| Re: [GMCnet] Tach wire [message #370071 is a reply to message #370065] |
Mon, 04 July 2022 11:20   |
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RF_Burns
Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada
Karma:
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Senior Member |
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Richard,
The GM 7747 ECM used in the Howell (and EBL) was designed for use with a CCD for spark control. The output of the CCD module to the ECM is a 5V pulse so that is what the ECM is designed for. Howell designed a little board to reduce the 12V pulses from either the points coil or the HEI Tach connection terminal down to 5V. The Howell board might work with a 5V pulse input, but I would not think it would. Its just a Schmidt trigger NAND gate with a voltage divider at the input (if I recall). When you pulled the 4 wires required for the CCD, you likely pulled out the single Tach wire with the circuit board in heat-shrink from the wiring harness.
The Tach connection for the CCD distributor is the same for both standard and ESC distributors. Was it the CIP tach that was reading 4000RPM intermittently or the Tach on the EBL What's up Display (WUD)? If it was the WUD, there is a filter to smooth the electrical noise from the terminal. Look for "RPM-Filter for 12.5 RPM, mine is set to 81.57%
FYI, if you a Vehicle Speed Sensor (VSS), there is a noise smoothing filter for it as well. A couple of years ago I noticed my engine would randomly seem to cut out, so with the WUD I noticed the Speed indicator was jumping by 20-50MPH. Turns out the speed limiting was coming ON even though it was set to some high speed. I replaced the reed switch in the VSS, then Randy Van Winkle pointed out the filter setting called "VSS- Filter, Filter %" mine is set now to 75.29 .
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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