hanomon Posted November 19, 2004 Share Posted November 19, 2004 After utilizing the search function and sifting through the 40 previous posts on the subject, I came close but no direct hit! The stock tach was recieving a signal from a 6point dizzy and calibrated to such, what would tell the tach to adjust itself for the 2 extra cylinders? I'm thinking an in line potentiometer in the OE blue wire that had the resistor. but between where in the signal from the optispark's coil pack to the PCM? I've checked in the Haynes shop manual for the 94' vette. Index reference, tachometer... and it dosen't show a schematic. It only states that the tach signal/display is a part of the onboard control center. I have access to a portable strob-a-tach we use to check brushes on the generator's exciter. So calibration would not be a factor once a signal is established. Its just figuring out where to grab the pulse from. Would it be the RED clip on the PCM, point No.10 WHT or white wire, 121 (tach) as on the LS-1 harness? Anyone been there done that here? TIA, Dave Quote Link to comment Share on other sites More sharing options...
hanomon Posted November 19, 2004 Author Share Posted November 19, 2004 Further investigation reveals circut#400 yellow wire, A32 on the Red clip to PCM showing a varying voltage signal with key in run position. This is the VSS. of the IC system on the vin "P" engine. Ring any bells? Quote Link to comment Share on other sites More sharing options...
80LS1T Posted November 20, 2004 Share Posted November 20, 2004 I have never heard of anyone trying to run their tack off of a PCM output on the old Datsun tach? You would think it would be possible? I mean if you can make the original tach read a V8 signal why not from the PCM? Now I dont have a wiring diagram for a 94' LT1 but I do for a 95' and 93'. For a 95' I would say run it off the "A13" which is the Red connector on the PCM. This is labeled as the "TACH output wire" Guy Quote Link to comment Share on other sites More sharing options...
hanomon Posted November 20, 2004 Author Share Posted November 20, 2004 Thanks Guy, I found a couple adjustment screws behind the tach. Assuming these were calibration points, as one of them was factory-tape sealed. I applied a low voltage signal across the leads and measured with a multi-meter as I turned the screw back and forth watching the voltage fluctuate only in one direction, the signal dropped when the other screw was backed out. So that tach can be calibrated rather easily verifying it to the hand held strobe-a-tach. I found the flow path in the (IC) circut tonight. This should work fine. Those rechargable hand tool battries are great for testing partrs at the junkyard prior to purchase too. Quote Link to comment Share on other sites More sharing options...
Dan Juday Posted November 20, 2004 Share Posted November 20, 2004 If you have the JTR book it has a good labled picture of the back of both 240 and 280 tachs showing clearly where the adjustment screw is. I have an EFI motor too but I just connected to the coil for the tach signal like a conventional set up. All you have to do is turn the adjusting screw to match your known good external tach (like you are doing). The adjusting screw is located under the hole that is between the two bulb holes near the top of the tach. Quote Link to comment Share on other sites More sharing options...
flibuoy Posted November 22, 2004 Share Posted November 22, 2004 OK, so far we determined that the tach is adjustable, as clearly shown and discussed in the JTR book. I used the 240 tach with a "normal" distributor in my last 350 swap for 5 years with no problem..very accurate after adjustment and almost perfect using the suggested "rough" setting. But this question still seems to beg an answer on which wire to best use for output from the LT1. PCM or coil? Mine happens to be a 94. Also reading both the JTR Datsun swap and JTR fuel injection swap books there seems to be a need for an additional resistor in the circuit for 260/280 tachs. Somebody out there has a definitive answer I think... My current swap is early 260/LT1/T-5 and about ready to head for paint shop after driveshaft is cut....incidentally mine is WAY back and driveshaft is only 19.5" center to center! I'd like to get tach working. I do have the swap pretty well photographed as it progressed, so if there is specific details you would like a shot of I probably have it...my version. John Quote Link to comment Share on other sites More sharing options...
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