My friend is an artist and he promised to help me with this so I figured I'd try to get the rest of the info here.

I've always wanted more than an 85 mph speedo and of course the logical choice is getting one from another car but unfortunetaly this isn't really an option because I want the 84 scheme (where the artist comes in). My idea is to switch the gears in the tranny to be 1/2 of what they should be for my combo and thus uses the same hash marks but the numbers will be off by 1/2. Now, that would be the end but I'd like to retain CC. I've taken the speedo fully apart and will try to explain it a little better than it has been

What happens is the speedo cable drives a I shaped bracket that is a magnent in the middle. It ecompasses a metal wheel that is spring loaded to bring the speedo needle down. The faster the bracket spins the more magnetic pick up on the wheel so it can overcome the force exerted by the spring. Now, the wheel is colored black and the bracket is silver. The silver passes by a hole where the VSS buffer "plugs" in (a LED and a reciever). If I just changed the cable gears the speedo would read 20, the car would go 20, but the VSS would see 10. the Ok, now, I have two choices here....

My first instinct was to attempt to make an algorithmic circuit to increase the VSS buffer's output properly.

My second idea is to mount two very light non magnetic bars perp to the original so it tricks the VSS into seeing twice as many turns/sec.

Anyone else see this? I made this small table incase you're confused.

Car SpeedA SpeedN VSS
20 20 10 10

When the car is going 20, the speedo will read 20 with the new art but the new 20 will be were the old 10 used to be and thus the VSS would be reading 10. If anyone can back me up on this theory it'd be greatly appreciated.


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1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 Trans Am T-tops
4-bolt main 350, performer intake, headers, Holley 650, T-5, hayes clutch, dual elec. fans and 3.23's.
Daily driver and restoration
13.98 @ 101

 

How about this:


I removed the needle, and insalled it back pointing almost straight down. then I re-wrote the numbers that 0 would be 10, and 85 is now 95. I added marks for 0, 5, and 10. Then for 100, 105, and 110. This way I don't have to change anything else. With your theory, you'd have to do what I'm doing, and more. I'll try to get some pics up asap.

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'82 Firebird, dead stock, 9 bolt disc rear, over 200,000 miles and still going strong, more to come...
http://www.spinfrenzy.com/stingerssx...easures.html#4

 

Both great ideas! With the first idea, you could use a one shot (555, 74LS122, or 74
LS123), trggered by both rising and falling edges of the VSS output, to create a frequency doubler. The output pulse width of the one shot would have to be shorter than 1/2 the period of the VSS output at maximum mph (the highest frequency seen at the output of the VSS.

Tim

 

Doh. Forgot to ask one of my main questions....would the VSS be damanged by reading 120+ (at the track) readings on an 85 mph speedo? Or does it have a higher limit?

If I changled the needle location my VSS would still be off by how many MPHs I made the offset.

------------------
1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 Trans Am T-tops
4-bolt main 350, performer intake, headers, Holley 650, T-5, hayes clutch, dual elec. fans and 3.23's.
Daily driver and restoration
13.98 @ 101

[This message has been edited by 84FTA (edited January 26, 2001).]

 

The VSS would actually be seeing the same input - its output would be doubled by the new circuit & its output is sent to the ECM. The question is, would the ECM be able to interperate the higher frequency (speed)? - my guess is probably so.


Tim

 

I meant with double the reflective area. I'm really leaning towards this one because it'd be easy to do and there is space for it. There is little space run an intercepting circuit without hacking the stock harness.

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1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 Trans Am T-tops
4-bolt main 350, performer intake, headers, Holley 650, T-5, hayes clutch, dual elec. fans and 3.23's.
Daily driver and restoration
13.98 @ 101

 

It would depend on the bandwidth of the optical receiver and its associated amplifier in the VSS buffer. Chances are that will work - only one way to determine if it will operate - spin the reflector & look at the output of the VSS & verify its amplitude & frequency are correct. The output of the VSS will probably require a pullup resistor if tested by itself - if I remember correctly, it has an open collector output that is pulled up in the ECM.

Tim