Speedometer not speedometering right

[PhattyX]

Re: How is that right?

Alright, so I'm almost roadworthy and I'm having trouble with the speedo. At a standstill (key off) it shows 35mph. When I start moving, it shows 65 - 70 mph and sits there shaking. Any thoughts what to check first? It worked before I started the restore. It wasn't removed from the vehicle, just pulled from the dash and tied up during painting.

'84 CJ7

Thanks,

X

 

[Inspector-Gadget]

RE: CJ Tach Wiring

Hi-Ya PhattyX,

Just a guess: your speedo is broken internally.

Regards,

Gadget

 

[88Wrangles]

RE: How

yep, that's usually what they do when they're something wrong inside...with the speedo it self.

When it was moved around, it could have easily shifted or broken something inside that was ready to go anyways.

 

[redrooster]

The cable just screws in the back of the gauge. Reach up in there and unscrew it to see if the gauge will drop back to zero. Shoot some WD40 on the ends of the cable at the top and bottom and see if that will help. You may even shoot some in the hole on the back of the gauge. If it's already broken then it wont hurt it any worse.

 

[mud4feet]

RE: Idoling too low

.............. If it's already broken then it wont hurt it any worse.

Great advice. That's my theory on most anything I repair and I've learned alot from it!

 

[Snitty]

RE: Re: RE: 2.5 swap to 4.2 what is involed?

Same here. That's how I learn how most things work. I find a broken one and mess with it to see if I can get it to work. When I'm done, I know how they work

 

[Sparky-Watts]

New Pope

Things are usually more broken when I'm done with them than when I started. And usually, I still don't know how they work, either. And I was a paramedic, too....... :lol:

 

[PhattyX]

Re: how can i get better gas mileage?

redrooster
Posted: Apr 19, 2005 - 03:01 PM

--------------------------------------------------------------------------------

The cable just screws in the back of the gauge. Reach up in there and unscrew it to see if the gauge will drop back to zero. Shoot some WD40 on the ends of the cable at the top and bottom and see if that will help. You may even shoot some in the hole on the back of the gauge. If it's already broken then it wont hurt it any worse.

Hopefully, I'll get to try that in the next couple days. After another ride around the block, it now sits at 25mph until it's turned on, then seems to run 25mph higher than actual speed. I'm hoping there's just crap in there and the WD40 will take care of it.

As for the rest of the post, you know what they say "If it ain't broke... fix it until it is."

Thanks!

-X

 

[Robrt-Stephens]

Alright, so I'm almost roadworthy and I'm having trouble with the speedo. At a standstill (key off) it shows 35mph. When I start moving, it shows 65 - 70 mph and sits there shaking. Any thoughts what to check first? It worked before I started the restore. It wasn't removed from the vehicle, just pulled from the dash and tied up during painting.

'84 CJ7

Thanks,

X

The phony little plastic gear inside the transmission is shot. No biggy. Get a new one and that will fix it if you get the whole gear, cable and bezel screw in nut, all as one.

 

[Inspector-Gadget]

RE: alright, hellcreek, rubicon express, or bds lift??????

Phatty, now that I have more time, here is an explanation of why I “guessed” that the speedo unit is broken internally.

There is no solid mechanical connection between a speedometer cable and the speedometer needle.

A speedometer of the variety used in an 84 CJ is actually a small electrical generator (dynamo) and voltmeter. The voltmeter is disguised as a speedometer by marking the dial of the voltmeter in increments of miles-per-hour instead of volts.

The speedometer cable spins a small shaft mounted magnet inside of the speedo. The spinning magnet induces counter-electrical currents in a coil or cup mounted on a separate shaft on which is also mounted the speedometer needle. The induced current causes a torque reaction in the speedo needle shaft. (This is a minature electric motor.)

The angular deflection of the speedometer needle shaft is proportional to the angular velocity of the speedometer cable driven rotating magnet. The twisting of the speedometer needle shaft is resisted by a tiny spring (hairspring).

It is that hairspring which returns the speedometer needle to “zero” when the Jeep is stopped. When a mechanical speedo “floats,” it is usually the hairspring that is broken or disconnected.

I suppose it is conceivable that the speedo was re-installed in the dash such that the speedo housing is torque-warped, causing the internal mechanisms to bind.

Before you declare the speedo DOA, you might try loosening the speedo in the dash and applying the time-tested repair method of tapping the glass while swearing.

Regards,

Gadget

PS: Artwork created in MS-PowerPoint. OK -- so I ain't exactly another Lenny DeVinchi. :wink:

 

[Sparky-Watts]

Before you declare the speedo DOA, you might try loosening the speedo in the dash and applying the time-tested repair method of tapping the glass while swearing.

Also known as "percussive maintanence".