VDO Tacho not working?

[mrfiat]

Ive been running a delco ecu in my fiat with the original tacho. The tacho would read fine (as fine as any 30 year old Italian gauge would) until i rev to 5k when it would drop back down to zero.

I had enough of this so at the wreckers i picked up a few VDO tacho's from n13 pulsars, (they are a close match to the fiat one in design and size with the same three inputs. Ignition, signal and earth). I managed to mount it perfectly in the original dash, you cant even tell its been changed... Shame it doesn't work . Ive wired it all up the same as the original but it doesn't do much, when the engine is cranking the needle bounces but as soon as the engine starts it stops moving, and occasionally when Ive got the blinkers on the needle bounces in time with the blinkers.

I don't know much about electronics but i did notice that on the original circuit board on the nissan, that the tacho connect to there were a couple of electrical components that were connected in the path of the ignition + and earth connections (will get some photos of im not making sense).

How likely is it that my wrecker spec VDO tacho is actually broken? (ive got a few i can swap to test) or is it more likely ive wired it wrong?

Any help is greatly appreciated, i can get photos if it will be of any help. thanks

[Jayme]

some tachos didnt read the 12v square wave, but read the high voltage spike off the coil negative terminal. ive found when doing swaps that get rid of the coil from the tacho circuit, like a v6 coil pack or a CDI ignition box, where the tacho only sees a 12v square wave, they cant read at all, and require a relay coil with the bar removed to be wired in to generate the high voltage spike.

what sort of ignition do you use? I have seen this type of tacho in VK's and GQ nissan patrols. wouldnt be surprised if the n13 used that sort of tacho too.

[mrfiat]

Im using the gm ignition module that holds the two holden v6 coil packs

[delcowizzid]

ive had issues with v6 modules running old tachos not enough grunt feeding to the tacho.you can run a diode from each coil neg under the coils so power can only flow one way and join them to one wire to drive the tacho or do as jay suggested and run a relay o create the high voltage spike

[Apples]

may be try this works on LS1 pcms and after market tacho.

tach_circuit_fix.jpg (60.23 KiB) Viewed 6490 times

[mrfiat]

Thanks guys, i just ended up hooking up an aftermarket tacho i had lying around which is working great.

[antus]

Old thread but just trying to keep the place somewhat organised. I found this thread looking for this circuit.. so I might as well drop this link here to help anyone searching in the future. The circuit jayme references can be found here: http://pcmhacking.net/forums/viewtopic. ... lay#p21551