blownmiata

I hooked up my '99 Miata digital speedometer direct to the T56 speedo pickup wires. It works but it reading WAY too fast - maybe 10x what it should be. I had the PCM programmed to match my tire size. Is there something else I was supposed to do?

loweredd

iTrader: (2)

If I remember correctly, GM puts out a 4000 ppm signal, your stock speedo must be different. Maybe 500 or something. Programing the computer for your tire size will just correct the computers 4000ppm from the tire size that was in there before.

busta9876

iTrader: (3)

You're T56 will out put a 17 pulse per driveshaft revolution. If you're speedo is hooked directly to the trans speed sensor, you'd get about 51,000 pulses per mile, with a 25" tire and 3.73 gears.

Use the 4000 pulse output from the computer, that will be about 12x slower.

blownmiata

OK - back to the computer - thanks for the info.

blownmiata

So if I understand it correctly, there are two wires going from the speedo pickup in the T56 to the PCM. Then there is one wire that goes to the speedo gauge?

My Miata has two speedo wires going to the instrument cluster. I have tried hooking the speedo output from the LS1 PCM to each of the instrument wires - one does absolutely nothing. The other makes the speedometer bounce around erratically for a few seconds when the car is started, then nothing after that.

Even if I do need a Dakota Digital to adjust the signal, I would think I should be getting a steady signal. Help?

busta9876

iTrader: (3)

Depends on what the stock speedo signal was. the T56 speed sensor outputs SIN wave signal, or AC voltage. The PCM outputs square wave, DC voltage.

blownmiata

So I went to check the data codes tonight, and finally figured out that I had the serial data hooked to the speedo (too many damn green wires!). No wonder it didn't work right. Now I have the speedo pickup wired back to the PCM, and will try to hook the correct PCM speedo output wire to one of the two speedo instrument inputs. Hopefully the other instrument wire is ground.

BTW - the only code I have is a bad O2 sensor - not too bad!