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Discussion Starter · #1 ·
Any way to trigger my Autometer Shift Light with my 7531 instead using the yellow chips? I was thinking maybe I could put my 3000 RPM test chip in it and use the shiftlight output to power or ground the shiftlight. Should work, right? If I can't find my test chip, can someone measure the resistance for me, please.

...failure to communicate
4,872 Posts
I wrote this up a LOOOOONG time ago so I can't confirm if it still works.

I used it so I could cut the wires off the shiftlight and splice into it instead of using the pills. I made a hobby box with an adjustable potentiometer and a bunch of DIP switches. I didn't run it off the MSD though, it was powered of the (-) side of the coil.

Volt/Ohm Meter measured values of Automer RPM Pills
3000 = 1880 Ohms
5000 = 3590 Ohms
5200 = 3790 Ohms
5400 = 3980 Ohms

From Darell
3000 = 1900 Ohms
5800 = 4400 Ohms
6000 = 4600 Ohms
7000 = 5680 Ohms
8000 = 6860 Ohms

To use round numbers (and only be off by 10-20 rpm), I determined the approximate formula in the 5000-6500 range to be:
RPM - 1400 = Resistance
RPM = 1400 Ohms + Resistance

5000 rpm = 1400 + 3600, so I need 3600 ohms of resistance for 5000 rpm
5500 rpm = 1400 + 4100, so I need 4100 ohms of resistance for 5000 rpm

What I did was put a variable potentiometer (10K) in and set it to 3600 ohms (5000 rpm).
From then on, each ohm of resistance = 1 rpm added to 5000 rpm. Units are now interchangeable.
The output from the pot goes to 3 of teh DIP switches. They are 0, 500, and 100, so if one is selected, that output would then be 5000, 5500, or 6000 rpm. That value goes into the other 5 DIP switches which have values of 100, 200, 300, 400, and 500.

The outut is then 5000 rpm (3600 ohms) PLUS 0, 500, or 1000 PLUS 100, 200, 300, 400, or 500. That gives the opportunity for any 100 rpm increment from 5100 (switches 6 and 1) to 6500 rpm (switches 8 and 5).
5100 rpm = 6 and 1 (5000+0+100)
5800 rpm = 7 and 3 (5000+500+300)
6500 rpm = 8 and 5 (5000+1000+500)

To get the resitance values, I used a number of resistors (they add in series and divide in parallel) on a circuit board. I put the whole thing in a small (~1.5"x3") box and cut the switch box out and a hole for the pot. I then spliced into the two wires in teh shift light that go to the place where the pill plugs in. I used copper phone line. I ran the wires down the A pillar and out underneath the seat and place the box there.

Realistically, you could use 7 adjustable pots for the resistance values instead of resistors, or just use one pot and no switches or resistors, but you need a VOM to read the resistance everytime you want to set it. I like being able to flip a switch at any time and change the light value.

365 Posts
Discussion Starter · #3 ·
Jay - Thanks for the info. I'm just getting too much stuff driven off of the tach output. I'm going to use the shift light feature on the digital 7 to switch the light with no coil or tach input to the shift light. I basically need it "always on" when powered, so my 3000 RPM idea won't work either. I'll try something with very low resistance. What's the resistance of buss wire!?!?
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