speedometer/cruise control problems - need help

[Andy911sc]

When I got the car back on the road last summer the speedometer was accurate. I randomly tried the cruise control and Speedo now reads slower than I am going significantly. I just had the Speedo in for a repair for the odometer gear and needed some other parts, I also just replaced the electronic speed sending unit on the transmission. My speedometer is still reading slow, the same as before. The cruise doesn't really work that well, but can something in that cause the inaccuracy? This is really starting to drive me crazy. I don't want to have to use my phone anymore for my speed.

[DRACO A5OG]

who did the speedo work? nhs and my local speedo shops both have rollers to test the speedos on the car. if local, take it back on the car have them check it.

[Andy911sc]

The speedo tested out just fine. It's either something with the cruise control electrically zapping some power from the speedo and causing it to read slower. Or the magnetic ring that trips the signal at the sensor lost some pieces. I was talking to my mechanic about it on saturday at our regions CPA-PCA first cars and coffee for the season.

[James Brown]

you can rase the rear wheels and with the car on, turn the rear wheels slowly while watching the spedo and count the "blips" on the as you turn the wheels. Mark a wheel so you note when you make a 360˚ turn. can't remember how many magnets are on the wheel inside the trans but think there are 8 (thanks Walt)

[Andy911sc]

James, thanks. I have been sitting here trying to think how I could test this. I will have to do this sometime in the next two weeks. I am hoping it's not that because I really am not in the tranny tear down mood.

[James Brown]

yeah not fun, i thought i had that problem, turned out a bunch of crud was behind the sensor on the tranny, cleaned it out and worked good

[Andy911sc]

Well looks like I am missing 4 magnets and it's weird that it's every other one. The odds of that happening are close to impossible. I will be putting the car up on jacks later this week and start taking the differential apart. I am going to rebuild the CV joints while I am in there (new boots, bolts and a repack on the joints). My mechanic also informed me that I could have some corroded magnets also. We will see, my deadline is to get the car back on the road before the April 20th Hershey Swap Meet.

Does anyone know where I can get new magnets or pick up a new carrier plate?

[schumicat]

how did you figure that out? did you already take out the diff? That is very odd and unlikely to happen by chance. I have a new carrier bearing and set of new retainers from the ring bolts if you need them. I ended up not needing them and waited too long to return so you could have them cheap?

Originally Posted by Andy911sc

Well looks like I am missing 4 magnets and it's weird that it's every other one. The odds of that happening are close to impossible. I will be putting the car up on jacks later this week and start taking the differential apart. I am going to rebuild the CV joints while I am in there (new boots, bolts and a repack on the joints). My mechanic also informed me that I could have some corroded magnets also. We will see, my deadline is to get the car back on the road before the April 20th Hershey Swap Meet.

Does anyone know where I can get new magnets or pick up a new carrier plate?

[Andy911sc]

Jacked the car up and hooked an multimeter up to the speed sending unit. Set it to continuity beep. Only had 4 beeps instead of 8 like I am suppose to have on a full tire rotation. Speedometer is reading slower which confirms that there are missing or corroded magnets not being picked up by the reed switch. I also testing the voltage and resistance on the cruise control per the bentley book and everything on that checked out fine. New speed sending unit and just had my speedo out for odometer repair. I am missing half the amount of magnets and my speedometer is registering approximately half the speed I am traveling. Too many things check out.