Tach issue when crossing forum

[DAMINK]

All D1 tachos drive off a belt driven alternator. That is really all the information you need to know. I've even converted a shiny new V8 alternator into a diesel one by swapping the front housing and pulley, so there's no difference in the internal design. The only point at which the engine makes a difference is when you drown the ignition distributor, and the OP didn't mention that event.

3 foot of water in a car does indicate something.
Alt shorting is likely but i would expect battery light to illuminate at that point.
Location of alt does matter. A low one could in theory short earlier than a higher located one so i dont agree with the dont matter theory.
Dead alternator bearing only binding enough so when belt is wet could in theory cause issues although very unlikely.
Old belt that has lost tension could be likely.
Its an interesting problem IMHO and look forward to the answer.

[bee utey]

3 foot of water in a car does indicate something.
Alt shorting is likely but i would expect battery light to illuminate at that point.
Location of alt does matter. A low one could in theory short earlier than a higher located one so i dont agree with the dont matter theory.
Dead alternator bearing only binding enough so when belt is wet could in theory cause issues although very unlikely.
Old belt that has lost tension could be likely.
Its an interesting problem IMHO and look forward to the answer.

To quote the OP:

Tach cuts out when crossing water (not to deep 3 foot). Once out of the water it works fine. rest of the time runs perfect.

I dont think water is getting up to the alt during the crossing.

As anyone who has ever ridden a bicycle across tram lines in the rain knows, rubber on wet steel slips like buggery. Therefore belt slip under load is the most likely answer. All that needs to happen is a substantial amount of water gets onto the crank pulley and the alternator is working normally. It won't be at idle, you know. And most likely its a serpentine belt so the tensioner is automatic.

Dead bearings would be howling loudly already, usually they wreck the armature against the field before they induce belt slip. A teeny weeny bearing is no match for a fully tensioned drive belt. A seized bearing usually spins in the alloy housing and instantly wrecks it allowing the armature to contact the field cores.

If the dash tacho was driven in the normal way off a dissy or a toothed wheel in the injection pump (like say a Rodeo) this question would never have arisen as the OP never mentioned the red light coming on.

As for shorting, 12 volts won't conduct anything much through fresh water. If you read one of the electric fans threads you'll see that little fact mentioned. Regulators are generally well sealed. Salt water is another matter and not be something you drive through on a regular basis without rapid vehicle destruction.

Oh and finally, I'd hazard a guess that the 3 feet was not measured with a ruler out of the window, so could be substantially less.

Lets hope the OP comes back and enlightens us on his choice of vehicle.

[loanrangie]

At 3ft of water a tdi alt will be submerged and a V8 alt half submerged, so when the op stated he didnt think water was getting that high then unless he has a hovercraft it was getting wet which is relevant to his issues.

[theNEWT]

I agree the slipping hypothesis has merit and not all electrics are sealed, possibly either?????