Rev counter showing 300-500 RPM too much

[pmansson]

My 3.8 has the rev counter generator at the rear of the intake camshaft.
The recently serviced instrument is showing 300-500 RPM too many depending on range (the higher the worse).

What does the expertize say about this?
Any simple fixes besides knowing about it when you drive....?

[Heuer]

Odd as usually they under register because the generator magnets get weak. Sounds like they have not calibrated it properly when it was serviced.

Alternative is to fit one of Dave Rawle's electronic rev counter boards which gives fairly accurate figures. Details here: http://etypeuk.com/forum/viewtopic.php?t=292

[daverawle]

My 3.8 has the rev counter generator at the rear of the intake camshaft.
The recently serviced instrument is showing 300-500 RPM too many depending on range (the higher the worse).

What does the expertize say about this?
Any simple fixes besides knowing about it when you drive....?

I would guess that either the generator or the gauge have been changed at some time. During production they were a matched pair - the adjustment taking the form of a precision wire-wound resistor in the gauge. It's unusual, as David said they mostly read low but reading high is preferable as long as you know about it.

Dave

[pmansson]

I suppose I could try a different generator, but changing them is a messy affair, is it not?
Oil pouring out and how is it sealed? Originally with a paper seal and nowadays to prevent leaks, using a seal compound in a tube ??

Got the point about the generator and instrument being factory tested and adjusted as a pair. 50 years, and many changes later, there is bound to be trouble...

Have heard about the adjustment possibility behind the instrument, but won?t play around with that just now. The kit sounds good, but I don?t have time for that right now.

[HarryW]

Hi, pmanson (it would be nice to have your name, by the way).
It's not clear how you know the rev counter is over- showing on revs but in any case the generator itself is not difficult to remove/replace - I recently had to remove and readjust mine when the rev counter 'failed'.
It is held on with three bolts, one being fiddly to access. I would be wary of replacing the old one, as new examples appear to have a poor reputation for reliability (and they're not cheap...). Sealing is not a problem, unless the 3.8 is different to the 4.2.
Another line of enquiry might be to check the Library section where David has posted a list of rev counter codes (see 'E-type File Data') to confirm that you do have the correct instrument.

[pmansson]

Thanks Harry,
sorry, my name is Peder. Scandinavian alternative to Peter, which also exists and is much more common.

I think my generator and instrument have been a pair since leaving Coventry, but of course I don?t know for sure.
The instrument was sent off to a specialist, whose answer you can see below:

Hi Peder,
REV COUNTER
Had a reply from John Ostick copied below.

Hello Derek... very rare 'RV' rev counters are faulty... the unit is only an AC volt meter (f/s = 30volts)
the camshaft generator output should be 5volts / 1000 crank speed (5volts / 500 camshaft revs) 6k = 30volts
as you say although the rev counter is ok it will read low if the generator output is low OR SLIPPING
ie. the drive dog not locating correctly in the camshaft slot
so reading 'high' is not normally a valid fault


It is annoying not having a correct rev counter. It was measured yesterday at a modern Mercedes workshop.

Judging by my notes while driving last week, it ought to have been the other and normal way, i.e. showing too few RPM.

I might have it tested in another place where I know the mechanic as he does my BMW coupes for me.

Peder

[Heuer]

Actually they are speedometer codes Harry. But as you say removing the generator is relatively simple; three bolts (Allen head on my car) one of which takes time to get out/in and no oil leaks unless you try doing it with the engine running

Peder

Modern generators are not good so worth buying a good used one from eBay, Stoneleigh or M&C Wilkinson from where I bought my replacement.

[HarryW]

Thanks, David - sorry about the bum steer re the rev counter code, Peder - but you could always check your speedo!
I didn't have an Allen Key for the bolts handy, so I used a small 's' shaped screwdriver which fitted snugly. I should have said there is a 'u' shaped washer which is located across all three bolts and is easy to drop onto the gearbox - a great inconvenience (ask me how I know )

[pmansson]

A Jaguar guy in the US sells these on Ebay:

http://cgi.ebay.com/ebaymotors/JAGUAR-E ... 5889f0477d

Anybody tried it?
Available in the UK as well perhaps...?

[Heuer]

I bought an original used but fully functioning one from M&C Wilkinson for about ?50. The re manufactured ones are probably going to be better than the new reproductions though.

[pmansson]

Thanks,
but the question might be: "shall I send off the rev counter for a second time, to be calibrated" perhaps with its existing generator....?
A winter project perhaps.

Or go all the way and install a modified rev counter that works directly on the coil, via a separate wire....

Anyone done that to the E S1 ??

I know they are for sale but haven?t seen the ones for the E-type with red between 55 and 60 (which is max. x100 of course).

[Heuer]

Peder

The 3.8 tachometers were red lined between 5,500 and 6,000 rpm, the 4.2 tachs between 5,000 and 5,500.

The only tachometer modification I would recommend is the one done by Dave Rawle as it uses the existing generator and wires and you can calibrate it yourself. Start fitting a tach running off the coil and you are really going off piste which will make diagnosis of future problems more difficult. Personally I would send your tachometer to have its calibration checked - I doubt they will need the generator.

[pmansson]

I know, and my car is a 3.8
Much erlier in the thread you can see the comment from the guy at the instrument shop that calibrated it about 4 years/2000 miles ago.
John Ostick runs the J.D.O. Instrument Engineers in W. Yorkshire.
I am reluctant to remove and send it off again....
I will have it checked at one more place, just to make sure.
I will also measure the output in volts from the generator.

[pmansson]

I went to a different garage today and it now appears that the rev counter reads 200 too many up to 3000RPM, then it reads 300 too much.

We did a test on the AC generator at the end of the intake camshaft.
It reads/gives 7 instead of 5 volts/1000 RPM.

I will now call M&C Wilkinson and see if they have a decent, original, unit which I will buy and test.

Or should I try something different??