Thread: 93 Hard Dash, Spark Plugs and Fuses

Originally Posted by bee utey

Does the tacho work consistently? It is wired to the alternator after all.

Yes tacho is working fine.
Are these alternators binary (i.e. good or dead) or can they drop volts slowly? I swapped this battery out of my work car and while I thought it was good, the work car did start a lot smoother without it. So the battery is not beyond suspicion but should be good enough. After a night on the charger it's running superbly.

I have a 2000km trip in another vehicle next week, so I won't be back to this one for a while. I'll be taking the car that'll burn 120L on that trip. Not 300L.

That style of alternator senses the charge voltage at the output terminal. If there is a high resistance between the battery and the alternator output a normally working alternator would deliver only a small current. I've fixed the occasional fried cable end on RRC alternators before. A new charge cable straight to the battery is a quick fix.

When you get around to checking charge voltage do it at the back of the alternator as well as the battery (headlamps on, 2000 engine rpm approx). I've seen lowish charge voltages but mostly they're either dead or alive.

Last fillup. 51 litres for 168km.

I'm hoping for double that on the next one.

Originally Posted by Dougal

Last fillup. 51 litres for 168km.

I'm hoping for double that on the next one.

This might be stupid question. But how does a tacho takes it reading from an alternator. I'm not sure if I should trust the tacho in the one here. It tells me the V8 is spinning at close to 3000rpm @ 100km/h ... that's with a 5spd manual gearbox.

What don't they just use the coil as a pickup like every other car in the world

seeya,
shane L.

Originally Posted by DoubleChevron

This might be stupid question. But how does a tacho takes it reading from an alternator. I'm not sure if I should trust the tacho in the one here. It tells me the V8 is spinning at close to 3000rpm @ 100km/h ... that's with a 5spd manual gearbox.

What don't they just use the coil as a pickup like every other car in the world

seeya,
shane L.

'Cos the tacho works on the diesel engine just the same. An alternator produces AC current internally which can easily drive a tacho, you just need to have the right size pulleys and it all works perfectly.

There's nothing stopping you from borrowing a regular tacho (or a timing light with tach function) for comparison purposes.

Originally Posted by bee utey

'Cos the tacho works on the diesel engine just the same. An alternator produces AC current internally which can easily drive a tacho, you just need to have the right size pulleys and it all works perfectly.

There's nothing stopping you from borrowing a regular tacho for comparison purposes.

That makes sense ( I think ). It would meausre the wave length of the A/C form I imagine.

OK, I need to get a normal tacho then The alternator on miine lives where the A/C compressor used to be. A much better spot for it IMO However we can't guarantee it'll be spinning at the identical rev's to the standard setup.

BTW: What's the maximum revs/cruising rev's of these things. It's seems quite bizarre there is no redline on any of the tachos fitted to anything with this old V8. The one here never seems to need to rev harder than about 3000rpm.

seeya,
Shane L.

"...BTW: What's the maximum revs/cruising rev's of these things. It's seems quite bizarre there is no redline on any of the tachos fitted to anything with this old V8. The one here never seems to need to rev harder than about 3000rpm."

...There's your answer.

If you need or want to rev much over that, - get another car.

The alternator driven tacho is also a very good clue to a dropped fan belt.

If you are cruising along and suddenly the revs drop to zero but the engine is still burbling along nicely I guarantee you have just spat the serpentine belt.

Depending on battery condition (and whether the belt-tensioner idler wheel bearing has just disintegrated causing the belt to come off) you have but a short time left to fit another one pronto...

Otherwise, enjoy your flat-battery and flatbed ride...

You wouldn't enjoy this sophisticated diagnostic heads up if the tacho picked up off the coil...