Tachometer & Oil Pump Bushing Question

[vande17941]

I need to know from someone who knows!

My tach was lagging behind the actual rpms when rapidly accelerating. I asked my mechanic to check the endplay in the bushing that drives the oil pump and tach....he said it was fine.

Therefore, I bought a new cable and had the tach cleaned and serviced at a reputable  shop. The shop said it was fine and it is perfect if I Rev the car at idle....while I accelerate, it still lags and jumps.

Unfortunately, the tach performed the same. So I checked the endplay myself on the oil pump & tach drive bushing and found it to be over 1mm!

Does this mean that each time the tach lagged or stayed lower than actual RPM momentarily that the oil pump was not providing enough oil and the engine was oil starved???

[Benz Dr.]

No, I think your oil pump is probably OK or your engine would be seized by now. There is info on this site about how to check and remove that end play. Make sure your cable doesn't have any sharp bends in it which can make it bind.

[vande17941]

I don't know if this helps.

1. RPMs move perfectly on tach (and fast) when reving motor at idle.
2. Brand new cable (old cable had same result).
3. Tach just rebuilt. Shop said tach was fine, just cleaned and lubed it since it was apart.
4. Tach/oil pump bushing has around 1.1-1.25mg of endplay when I put a small screw in it and pull up/down.

Would the small amount of play have anything to with tach lagging and then hopping up in RPMs while the vehicle is accelerating under load? The tach keeps up perfectly fine when reving at idle....so it can't be the tach or the cable.

[ja17]

The endplay in the bushing does not usually effect the operation of the tachometer. You might want to remove the bushing from the block and actually check the vertical timing gear for wear.

[vande17941]

Great. Will do. Thanks.

[vande17941]

Figured it out. Tach gear bushing was only slightly worn
..shimmed it. New cable was fine. Tachometer was rebuilt but the tech later found too much shaft endplay. Tach was working fine while car was rev'ed when stopped...but was hanging up and bouncing while accelerating. Since the gauges sit exactly perpendicular to the vehicle with no forward lean as in most cars, acceleration was causing the magnetic cup to slide rearward because of G-force on the tachometer drive shaft that had too much play. Once the shaft endplay was taken out the tachometer works perfectly

[Benz Dr.]

That's a new one on me. BTW, the cup isn't magnetic but the drum spinning inside is. ( at least I'm thinking it is......... )