Need help with tacho issue.

[shifty6]

Ok well I just took the belt off the alternator and removed all electrical connections from it. No difference. This is really starting to piss me off. It worked in this car when first installed, then just developed this issue. But still works on other holden 5L cars.

[vlt304]

I know this might sound nuts but what fuel pump are you running? where does it get its power from? I had a chinese 044 mess with a tacho in a race car. solved it with a noise suppressor capacitor.

[Dylan]

I've seen some cars run noise suppressor cap's on the ignition coil and alternator.

[shifty6]

I know this might sound nuts but what fuel pump are you running? where does it get its power from? I had a chinese 044 mess with a tacho in a race car. solved it with a noise suppressor capacitor.

Just stock in tank pump, battery in boot with power to pump running through relay switched by ecu. Don't think that's gonna be it.
The weird thing is it has worked before so even if u suppression cap fixes the tach I'd like to know what's actually causing the problem.
I connected a stock capacitor (0.5 uF) from the VR 5L from coil pos to ground. Did nothing. Tried coil neg to ground and that also did nothing.

Thinking I'll have to look for another loom soon.
Are there any earths that should be separated? The loom has been shortened and modified and pretty much all earths are joined together in the loom and then earthed at the block, ignition module mounting point etc. but most diagrams I've seen show pretty much all the earths joined together anyway.

[vlt304]

ahh the earths may well be your problem it should have a noisy earth and quiet earth or you could get earth loops happening. they both go to the back of the head but under separate bolts.

[shifty6]

ahh the earths may well be your problem it should have a noisy earth and quiet earth or you could get earth loops happening. they both go to the back of the head but under separate bolts.

Ok. I would have thought that provided all the earth connections are solid it would have eliminated any voltage difference between the earths anyway. Do you know which ones I need to try to isolate from the others?

[VL400]

I know the tacho works in another car, but have you tried any other tachos?

[shifty6]

I know the tacho works in another car, but have you tried any other tachos?

I just remembered I had a calibre tacho a while ago and it did the same thing. It was second hand and I thought it was stuffed so I chucked it out. Bought this speco one brand new and it was good for a while, but now it's not. Can't really remember when it turned bad I think I just assumed loose connection etc at the time.
Don't have any others laying around to test now though.

[shifty6]

Well I pulled the loom out and completely rewired all earths. I had a factory VR loom to compare with, and on that loom there were only two earth points. As far as I could see the o2 sensor had its own earth wire which ran separate from all the others until it was crimped into an eye lug with the other loom earths, which bolts to the back of the head. Only other earth point was near the oil pressure switch. All earths in the loom were interconnected within the loom except the o2 sensor, which was only connected at the point where it bolts to the motor.
Anyway I tidied up the whole loom and set the earths up the same as the VR one, still no difference. Basically wasted my weekend stuffing around with it.
Disconnected O2 sensor from loom while running in case it was interfering, no difference.
I'm out of ideas and ready to throw a match at it.

The plugs are black as. Could running rich be causing this? I've replaced the plugs before and it immediately had the same problem.

[HQ355]

I could be wrong but it may be the brand of tacho's your running, speco and calibre are really nothing special, a mate had the same prob years ago with delco injection on sbc, went through atleast 5 tacho's until he got a decent one and never had prob again