Re: [GMCnet] Tach wire [message #370070 is a reply to message #370069] |
Mon, 04 July 2022 10:38 |
rjw
Messages: 697 Registered: September 2005
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RF_Burns wrote on Sun, 03 July 2022 21:45Now here is an example of where I think I'm missing the original question from the email side. I only use the web-based forum.
This is the first message in the thread that I see from Emery:
I know the tach wire goes to the end of the distributor but I cannot find it. Can someone tell me the color?
It is not shown in the wiring diagrams.
Emery Stora
emerystora@mac.com
So I'm not sure what Emery is hooking up... is it just a Tachometer, or is at an EFI system, or is this some piece of test equipment. Regardless, the tach connection is usually to the points side of the ignition coil. On HEI systems, the TACH connection is marked on the distributor cap and the connector is parallel with the HEI module's open collector output to the coil. OEM, the GMC had nothing connected to the TACH point.
Be careful that what you are hooking to the Tach connection is compatible with 12V switching to ground. In the case of a Howell EFI, there is a small circuit board in the wiring harness that level shifts the 12V Tach signal to 5V. This circuit board gets removed when a spark-controlled distributor is added because ESC output is 5V.
I didn't know that. I was using the HEI tach connection to my Custom Instrument Panels tachometer with no issues starting back in 1999. Worked fine. In 2012, I added to my Howell EFI (+EBL) a computer controlled distributor and found tachometer worked OK but sometimes acts weird. Like registering 4000 RPM when idling on occasion. I forgot about the PC board in the Howell wiring harness until I read this. I don't recall removing it when I went to CCD but then again that 10 years ago and I sometimes I don't remember what I had for lunch.
Richard
76 Palm Beach
SE Michigan
www.PalmBeachGMC.com
Roller Cam 455, TBI+EBL, 3.42 FD, 4 Bag, Macerator, Lenzi (brakes, vacuum system, front end stuff), Manny Tranny, vacuum step, Tankless + OEM water heaters.
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