Thread: Wacko Tacho

Originally Posted by 110Landy86

So I recently wired up a VDO tacho and now its doing strange things. Firstly id just like to mention that the alternator currently fitted didn't have a "factory" W connection so I had one installed at a local auto electrician.

Back to the strange behaviour... Last night (lights on) I went for a cruise to test everything out after the install...Everything appeared to be going well and apart from the fact that rpm rose surprisingly quickly (i.e. seamed like 3000 rpm came up way to quick), i did considered it to be somewhat accurate...

After driving the car this arvo (lights off) tacho hovered around 1000 rpm and struggled to rise above about 2000rpm.

After turning on (only one at a time) any small-like appliance such as parker lights, interior lights etc (other than full lights and a/c -i.e. anything that doesn't draw heaps of current) the tacho responds, what i think to be, more accurate.

Calibration was carried out by the auto elect as they have access to hand held tachometer and no what they are doing w.r.t running a W terminal. I installed the unit.

Any Ideas???

P.s. While on the subject, what gear ratio should I use for calculating rpm assuming i have an LT85. According to my tacho last night i was sitting on ~3000 rpm for 120km/hr *private road, no traffic*

Cheers Joe

The formula for calculating road speed is:-

rpm x 60/tyre revs per kilometre x overall gear ratio. The overall gear ratio is 3.54 x the overdrive ratio. Assuming you are looking for the figure for high range 5th.

As for the tacho. Get the real thing. A genuine Isuzu tacho is available from the industrial engine dealers like Zenith Engineering in Brisbane. This has a transducer that repalces one of the screw in plugs underneath the injection pump.

For those who are interested, I found out that the erratic behavior of the tacho (when there was no load on the alternator) was due to fluctuations in the voltage supply from the ignition source to the voltage regulator.

The problem was rectified by simply replacing the wire.

With 3.54:1 diffs and an LT85, standard tyres, your tacho should be indicating 2000 rpm at 100 k's, 2200 rpm at 10 k's, 2400 rpm at 120 k's.

Thanks Brian,
Using the formula you provided i calculated that ~2500 rpm in 5th gear equated to 100km/hr, assuming a final drive ratio of 1.02.

However if what you've just said is true then my tacho is wayyyy out! I'm hitting ~2600 rpm for 100km/hr. Which personally sounds high exacerbated by the fact that when going through the gears 3000 rpm seams to come up way to quickly and also its just too quiet those kinds of rpm imo! The auto elect did say that the tacho was out by about 150 to 200 rpm, when compared to the hand held tachometer. VDO state that there tacho tolerances are general in the order of this magnitude.

For the record, i'm running 16in defender rims, 235/85/16R tyres & LT85 5 spd gear box in 86 110 county, N/A (not for long )

Cheers Joe