Tacho under-reading

[cindersailor]

I have a VDO 0-3000 rpm tachometer sensing the alternator. When I start from cold it under reads by about 50% for 15 minutes or so and then it is fine. Initially I thought it was due to the alternator belt slipping while the batteries charged from the starting, but I have tightened the belt and used some anti-slip spray and still no change. No problems when restarting once it has sorted itself. Any suggestions?

 

[snowleopard]

when it starts to correct itself, does it suddenly jump up to the full reading or creep up over a period?

 

[ShipsWoofy]

What state are your batteries?

The tacho using alternator sensed require a charging current. Though, even with full batteries I have never noticed a problem, but the blurb does suggest it could cause a problem.

When it is under reading are you getting a full charge 14.4v across the battery terminals? If not it could be the alternator.

If you are it could be a bad battery so little current until it warms. Can you substitute your car battery for testing?

Does the VDO require the capacitor across the terminals of the tacho, this might if fitted be suspect, though it is unlikely a capacitor will fail then work like this, but never say never.

Just some thoughts.....

 

[cindersailor]

Thanks for your replies. The batteries are almost new. I get a good charge just after starting (30-40 A @ about 14.6 V), but this drops to about 5 A after a few minutes once the batteries have recovered from the effort, so I do not think there is anything wrong with the alternator or batteries. I have not witnessed the recovery to normal opperation so cannot comment. I will have to sit and watch it!

 

[dmayes]

Just a thought, check that your brushes on the alternator are not sticking. They may not be delivering the correct charge until they free themselves.