Eccentric Rev Counter

[stuzbot]

Here's an odd one that my van's just started doing over the past week or two:

1: When I first startup the van and drive it, the Rev Counter has started behaving really eccentrically. Here's a clip of it while the van is parked up and idling. But, while driving, the swings are even more pronounced; I've had the rev counter drop to zero while I'm driving slowly and almost go into the red when I've got the revs high enough where I'm just about to change up a gear --at an engine speed when normally I'd be in the middle of the green zone.


Link


Now, I supposed the obvious thing anyone is going to say is that the Rev Counter might not settle down right away when the engine is cold. But it's never done this before and it will continue to behave like this even after I've been driving for 15 or 20 mins and the engine has come up to working temperature.

Neither is there any oddness in the way the engine is running. It's running as smoothly and regularly as it always does [you may be able to hear it purring sweetly in the background].

Now, here's the odd thing. If I park up the van and leave it for 5 or 10 mins and then come back and start up again, the Rev Counter will be good as gold and solid as a rock again. Here it is after letting the engine sit for a few minutes and then starting again:


Link


It's almost like parking it up allows the heat from the engine bay to dissipate <somewhere> that fixes the wobbly Rev Counter needle --until the engine is left to go cold again.

One thing that would help tracking it down would be if I knew how and where the Rev Counter reads the engine speed from. I'm presuming on a van of this vintage it's a purely mechanical sensor and the flywheel would seem the obvious candidate for measuring from, but I don't see any likely looking cable going into the engine bay anywhere.
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Click to view image
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1992 VW LT35 2,4D [SOLD]
1993 VW LT35 2,4D [SOLD]
1992 VW T3 1,6TD Syncro
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[MarkWard]

Diesel? Diesel tachs got a signal from the W terminal on the Alternator. I don’t believe any of the diesel vanagons sold in the US market had tachs. So you are either sporting a diesel tach from somewhere else, or it’s a gasser tach that’s been modified.

Makes sense that the heat soaking of the alternator might improve tach function.
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1982 Vanagon Camper with ALH TDI conversion.
1990 Vanagon Camper Syncro conversion.

[stuzbot]

[crazyvwvanman]

The 2.2 marked on the face of the tach tells that it is an alternator W lead tach, calibrated for a 2.2 to 1 pulley ratio.

Mark

[Gnarlodious]

Has it been very wet there recently? Sounds like your diesel tachometer connections got wet and are shorting out. After some engine heat they dry out and it works normally.
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Vanagon ’83 diesel AAZ w/Giles injection, 5spd 4.57R&P+TBD.

[stuzbot]

[djkeev]

[Gnarlodious]

Yeah. I would recommend lubing that alternator W connector with some dielectric grease smeared on the contacts. Should help maintain good conductivity and keep out some of the humidity.
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Vanagon ’83 diesel AAZ w/Giles injection, 5spd 4.57R&P+TBD.