Rev Counter Fault

[tannera34]

I have a non working rev counter in a 1988 T25 westy 1.6td jx.

Looking at the web the wire from the alternator (red black) goes to the binnacle (it does I checked the connectivity). How do I determine whether it is the alternator that is not sending a pulse or the rev counter which has gone? I only have a basic mulitmeter.

If it is the rev counter, can it be repaired? If not where would I get one from? (The alternator I would replace)

[ambivert]

If its never worked then consider: diesel rev counters don't work in petrol vans and vice versa (at least not without modification to the circuitry in the van). First thing is to check you have the right one. Not sure how to do that, maybe check the part number on the rev counter (and possibly on your alternator too) and compare these to standard. Someone on here will know what's standard ... or how to find out!

Failing that, then something may be faulty... So how to find it? Here goes...
I assume that your alternator is meant to be outputting some kind of periodic signal such as a square wave or pulse train - and that a basic circuit in your rev counter (or in the PCB in the dash behind it) is meant to be converting the frequency of this signal into a steady voltage which drives its needle magnet.

In order of probability, it could be
1. The PCB
2. Loose or corroded connection in the wire between alternator and dash
3. The alternator
4. The rev counter

Unfortunately the PCB is the hardest thing to test and to fix (and to find a replacement for!) One option is to get rid of it totally, by replacing it with wires - https://club8090.co.uk/forum/viewtopic.p ... 14&start=0" onclick="window.open(this.href);return false;

#2 is easiest to rule out either by running a new wire to bypass it or use your multimeter to test continuity (very low ohms resistance) of the old wire.

If your multimeter has a AC-volts setting, you might be lucky and it might (although not definitely) be able to indicate something coming out of the alternator, which could rule out #3. However multimeters usually expect AC at mains frequency (50Hz = 3000rpm) so you'll need to rev it a bit (shame your rev counter's not working 3000rpm is about 1/2 of the way between idle and the red line for a 1.6 I am guessing... You'll need to put the meter into a pretty low voltage range to see anything, I would also think (start high and work your way down!)

Even if your multimeter only has a DC-volts setting, you MAY be able to see something which could indicate output from your alternator... If your meter is analogue that is. If its digital then forget it. But analogue (due to the inertia of the moving parts) will smear out the waveform into a time-averaged signal so if for example the signal is an equal ratio square wave between 0 and 12 volts, you may see an average reading of 6 volts.

But you really need an oscilloscope to inspect a waveform, if you're near bristol you can pop over and hook it up to mine. Without seeing the waveform you can only make guesses.

Or you could forget about all this mucking around and ask someone from this list who has the same engine as you if you could go to theirs and temporarily swap alternator or rev-counter (one at a time!) in order to see if they actually work in someone else's van, in order to pin-point the faulty component.

In fact you needn't take the components out to 'swap' them, simply get enough length of wire and run the output of your alternator to their rev counter and vice versa.

Good luck!

[tannera34]

Thanks, that looks like a good list and I take your point about finding someone with a working van, if I can find a local one that has to be the way to go. All I think I need to do is unplug my red black wire and connect it to their alternator using a loose wire? Earth the vans together as well - Probably best.

I did take the binnacle apart and test the connectivity between pin and the back of the rev counter - it is good. I did the same with the red balck wire to the binnacle connector - also good. So I agree with you rev counter or alternator. I also agree about the oscilliscope! Shame I dont have one - although thinking about it I might be able to blag the use of one from my old university.

Thank you for your very prompt reply.

[ambivert]

yep just swap the wires over, you'll probably also need to switch the ignition on.

[AngeloEvs]

Diesel Tacho wave form from the Alternator will be Sinewave (Some Alternators output half wave but it doesn't matter for confirming you have a pulsetrain) and the frequency is around 400hz for a tacho reading of around 2000RPM. You can use your standard multimeter by setting it to read AC Voltage at a voltage range of 0-25v or thereabouts. you need a minimum reading of at least 6v AC from the W terminal to trigger the tacho but I have known some to trigger at lower voltages. A higher reading is normal and to be expected. If you get a reading then check you have the correct binnacle (it should have a glow plug LED in the warning display). If you do then I would be checking that the alternator signal is arriving at the Multiconnector pin 9 and that the tacho is receiving the 12v supply via the membrane panel at the rear of the tacho itself.