Not entirely sure why but my Isspro R5535M wont work with the factory tach feed in my 1992 dodge pickup. I had Isspro test it and it works they suggested a R8912-92 adapter however this would cost $40 and this tach doesn’t require the filter so its just female spade connectors. This wouldn’t solve my problem though. I’m guessing I need a new pcm, but my cruise, alternator and air conditioning all work so do i really want to spend $300 for a new pcm just to get my tach going. I found a R8912 (magnetic pickup [i wouldn’t buy one from isspro; they raise the price $15 every 2 months]) i had laying around and modified the bracket so it bolts next to the factory engine speed sensor. when this decides to give up ill play with the wires coming off the factory sensor and try to tap one of those.
Update.. I bought a Isspro R5503 and tested my tach feed and it works without a problem. I like the isspro gauges but their tech people make me wonder.