Tach issue

[Mike Loveday]

6BTA 315 Nippon Denso with VDO Tiara supplied gauges in a 1995 Tiara.

One tach doesn’t work at all until engine revved above 2000rpm. Works fine at low rpm after being revved up.

Swapped tachs around to confirm the issue isn’t in the tach itself.

Swapped bell housing senders between engines with no change and triple checked that sender depth is correct/ as close to ring gear as possible. Checked induced AC voltage OK at sender. Same on both sides.

Separated and cleaned engine harness main connector to gauge cluster.

Tried swapping an extra gauge cluster to engine harness I had lying around and it does the same thing, so I know the issue is in the main engine harness/sender.

Never seen an issue like this before. Very limited access to the harness above the preheater module with heat exchanger installed as it’s the stbd tach giving me grief.

Hoping I’ve overlooked something simple as I have way too much time into this. Anyone seen this before?

Topic

[Rob Schepis]

You’ve been diligent and thorough. Maybe it’s time to run a dedicated feeds from the sensor to the tach and 12V too. You can loose run it all for testing purposes.

[Mike Loveday]

I should add- red/black wires are connected at the bell housing senders and the green/white wires were disconnected on both sides since the first time I saw this boat. If I hook up the green/white connectors, the preheaters won’t go into post heat cycle. I don’t think it has anything to do with this since the tachs should work without them hooked up.

[Author]

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