Tach issue w/Redline EFI

[63fiberbuggy]

My VDO 2 1/16” Cockpit Series tachometer worked fine connected to MSD 6AL ignition controller; however, when I switch to Redline Weber VW EFI, the tach only “blips” to maybe 100 rpm during cranking, then drops/remains at zero after engine fires. I tried all possible combinations of dip switch settings on the tach with same results.

I contacted VDO tech, and John suggested maybe the ECU signal is weak and I should install a “step up resistor” in the signal line. I found a product from Electromotive.com “tach signal amplifier” which says it works with “12v square wave output”.

Does anyone know if the Redline ECU tach signal is a square wave (versus pulse, etc)? Or know what the right solution to this issue is?

Thanks, Bill

[Steve Arndt]

Are you using the tac output from the ECU? Are you still using the MSD and distributor or is it crank trigger?

[63fiberbuggy]

Hey Steve, yes I’m using tach output from the ECU….I removed MSD 6AL & MSD distributor; and installed a crank trigger. Also removed blue coil and used the Redline coil that came with kit

[Steve Arndt]

I use an Autometer tac with my similar setup. Haven't tried a VDO.

[nbuscemi]

You need to get your hands on an oscilloscope and measure the Tach output of the control module. I'd bet the signal is too weak or incorrect to drive the tachometer. I had to modify my Megasquirt with a tach driver circuit in order to run my Autometer tach which was designed to use a Coil Negative pulse. Perhaps there's a setting on the Redline unit that can change the tach signal.

-Nick-

[63fiberbuggy]

UPDATE:

Posting solution in case someone else has similar issues & this might help. I purchased the Electromotive.com “tach signal amplifier” & that solved the issue. Hats off to Electromotive for offering "up front" that if it didn't work, they'd take it back for full refund....good folks

[Clonebug]

Some tach's required a little higher power to trigger it. My "Mac Tools" signature tach needed that. I put two resisters inline to a ignition switched power source and connected it to the feed from the ECU to the tach and it worked.
I can't remember the exact resistors I used but it did solve the issue. I am now in the middle of swapping from MS-2 to MS3Pro so will see if I still need the jumper.