crazy speedo when warm

[Locutus]

the speedo gauge in my NA8 reads over 2x actual speed when the gauge itself is warmed up - say after it's parked for 10 mins after a long drive or after it's parked in the sun in summer.

gauge readings return to normal after it's cooled down. anyone else experienced this? the odo and trip distance don't seem to be affected - just the speed reading on the gauge.

any ideas what's going on here? i haven't taken the cluster out of a 5 before, but in other cars it looked like one sealed unit so am i correct in assuming i wouldn't be able to open it up for a looksee?

[Garry]

i haven't taken the cluster out of a 5 before, but in other cars it looked like one sealed unit so am i correct in assuming i wouldn't be able to open it up for a looksee?

Yeap, you can open it up and just take out the speedo guts if you like. The clear plastic face and front black plastic trim are one piece held on by a few clips. Once you have that off the speedo is attached with a couple of screws from behind. You may need to take the speedo face off for a good look, I cant remember from my NA days, but thats no big deal.The NA's use a mechanical speedo so you should be able to see if something is broken, possibly the needle return spring? I've heard of speedos reading slow but never fast like your car.

[Locutus]

took the speedo out today while i was swapping oil pressure gauges: http://www.mx5cartalk.com/viewtopic.php?p=379428#379428

the speedo mechanism itself looked ok but my cluster and part of the dash behind it was absolutely CAKED in grease. it looked like a lot of it turned to liquid on a hot day and covered the bottom of the instrument cluster and leaked out. whoever took the cluster on last went absolutely crazy with the stuff. there were even good sized grease chunks inside the speedo mechanism on parts which don't rub against anyhting else.

so i'm convinced my wierd speedo behaviour is a result of severe grease poisoning.

i wiped as much grease off as i could and left ample amounts on the gears in the speedo mechanism and the speedo cable itself. i hope that solves the problem but only time will tell.