How does a petrol rev counter work on a BX?

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David
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How does a petrol rev counter work on a BX?

Post by David » Thu Jun 19, 2014 6:09 pm

I was having a mess with a petrol clock the other day. On the diesel it has live, neutral & 2 "sensor" wires. One of these goes to the sensor hole in the pcb of the clock & the other goes to neutral. when you touch the wires together & pull them apart quickly the clock jumps up.

On the petrol it has live, neutral the same, & on the plug, 2 "sensor" wires. On the pcb both of these are joined together. I tried pulsing neutral to the sensor the same as the diesel clock works &... nothing.

So, are they PCBs wired completely different? & what would I have have to send to the sensor wire to make the petrol clock move? (live or neutral & if the former how many volts).

Trivial I know, But I want to see if the clock works.
1992 Citroën BX 1.9 Diesel Meteor 4x4 - The Project.

1992 Citroën BX 1.9 TXD (with GTI engine; Mulleys old car) - Parts car.

2004 Citroën Xsara Desire. (Now gone).

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Re: How does a petrol rev counter work on a BX?

Post by mat_fenwick » Thu Jun 19, 2014 7:11 pm

Try connecting +ve and -ve to a 12 V battery, then connect the -ve of the battery charger to the -ve of the battery, and the +ve charger lead to the signal input of the rev counter. Assuming it's an old style transformer based charger, there'll likely be enough of a mains ripple on the output. This will be 100 peaks a second at 50 Hz. For a 4 cylinder engine this should correspond to 3000 rpm. Never tested it on a BX rev counter, but have on another make when I was recalibrating it from suiting a 4 cylinder engine to a V8.
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Re: How does a petrol rev counter work on a BX?

Post by citronut » Fri Jun 20, 2014 9:02 am

mat_fenwick wrote:Try connecting +ve and -ve to a 12 V battery, then connect the -ve of the battery charger to the -ve of the battery, and the +ve charger lead to the signal input of the rev counter. Assuming it's an old style transformer based charger, there'll likely be enough of a mains ripple on the output. This will be 100 peaks a second at 50 Hz. For a 4 cylinder engine this should correspond to 3000 rpm. Never tested it on a BX rev counter, but have on another make when I was recalibrating it from suiting a 4 cylinder engine to a V8.

you will of course require a very very long extention lead for this, although it does depend how far from home you intend to drive :twisted: :roll: :shock: #-o :rofl: :rofl: :rofl: :wink:

regards malcolm
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Re: How does a petrol rev counter work on a BX?

Post by David » Sat Jun 21, 2014 7:25 pm

mat_fenwick wrote:Try connecting +ve and -ve to a 12 V battery, then connect the -ve of the battery charger to the -ve of the battery, and the +ve charger lead to the signal input of the rev counter. Assuming it's an old style transformer based charger, there'll likely be enough of a mains ripple on the output. This will be 100 peaks a second at 50 Hz. For a 4 cylinder engine this should correspond to 3000 rpm. Never tested it on a BX rev counter, but have on another make when I was recalibrating it from suiting a 4 cylinder engine to a V8.
I have connected it up as you suggested & nothing. I have even tried 2 different battery chargers. Am I missing something or is it likely the clock doesn't work? It doesn't even jump up slightly when you disconnect power from it like the diesel ones do.
citronut wrote:you will of course require a very very long extention lead for this, although it does depend how far from home you intend to drive :twisted: :roll: :shock: #-o :rofl: :rofl: :rofl: :wink:
This is only a test. Why would you need to drive the car around like that? :roll: [-X
1992 Citroën BX 1.9 Diesel Meteor 4x4 - The Project.

1992 Citroën BX 1.9 TXD (with GTI engine; Mulleys old car) - Parts car.

2004 Citroën Xsara Desire. (Now gone).

2011 Ford Focus Zetec - Daily Driver. (Absolute bone shaker).

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Re: How does a petrol rev counter work on a BX?

Post by mat_fenwick » Tue Jun 24, 2014 1:02 am

David wrote:Am I missing something or is it likely the clock doesn't work?
Unfortunately the test won't prove for certain the rev counter is dead, unless someone has tested a working one this way and found that it gives a reading. It may be that your chargers have a smooth output with very little ripple, or the rev counter needs more sharp edged pulses to trigger it.
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Re: How does a petrol rev counter work on a BX?

Post by citronut » Tue Jun 24, 2014 8:23 am

the rev counter should twitch/jump as you turn the ignition on before cranking the engine,

regards malcolm
curent ride
K reg BX 17TD TZD est
also own
K reg D special

no longer have
H reg CX saffari 2.5 TRI (now gone to Malaysia)
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Re: How does a petrol rev counter work on a BX?

Post by citronut » Tue Jun 24, 2014 8:25 am

David wrote: This is only a test. Why would you need to drive the car around like that? :roll: [-X
#-o :o :rofl: :rofl: :rofl:
curent ride
K reg BX 17TD TZD est
also own
K reg D special

no longer have
H reg CX saffari 2.5 TRI (now gone to Malaysia)
R reg xantia 1.9TD est (gone to meet its maker)

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Re: How does a petrol rev counter work on a BX?

Post by electrokid » Tue Jun 24, 2014 10:15 am

The rev counter has some electronics in it so when testing you need to supply it with at least a ground / 0 volts / chassis connection and a constant +12 volts / power connection and some kind of signal which represents the engine speed.

I've only bench tested the diesel version - not sure how different the petrol version would be but it will depend on where the 'signal' connection comes from. On the diesel it comes from a sensor (someone will be along in a little while to tell you where that's likely to be :D ) which, I believe, can be retrofitted to the petrol engine if required. Rev counters on petrol engines were typically connected to the contact breaker connection - although that may appear to be a 12 volt signal, there is a resonance effect (which is needed to make the sparky thing work) which makes the signal pulses much higher than 12 volts. Rev counters designed for this type of connection have their 'signal input' connection well protected against high voltage spikes - indeed they may rely on them to sense the signal.

I expect there are some rev counters for petrol engines that would take their signal input from an ECU - the signal input on these types may not be so well protected - may even rely on a 5 volt logic signal.

When I did the bench test I first determined which connections were ground and +12v battery and hooked those up to a power supply. I found which connection was the input and fitted a resistor (4.7K IIRC) in series in order to provide some level of protection for its internal electronics. I then fed a signal from a signal generator - I thought the siganal voltage required to make it respond was quite high - around 15 volts IIRC - which suggests to me that the instrument was originally designed for a petrol engine (with a 12 volt 'spiky' input) and then simply mated with a sensor with sufficient output for the diesel.

Mat's idea of using a signal from the mains is a good one - you could try using a transformer - possibly a 'wall wart' type but there are many different types of wall-warts and most these days will just provide DC so be sure to use one that put out AC - you'll probably need a 9 or12 volt version. On the wall wart you'll see the symbol for the output plug which is a dot with one connection and an almost complete circle for the other connection which represents the inner and outer parts of the power 'jack'. DC supplies will have two lines - one on top of the other - one of them solid and the other dotted. AC supplies will have the AC symbol ~ like the letter 'S' on its side which is represents wobbly electricity :)

You'll need to connect one of the AC connections to ground / 0 volts to provide a 'reference' while the onther connection should go to the 'input'. If you want to provide some protection for the rev counter input and don't have a resistor handy then a small bulb such as the 1.2W dashboard lights in series could work ok.

One thing to mention - when the rev counter is running on the bench it is surprisingly noisy - the noise is at the frequency you are feeding it so in the car it is the same 'note' as the engine so it goes unnoticed. On the bench, however, it's a bit disconcerting !
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Re: How does a petrol rev counter work on a BX?

Post by citronut » Tue Jun 24, 2014 1:58 pm

i believe the earlier petrols picked the signal up from the negative side of the ignition coil, then the later ones much the same as the diesels

regards malcolm
curent ride
K reg BX 17TD TZD est
also own
K reg D special

no longer have
H reg CX saffari 2.5 TRI (now gone to Malaysia)
R reg xantia 1.9TD est (gone to meet its maker)

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Re: How does a petrol rev counter work on a BX?

Post by electrokid » Wed Jun 25, 2014 9:23 am

That would make life easier - they both should have about the same signal input voltage then.
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Re: How does a petrol rev counter work on a BX?

Post by Kitch » Wed Jun 25, 2014 2:29 pm

It works like this:

When the engine is switched off, it reads zero. When you start the engine (warm) the needle should sit at around 1000rpm. As you rev the engine up, the needle sweeps round counting past numbers 20, 30, 40 etc up to 80. They are multiples of 100, and represent the number of revs the engine is doing.
In most BXs, when you're pressing on, and the needle gets to 40 or 50, you're going to need to get ready for a gearchange. The only model which is different is the 16v, which won't actually move forwards until it hits 4000rpm.

HTH

























Wait, did you mean? Like, the mechanical workings of a.................................

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Re: How does a petrol rev counter work on a BX?

Post by David » Wed Jun 25, 2014 5:14 pm

Thanks for the replies. The clock was out of a 1990 (H reg) car, so I assume its the newer type. I don't have any more battery chargers or other 12V AC transformers. (no not the robot transformers off TV) [-X . So looks like I would have to borrow a petrol BX with a rev counter in to test it...
citronut wrote:i believe the earlier petrols picked the signal up from the negative side of the ignition coil, then the later ones much the same as the diesels
I have tried connecting it up to a diesel sensor with no success. I even moved the wires around so it was wired the same as a diesel clock.
Kitch wrote:It works like this:

When the engine is switched off, it reads zero. When you start the engine (warm) the needle should sit at around 1000rpm. As you rev the engine up, the needle sweeps round counting past numbers 20, 30, 40 etc up to 80. They are multiples of 100, and represent the number of revs the engine is doing.
In most BXs, when you're pressing on, and the needle gets to 40 or 50, you're going to need to get ready for a gearchange. The only model which is different is the 16v, which won't actually move forwards until it hits 4000rpm.

HTH
Ah, That explains a lot. :!: I was wondering why it kept showing different numbers to the clock on the left. Mr police man told me it wasn't how fast I was going when I said it was pointing to 30, but he didn't explain what it did do! :roll: :lol:
Kitch wrote:up to 80
But the clock in my car only goes to 60. scratch... :-k :lol:
1992 Citroën BX 1.9 Diesel Meteor 4x4 - The Project.

1992 Citroën BX 1.9 TXD (with GTI engine; Mulleys old car) - Parts car.

2004 Citroën Xsara Desire. (Now gone).

2011 Ford Focus Zetec - Daily Driver. (Absolute bone shaker).