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VDO Tachometer on Lucas A115 alternator


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The Vdo tachometers have multiple operation settings, they will work off the W terminal on the back of the alternator and have a potentiometer for calibration to read the correct R.P.M.

Also you could fit a diesel tiny tach, which uses a clamp around an injector line and measures the pulses to give you a very accurate reading off an LCD screen.

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What a mucky place! :-)

I haven't worked on one of these alternators, but I'd have to say 'No'.

Those bolts seem to hold three diode packs in place, they probably screw into the case (frame, chassis), check with a multimeter. The bolts are probably insulated from the diode pack.

My advice is, with the aid of a small but bright torch, to peer into the dark and murky depths.

Try to locate the thick wires coming from each stator (field winding). You may only be able to see one wire from each, but that's fine. Those wires will be heading towards those diode packs. They 'may' even be electrically connected to those bolts (making my 'No' wrong), but I very much doubt it.

It is one of those wires you need to connect to, perhaps you will have better access from the side, rather than the end, shown in your view.

Good Luck

The attached diagram MAY help, but you probably need additional information, as it doesn't show the connection of the stator wires to the 'diode plate'.

post-118-0-26738500-1385028561_thumb.gif

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OK, I did the written answer, and then went looking for more information, so it was some time later I found the exploded diagram.

When I saw there was a 'diode plate' I did think my surmise of those screws going into the frame might be wrong.

Two alternatives come to mind, one is dependant on the access available while the alternator is mounted.

On the bench, see if you can get one meter lead on the point the wire terminates, and check for continuity to the bolt head. You might have to determine which screws hold the diode plate in place, and remove them so you can lift the diode plate away to give access.*

The other option is to mount the alternator with the cover off, then run the engine and check the voltage on those bolt heads.

If it exists it will be AC, a lot less that 12 volts, and won't vary much with engine speed, as it's the frequency of the AC that shows the change in engine speed.

Getting into intimate electrical contact with a running engine gives plenty of scope for accidents to happen. Double check nothing will get caught in the drive belt before starting the engine.

* Writing that made me think that a variation to Googling 'W terminal on A115 alternator' (and finding very little), might be to Google 'changing diodes on an A115 alternator'.

As the stator windings will have to be disconnected to do this task, any instructions should give a reasonable idea of where and how they are connected.

Good Luck.

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Just to close out this thread. The bolts shown in the picture are indeed isolated from the diode packs and stator wire by a plastic sleeve and plastic washer. I attached a ring terminal around the plastic sleeve and under the plastic washer, which causes it to contact the same plate that the stator wire is soldered to. Works great. I have the 6000 rpm VDO gauge and the dip switch settings of OFF-ON-OFF seem to be about right.

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Thanks for the feedback.

Did you have to loosen the diode plate from the alternator casing, or was this all accessible from the access in your picture?

If you want to check the speed, non-contact optical tachometers seem pretty cheap, if you 'gamble' on a purchase direct from China (or other far Eastern country). Obviously there is a paint mark or reflective sticker to be placed on the crankshaft pulley.

HTH

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No I didn't have loosen the diode pack. Remove the bolt. Remove plastic washer. Put a ring terminal down around the plastic sleeve. Replace plastic washer. Replace bolt. The ring terminal rests on top of the diode pack and is held down by the plastic waster. All accessible just by removing the back cover plate. Wish I had taken a photo when i attached it.

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