'78 speedo fault

phorbiuz

phorbiuz

Regular user
Hi all.

After some help/advice please. It looks like the actual speedo on my '78 C3 Anniversary has failed as the cable is good but the needle pins at the maximum 140 reading.

I'm not having any joy finding a replacement so I'm looking for clarification on what fits please? I believe anything '77 and earlier won't work but would any year from '78 to '82 be OK?

I suppose another option would be to have mine repaired but I don't know anyone who could do
 
kentvette

kentvette

CCCUK Member
The speedos for Vettes from 1980 are calibrated to 85mph max, so not good for a 78 I don't think.
You could try Speedy Cables, or Speedograph Richfield for a repair, which may be a cheaper option. Or have a word with Scott at Corvette Kingdom to see if he knows a repairer
 
Daytona Vette

Daytona Vette

Well-known user
phorbiuz said:
Hi all.

After some help/advice please. It looks like the actual speedo on my '78 C3 Anniversary has failed as the cable is good but the needle pins at the maximum 140 reading.

I'm not having any joy finding a replacement so I'm looking for clarification on what fits please? I believe anything '77 and earlier won't work but would any year from '78 to '82 be OK?

I suppose another option would be to have mine repaired but I don't know anyone who could do
Click to expand...
Not all the later (Big Rear Window) Vettes have 140mph Speedometers - The 1980 has a Speedo that tops out at 85mph - You can get higher if you want I have one that tops out at 165 mph - But the Needle shakes when it gets there.
How do you know it is the Speedo that is faulty - Have you checked the Plastic Cog on the end of the Spindle Drive as it locates in the Gearbox?
 
phorbiuz

phorbiuz

Regular user
Thanks for the replies.

I've done a fair bit of testing TBH. I unplugged the cable from the auto box and put the car in drive so I could see the worm drive was working fine. I then disconnected the box to cruise module and tested that spins fine. I then tested the short cable from the cruise module to the back of the speedo and that spins fine. That only leaves me with the clock itself. It's a bit weird really as the slightest bit of twist even as little as 1 anti clockwise revolution of the cable in 1 second and it will ping straight over to maximum reading and stay there. A spin clockwise will bring it back down to zero.

I had a quick chat with someone with more experience than me with these motors and it seems to me that as the cruise has never worked in my ownership anyway, if I get a clock or get mine fixed somewhere then I'll just get a longer cable and bypass the non-functioning cruise too.
 
phorbiuz

phorbiuz

Regular user
I've had a thought last night in bed (as you do, lol)....

Are there any electrical connections that assist in the speedo workings, apart from the illumination bulb? Electro magnet for example to provide resistance? Just in case a wire has broken or anything, and yes I am clutching at straws seeing as the availability of a replacement clock seems limited.
 
kentvette

kentvette

CCCUK Member
It sounds as though the return/control spring in the speedo head has broken? I think it's a magnetic drive system, so as soon as the cable spins it turns the needle. But, it needs the spring to hold it to the desired speed? (I think!)

The cruise system is controled via engine vacuum and an electrical current. If your cables are running through the "Transducer" (the item under the brake master cylinder), it is probably possible to get the system working. There are some good papers available and if you ant to go down that route later I can send them over.
 
phorbiuz

phorbiuz

Regular user
A bit of an update...

So I removed the speedo unit itself from the dash which was an easier job than I expected as half the screws weren't even there. As already said, the cable side checks out fine so I expected to find fault with the clock itself, and expensively for me I think I'm right. You can literally give the needle the slightest of taps and it will spin to the end in either direction, and even rotating the whole clock through 360 degrees the needle will hang in one position like a plumb line. In showing a mate this I clumsily broke the plastic needle too so now it defo needs repairing.

I've got an importer of these looking for a replacement for me but he isn't positive about it.

Speedograph Richfield have declined the repair as they can't get the parts. Speedy Cables have asked that I send them the unit and once stripped and analysed they can see if they can help, but they can't give me even an estimate for a refurb.

I've mailed Claremont and Corvette Kingdom to see if they can help or point me in a better direction and I'm hoping to get a reply soon enough from one of them.

Ebay searches have so far only found one supplier called Corvette Mike Midwest but he hasn't replied to my question - his listing says it's used and in km, but I think it's a library pic as it looks new and in mph. Looking through other suppliers in the USA it seems not to be an easy part to find.

So... If anyone has any more suggestions I'm all ears.

Thanks.
 
phorbiuz

phorbiuz

Regular user
The thread that just keeps on going, lol.

So, no further replies from any suppliers though I only used the JD Corvette contact page on Wednesday.

I do have someone over in the States looking for me (more details on that if/when it comes to a successful conclusion) who thought they could get me the correct 140 clock but it turns out they couldn't. Apparently the 160 clock doesn't fit the Anniversary '78 bezel. We're looking good for the 85mph clock though from an '80. I'm guessing that not many, if any, UK peeps will have gone from the 140 to the 85 clock so I'm not hopeful of a quick answer, but.....

Pretty certain the reading will be out. My thinking is that I'd have to either get the unit re-calibrated or I could possibly get away with changing the gearbox gear/worm drive. Would anyone know which gearbox drive would be correct for a '78 3sp auto, or possibly point me to a good USA based info source who is more likely to have done this?

Thanks (as always) in advance.
 
phorbiuz

phorbiuz

Regular user
I need to update the thread here as I've done Scott a disservice. Scott did indeed reply as far back as the 31/1 but my mail system flagged it as spam and took it from my inbox. Sorry for any negative vibes there.
 
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