Lazy speedo
 

The car is a '78SC ...today whilst driving normally my speedo started to read roughly a third of the cars
estimated speed judged by looking at the rev counter . ie at x revs in top it used to read 90mph , now it reads 30mph but otherwise operates normally . Does anyone know if this is the gauge or a cable problem .

bump

maybe start by checking the performance of the tranny sender

bump

I had that happen on a 66 Dodge many years ago, ended up being a broken cable. I had an additional symptom (aside from slow speed reading) of a bouncing needle when accelerating though. This assumes of course you have a mechanical gauge and not electrical.

Bones, did you ever solve your problem?

I'm experiencing a lazy speedo on my 86 right now. When taking off from a stop it doesn't start reading untill about 35-40mph, then picks up right around where it should be. Anybody else had this kind of problem?

Nope..
maybe some electrons get lost at low speeds inside the case. Warren might suggest a scope.
I would probably borrow a known good speedo for info.
... Ron

No Matt I haven't got to the bottom of it, I'm thinking it's the sender as it's electronic. I would have already replaced it but it seems like a bit of an involved job. Has anyone else replaced the sender and if so how difficult is it?

Thanks Ronnie. I'll pull it out and take it to Palo Alto Speedo when things slow down around Winter.

Bones, if you change the sender, let me know what a PITA it is or isn't.

the sender is easy on a 915. a spring clip on the tranny and a plug in the cabin tunnel. and there is a sender test in my books somewhere. something about jacking up the rear wheels and turning them about 6 times and watching a volt meter jump a fixed number of times.
and the good news is that the car is now, finally, prepped for carbs.........Ron

Your're going to make the switch? Did you change the cams and pistons?

Not to freak you out, but your symptoms sound strangely familiar.

There are a series of magnets in the tranny that send a pulse to the speedo sensor at each passing. If one of these were to fall out of its housing...guess where ends up....flopping around somewhere in your tranny.

See:

http://forums.pelicanparts.com/showthread.php?s=&threadid=91745&highlight=differe ntial+magnet

Edit: Just noticed your speeds dropped more than half.....doubt more than half the magnets fell out. Just consider this an FYI then;)

Yep..
been prepping the car for this.
it already has the RS 8.5 pistons.. the cams are here already.. had them brand new for over a year. so my next act is to start getting the parts together for this, thus PMO, etc. now.
I want a fun car again.......Ron

I had the same problem, first intermttent loss of reading and eventually no reading at all. Replaced the sensor and now I get a doubled reading. The sensor is easy to replace and I have heard that the retaining clip should be either replaced or carefully positioned or you may get a faulty reading. I will be adjusting my new sensor soon to see if I can get a correct reading.

(Bones, I really didn't mean to hi-jack your thread)

Charlie, I read that thread when it was first posted, definatley spooky, and I hope that's not what has happened to me.

Don't worry this is all interesting stuff just like most threads here. lol