On Speedo..Can't get past 55!

[Buzzbomb]

Hey guys, 

 

I wonder if some of the all powerful Datsun minds here might be able to point me in the right direction. I was out on the highway today, and my speedo was fine on the trip up at all speeds, including over 65 MPH. When I was coming home, I noticed I was keeping up with 80 MPH traffic and my speedo was reading 55! Kinda glad I had the speedometer Android app on my phone..it showed I was around 65-70. Anyway, I pulled into town and all was fine from 55 down? I checked it vs. my GPS app on my phone (which is luckily VERY accurate, BTW, I tested it vs. a '09 Maxima) and my cluster was off by 3-5 MPH around town, which has always been the case. You guys think my cable is reaching the end of the line :P ? It has 187,000 miles on it. I was thinking maybe it's just flat worn or perhaps filled with crud. It's not a "smooth" reaction on the guage, but it's not jerky. It's almost like it's running a tad slow; in other words it's smooth but seems to be binding? Is it even worth trying to clean it? Here's to hoping it's not a real b*tch to get in and out.

 

Thanks for any ideas!

[datzenmike]

The speedometer cable spins a magnet on the end that induces eddy currents and an opposing magnetic field in a metal disc with the speedometer needle on it. The disc has a return spring and is pulled around by magnetism the faster the cable spins.  It's not physically connected so I would say that some foreign object is caught in it.

[Buzzbomb]

That's interesting, DatzenMike.. So the magnet is on the end of the gauge side, right? And I assume the return spring is to return the needle to 0? Man, I sure don't want to have to pull that dash apart for a problem on that end :( . That looks like a really involved job. I don't know how in the heck anything could have gotten in the back of the housing? There sure isn't anything in the front of the housing obstructing the needle. It's pretty odd that it hangs up at 55, though. I sure appreciate your suggestion. Who'd have known the intricate details of these old speedo's other than a self taught Datsun expert like yourself :D

[datzenmike]

The hardest part of removing the dash cluster is getting your hand up in there to un-clip the speedo cable from the back. I pull as much slack as I can up into the engine compartment. Loosen the retaining screws holding the gauge cluster and pull it outward with the extra speedo cable lack. Then you can get your hand in there. The rest is just unscrewing stuff. Also helps (but you can do this without) if you remove the steering wheel.

[Buzzbomb]

Thanks for the tips, DatzenMike! I guess it's not as tough as I first thought. I'm hoping if it's something in the way, maybe it'll work it's way out. I know these old interiors sometimes like to turn to dust :P

[difrangia]

Gotta be the 'Anti Sammy Hagar' syndrome.

 

https://www.youtube.com/watch?v=RvV3nn_de2k

[Charlie69]

Pulling the speedometer cable on a 720 is fairly easy. 

If it is a tilt column put it in the lowered position if it not remove the 2 nuts on the column retainer clamp

Remove the column clam shell

Remove the 2 screws for the bezel

Remove the 4 screws holding the cluster to the dash

wiggle the cluster a little just to be sure it is loose

Open the hood and take the speedometer cable out of the positioning clamps along the fire wall

Push the speedometer cable into the cab gently releasing the cable boot from the firewall

Now you should be able to pull the cluster out of the dash far enough to release the wiring and the speedometer cable

Most people when pulling a 720 cluster do not free up the speedometer cable from the firewall.

[Buzzbomb]

I was thinking about that song over the last few days..LOL!

 

I sure appreciate the instructions on this, and I'm going to print them off just in case. Strange thing happened the other day, though. My speedo was fine..until I hit a weird spot on the highway! This has happened to me before on this exact same spot albeit my speedo didn't get stuck to where it couldn't pass 55. The highway has a dip and bump right after it, and when I hit that spot, it acted a little odd. It acted almost as if there was no resistance on the needle, but it leveled off to normal after a few seconds. DatzenMike's explanation of how the speedo works makes me wonder if there is "flex" or something at the back of the dash or in the cable that is causing this? I rechecked it with my phone speedo, and it was the usual 5-7 MPH off. Pretty weird...

[Charlie69]

The 720 speedometers go out quite often.  I have put 3 in my 86 and yes I am buying used or buying complete wrecked trucks and taking the speedometer out.

[Charlie69]

On 1 truck I had to remove the speedometer cable from the transmission to get enough slack to pull the cluster far enough out to get the speedometer cable loose. I think the reason for this was it was an after market cable and was shorter.