I was just wondering if anyone knew if the stock tach works on amperage or on voltage? I want to hook up a stock shift light without the ECM connected. I can do the electronics work myself, also, it won't tell me to shift too early like real stock ones .


Thanks for any help.

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1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 WS6 Trans Am T-top car
4-bolt main 350, headers, Holley 650, T-5 and 3.23's.
Daily driver and restoration

 

I believe it's neither, it's pulse modulated.

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1984 Silver Z28, 383 cid 4 bolt, ARP Fasteners, 2 1/2" Cowl Induction Hood, 600 cfm Performer Carburetor, Torker II Intake, Performer RPM Heads, XE268H Cam, Magnum Roller Tip Rockers, MSD6AL Ignition, Blaster2 Coil, Recurved HEI Distributor, 700R4, TransGo SK, 2.73:1 Limited Slip Dana 44, Terminator Headers, Custom 3" TIG Welded Stainless Exhaust, Flo-Pro Twister Muffler, 4 Wheel Disc Brakes, Oil Cooler

 

A tach uses a frequency to voltage converter to drive an analog meter. It is possible to tap off of the output of the F to V circuit & drive a voltage comparator circuit that would operate a light.

Tim

 

How about if I came off the coil feed and ran a low pass filter to operate the light?



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1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 WS6 Trans Am T-top car
4-bolt main 350, headers, Holley 650, T-5 and 3.23's.
Daily driver and restoration

 

A simple LPF would not do the trick. You could build a F to V converter with an LM122 (precision timer), or an LM2907 (frequency to voltage converter), or a simple system, with discrete transistors. You could also buy an MSD RPM activated switch module (< $100, from Summit) to control a shift light as a function of RPM.

Tim