Whilst carousing the local pick-n-pull this afternoon, I came across an 89 GL with a tach. Rather than trying to transplant the tach head into my DL, I just yanked the whole cluster. Everything went fairly smoothly. Reset the mileage, replaced the odometer gear with an aftermarket one while I was at it too.
I remembered reading that the red/white wire is the guy that gets plugged into one of the lower connections on the tach. That works. There was, however, a yellow wire left over. There was no obvious place for it to go. The connections were different on the two clusters. I plugged it all in, started the car up, and everything seems to work. I've driven it around this evening and can't find any obvious missing functionality. What was the yellow wire doing?
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"If you tell the truth, you don't have to remember anything." Mark Twain