Slow Car Fast
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December 20, 2010 - Okay, so now that I can steer, it's time to find out how fast I'm going.
The T56 transmission puts out an electronic signal. Naturally, the MG speedo uses a mechanical drive. There's a solution to this that involves a little motor in a box that spins a speedometer cable to match an electrical signal, but it's fairly expensive. For less money, I found a Smiths programmable electronic speedometer! I've been waiting for this part to show up for well over a month now, it seems they're made to order by British craftsman and, well, it takes time. But it'll fit into the MG's instrument cluster better than any other gauge, right down to the font on the numbers. The black center on the needle is the biggest giveaway really. I had my choice between 140 or 170 mph ranges, and I figured the 140 would be easier to read in the sane ranges - and above 140, I need to keep my eyes on the road!
When I went to hook it up, I found another advantage to using a Smiths part. They still use the same Lucas wire colors! Switched power is green, instrument lighting is red with a white stripe - makes it easy. But I did have a bit of trouble.
The GM engine computer (PCM) puts out a digital signal to run the factory speedometer. So that's easy, just plug it into the input on the speedo and away we go. Umm, no. With the rear end up on jackstands, I ran the car in gear and nothing happened. I tried the "low voltage" setting and nothing.
A bit of research on revealed that the PCM puts out a square wave signal that doesn't always play well with aftermarket speedos. The solution is to pull the signal right off the transmission. Okay, that's easy enough. I spliced into the transmission VSS signal and tried again.
This time, the speedometer woke up but it was acting odd. It would pin, or just hover mid-range but not consistently. Finally, I discovered that by letting the car idle in first gear I could get a 70 mph reading. According to the OBD-II reader, the PCM thought I was going 7 mph. So my calibration was off by an order of magnitude!
I quickly set up the speedo to count 40,000 pulses per mile and it settled right down. It's still not properly calibrated (what would be the odds?) but it's in the ballpark now. I just need to check some numbers, do the math and input the proper calibration. Getting closer...
tags: instruments, speedometer