labhound

Since I've had my '74, almost 18 months, the speedo never worked and the po kept notes on about all services and problems with having the speedo "fixed" four times then just disconnecting it altogether.
I just tried to diagnose it my self and so far the cable is good and the speedo works when hooked up to the cable, by using a drill to turn it, the drive gear looks good to.
Now then, the drive gear is an 18 tooth brown and by checking a chart it says it should be a 19, would that make a difference?
http://willcoxcorvette.com/instructions/1974.JPG

MelWff

the gear you are referring to that attaches to the speedo cable is the driven gear, the drive gear is on the transmission output shaft. If the speedometer works when attaching a drill to the cable at the transmission end and the driven gear has no damage, try to see if you can see the teeth on the drive gear inside the transmission case.

doorgunner

Also.....the previous owner may have installed an 18 tooth gear because of the tire diameter.....you need to rotate the drive gear that is in the transmission to see if any teeth are damaged.

Here is a sample gear set-up: The white lower gear is the driven gear (some vendors refer to it as the DRIVE gear. The green gear is actually the DRIVE gear as MelWFF said.

doorgunner

While we are discussing gears.....

my speedometer reads 20 mph too fast at 70 mph.......

(TH400 A/T....3.36 Diff. gears....15 X 295 X 60 series tires)....



do I need a DRIVE gear with more teeth/or less teeth?

7T1vette

With your symptoms, the most common fault with the speedo system is that the drive gear (cable connection at the transmission) has developed a crack at the end where the cable is inserted. The crack will be at one of the four corners of the 'square' recess where the cable fits. Because of the drag resistance on the cable system, the cable will just stay in place and the drive gear will rotate around it with the crack widening to allow the slippage. Remove this gear and replace it with a gear having the same number of teeth. You can also refer to it by 'color' for that model transmission. (I'm assuming that it is still a THM-400 trans.)

Since the primary cause was that there was excessive cable drag, you will also need to lubricate and/or replace the speedo cable to make sure that it doesn't happen again. There is a "squeeze" clip at the connection with the speedo gauge. Press this and pull the cable housing loose. Then you can pull the cable out of its sheath to inspect and, if OK, lubricate with dry graphite powder [only...NO liquid lubricant]. If you find that there are worn/frayed spots on the cable, it should be replaced also.

Not sure if the '74 car used two short sections of cable housing (instead of one long section). If so, try to find cables for those two sheath sections; installing a single, long section will present problems of its own, with cable 'whipping' inside the long sheath. That will result in needle bounce on the speedo.

labhound

The latest, the gear in the transmission looks good, I rotated the wheels and all gears are good. Now then, the drive gear does rotate when installed and no cracks are seen, it has a good fit into the adaptor and the adaptor rotates until it is installed, so that may be the problem'
with everything except the cable installed onto the trans, I didn't see any rotation movement from the adaptor, but yet it moves freely sperate.
It's a 4 speed by the way.

doorgunner

AAAAAAAAHAA.......a 4-speed......I'm clueless about manuals even more than A/Ts!

I think that the driven gear should rotate with finger pressure in the adapter....then when that assembly is installed into the transmission---the driven gear should NOT rotate at all because it is meshed with the drive gear that is mounted onto the tailshaft.....if the driven gear DOES rotate after the assembly is installed, I would think that the keeper is defective/broken that holds the drive gear in place onto the tail shaft....(automatic shown--but the same principle of operation)

labhound

What I'm wondering is that it could be the adapter/gear reducer thats may be culprit.

7T1vette

The "adapter" is just a carrier for the drive gear. There is no other mechanism inside it.

labhound

I'm talking about the 90 degree adapter gear reducer.

robert.sperry

I had a similar speedometer issue with my son's 74' 4 speed and it took me about a year of inspecting and replacing parts before the problem was solved. As you know by now the speedometer adapter is held to the side of the transmission by a small rectangular metal plate with a single bolt hole at one end. I noticed that the adapter unit was a little wobbly as installed which I attributed to wear in the slot in the side of the adapter into which the metal plate engages. So I took the adapter to a hardware store and tried various small metal pieces until I found something that was a tight fit in the slot in the adapter. I bought the metal piece, took it home, and fabricated a new metal plate using the old one as a template. I reinstalled the new metal plate and the adapter and the problem was solved. Hope this helps.