78 140 mph speedo - will it work in an 81?

[Black81Vette]

hey guys, i just got a 78 parts car (i have parts if anyone needs them, email me) and it had the 140 mph speedo i wanted. i have cruise and the parts car doesnt. is the gear different in the speedo for the cable? can i just bypass the cruise and run single cable the the speedo , but will the tranny gear be correct?what do i need to do to make it work?

 

[Black81Vette]

ok, the parts car does have cruise, but its the single button cruise. can i just transplant the tranducer and cables along with the speedo without having to change drive gears in the tranny?

 

[Yoda]

Hi BlackVette,

I am not sure about the speedo correctness, but as for fit, the 78 should fit in the bezel of you 81. I ordered a 140mph speedo from Redline Gauges and it works correctly. I told John I had stock rear, tranny and 255/60/15 I don't know that made much difference since Rare is stock 4-speed.

I would think, (here is where I usually get into trouble) that there would be no difference if the exchange was for Automatic to Automatic or 4-Speed to 4-Speed other than tire size, which may make a couple or mph difference and no more than 5mph difference at the most.

You don't need to run the transducer if you are not going to use the cruise, you can get the cable for a 4-speed and go direct from the transmission to the speedo housing.

A little trick in changing out the the old cable with the new one... disconnect the cable from the tranny, tape the new one end to end with the old one and have someone pull it back up through along the firewall, with you or someone helping to guide it up through the engine compartment... and be aware of the sharp angle once you enter the speedo housing area

Bud

 

[Black81Vette]

A little trick in changing out the the old cable with the new one... disconnect the cable from the tranny, tape the new one end to end with the old one and have someone pull it back up through along the firewall, with you or someone helping to guide it up through the engine compartment... and be aware of the sharp angle once you enter the speedo housing area



thx for the reply. yes, i have already replaced 2 lower speedo cables on mine. i think the transducer sticks in reverse because it twists and shears off the cable end when i go in reverse to fast. i'm going to try to get it all knocked out before the weekend and see if it works. also, one more question. do you have any trouble putting the lower end of the cable on the tranny? i always tear up the screw on aprts because i have to turn it on with a screwdriver 1/8 turn at a time. and a pair of pliers doesnt fit up there and i dont have a lift. any sugguestions?

 

[Yoda]

I changed out my speedo and installed a new cable last year, I don't recall any problems with the tranny connection. I saved the aluminum guideway the cable fit in from the old cable, and used it with the new cable.

The aluminum piece I speak of fits on the cable as it comes out of the transmission... NOTE: my car is a 4-speed, I'm not sure what the difference is in the transmission connection on the Automatic tranny..

I also jacked mine up and used jack stands to get under the car for the transmission connection process

Bud

 

[Nut]

I did the same thing as Bud's with my 165mph speedo swap and no problems with the cable connections at either end. And you definitely want to save the aluminum guide rail. It makes the tranny connection a LOT easier. Although with a big mitt like mine getting in there to align and tighten the locking sleeve was no easy matter.

Good Luck.................. Nut

 

[Rudzvetz]

If you have your stock ECM and trans with lockup converter you need to keep in mind tha the 1981 Corvette has a vehicle speed sensor built into the back of the speedometer that controls converter lockup. I don't know if this is something that can be removed and transplanted onto an older speedo or not. It's been awhile since I had mine out.


-Justin

 

[Nut]

Yeah you're right Justin. I lost my TCC when I swapped it out. I'm doing a little electrical schematic analyzing to see if there is a way to by-pass that sensor. I had a manual switch put in mine years ago so I could switch the TCC on and off as I pleased. But now it's not working because I had nowhere to put that sensor plug in the back of the 165 speedo. Stay tuned if I can get this to work.

......... Nut

If you have your stock ECM and trans with lockup converter you need to keep in mind tha the 1981 Corvette has a vehicle speed sensor built into the back of the speedometer that controls converter lockup. I don't know if this is something that can be removed and transplanted onto an older speedo or not. It's been awhile since I had mine out.


-Justin

 

[like2drive]

Talking about "a stitch in time"! My NEXT (drum roll please) project is the speedo change and you guys have already , without me even asking!, gave me a heads up on this. thank'uns again. Incidentially, I WAS planning on the 140 speedo from Vips. Now know another source! Jimbo

 

[pbcanney]

Well....


This thread has me wondering... I'm swapping in an M21 from a 79 into my 81, I wonder what the speedo will say when I'm actually doing 55..

hmmmmmmmmmmm

 

[Rudzvetz]

You can forget about needing the speed sensor on the speedo on 81's and use a Bosch relay and a GM vacuum switch to handle your lockup automatically instead of the ECM.


-Justin

 

[81' Corvette Guy]

Ok guys if I have a stock speedo on my 81 and want to convert to a redline 140 mph speedo..I will have a problem with the ecm because of the speed sensor..what happend to the original speedo speed sensor after the upgrade to new speedo??? Thanks ..Joe!

 

[like2drive]

speedo swap and vehicle speed sensor

recently made the speedo swap, sort of. Corvette Clocks by Roger (Tennessee) took my 81 , 85mph and converted to a 165mph for less than the 140mph speedo would have cost from one of the catelogues such as Midland, Corvette Center. It looks like new and it still contains the same hook-up on the rear and whatever's inside to activate the speed sensor.
You can buy from the catelogues tiny, plastic gears to swap into the transmission if the speedometer is off. From what I've read here on another thread, there is a different speed sensor for the manual than that for the automatic. The only reason I mention this , is there must be another reason for the speed sensor other than to facilitate "converter lock-up"...but I yet don't have a clue what it does for the ecm. Incidentially, the sensor itself is blue, mine is made by AC, and is located at the driver-side , front of center console. According to the
Assembly Manual, it connects individually, at the top of the ECM. It didn't appear to be that way on mine, so for the time being, I'm leaving it alone.
regards, Jim

 

[GerryLP]

When I asked John, From RedLine, to help me keep the TCC and cruise control function, he said "no problem", and I believe that he swapped the aft speedo body section (with the square hole for the sensor) from another speedo, and returned to me a working 140 MPH speedo that works just fine in my auto tranny Vette. I am happy!

GerryLP:cool

 

[81' Corvette Guy]

Gerry..what is the TCC you are eluding to...know the cruise function..glad to know this is a workable mod....thanks! Joe!

 

[GerryLP]

The reason that the speedo sensor is there, I believe, is two fold.

One, is to inform the computer command control when the vehicle is slowing down, accelerating, and when it has reached cruise speed. This information is used to engage the torque converter coupling (TCC) and lock-up the converter (the lock-up provides a no-slip operation of the torque converter).

Two, the cruise control function.

Operating the L81 without the lock-up seems to reduce milage. Now, this could be argued either way, but I believe that its use saves 1 to 2 miles per gallon. This condition is magnified when using a higher stall converter. I needed the higher stall converter due to cam options I implemented (back when the L81 was still in my car; now it sits on a stand in my garage). The other engine I have now requires a slightly higher stall....off the subject...:W Anyway, The lock-up features still allows me to maintain a decent mileage.

GerryLP:cool