks in advance for the help.

Paul
1979 Porsche 928
50,000 Miles

 

an additional data point I left out of the first post.

when the speedometer is 100%, the odometer also is 100%

when the speedometer is at 85%, the odometer either runs at a reduced rate, or occasionally stops all together.

I estimate my total miles are now short by about 400 while this problem has been developing.

my continued thanks,

Paul

 

Still planning on cleaning all ground point this weekend along with verifying the connection at the transmission is sound.

Other recommendations are welcome and appreciated.

Thanks,

Paul
1979 Porsche 928

 

Paul,

I recently did the hard wire conversion on my pod as the flex film was shot. I also replaced the odo gear while I was at it since the odo was not working. When I put it back togather I had no speedo or odo

I went back to the wirng diagram and I realized I had missed a ground jumper from pin 7R to 11R on the car side connector to the instrument pod during my hardwire modification. 11R is a ground wire that runs to pin H2 at the CE panel which is in the speedo circuit.

Once I fixed that ground issue she has worked flawlessly.

Perhaps it is the ground connection at the pod flex film.

 

Thanks Jim,

I'll start there.

Paul

 

Keep us posted on your progress and post lots of pictures....

 

Yes, chasing an electrical gremlin is one way we really learn our beloved auto's....

My Speedometer problem has always been the connector in the spare tire well.....

I clean the pins and it works for another year.......part of my yearly maint. plan, now.....too lazy to change it out......