LM-1 w/ LM-3 aux box questions

[356RS]

Trying to get the tack signal down to the LM-3 aux box for RPM. Took the tack signal from the tack and hooked it up to #3 on the aux box. Set the box as per instructions and "no signal". This is a CDI 6-pin system. I would love some help on this.

[304065]

Did you hook up the ground? A floating TD signal isn't much use.

[356RS]

Thank You John. No, I didn't. I'm using CH #1, so the ground would be CH # 1- and the tack signal should be on CH #1 +, right?

[304065]

Yes. The Aux Box works off of "ground referenced" signals-- rather than tying everything to ground, it allows you to use floating grounds off individual sensors.

Anyway, the "ground" for the Tachometer Drive or TD signal which runs the tach in a six-pin is chassis ground. There are some good ground terminals under the dash, make sure the metal is bright where you attach. Good luck.

[356RS]

Still can't get the signal. Found 7.2 volts dc at the tack signal line? Aux box only wants 0 to 5 volts. I'm running the twin plug CDI set-up, would that make a difference?

[304065]

Hmm, 7.2 volts is really the average from the duty cycle of the TD signal which I think is an 11 volt square wave.

How about a 7805 voltage regulator with a 5V output? You don't care what the input voltage is, you just care about getting a 5V pulse out of it. That's what I would do. Google 7805, these are commonly available at Radio Shack for a couple bucks.

[356RS]

Thanks again John. I'll let you know how it turns out. I'm trying to dial in my 2.7 short stroke MFI engine. It's really hard to know where your AFR is without the RPM data.

[fredmeister]

How did you tap into the tach signal, from the dashboard gage wire and down to the Aux-box?
Or did you tap off the wire in the engine compartment? If in engine compartment which wire is the tach signal, I mean what is the color code on the correct wire?
Great info, as I have been trying to figure out how to do this exact thing on my 1979 930 turbo with stock CDI ignition.
Thanks,
Fred

[356RS]

Fred, I taped into it at the gage end and also got a good ground near the gage and ran the wires down to the aux box. Going to RS today and get the parts John suggested.

[356RS]

Well, still no luck on the RPM signal. The voltage at the Aux-3 box did go down to 4.85 volts, but still not showing the decimal point to confirm RPM signal. I'll try a 3 volt regulator next. Lots of money spent on this LM-1 & 3 and still you have to spend lots of time trying to make it work. Is there something for sale out there that works right out of the box?

[The Turk]

Hi Mark,
I run an LM1 and AuxBox in my car, I log RPM, Speed, AFR, throttle position and boost from two different locations.
Setting the LM1 / AuxBox up can be challenging but is worth it once your done.
The 7.2 volt signal off your tach doesn't need to be reduced and the input shouldn't need the ground, I have just the one wire from the tach hooked to the #3 input.
I spliced into the 7.2 volt signal behind the tach and ran it directly to the number 3 input, using either the "LM Programmer" software or the buttons on top of the AuxBox you need to set input 3 to "F" for frequency.
Give this a try, it should work fine.
Good Luck.