Anyone know what exactly the "pulse" for S4 speedo is?

anglospider · #1 (**Post Link**) 02-12-11

I want to do some bench testing of a couple of S4 speedo units with a view to cannibalising and repairing to get one good one. I am happy to buy a signal generator to do it, but I've no idea what I should be buying

Does anyone know what type of signal it should be or what bench generator I would need?

And yes, I know it would be easier/cheaper to find a working unit but this is a challenge and a nice project for the dark winter evenings!

karlssondesign · #2 (**Post Link**) 02-12-11

I suspect that the system works either via pulse width modulation or pulse peak modulation. Either way your best bet would be to get an old ScopeMeter. As this will be the best way to measure the pulse. As the pulse will change in width or height according to the specific speed generated by the gearbox output cable.

Are you trying to find a method by which you can bench test repair and ultimatly calibrate the repaired speedos ? There is a really comprehensive thread on calibrating s4 speedos on alfabb, without the use of a scope........ But you will need a gps sat nav or android phone with gps speedo.

anglospider · #3 (**Post Link**) 03-12-11

Thanks Karlsson. I'm not worried about the calibration as I can do that in the car once I have a working unit. In fact, the speedo part is accurate.

What I have is a working speedo with a defunct odometer. I'm trying to buy a non-working speedo with a working odometer to switch parts.
I'd like to be able to sit at a bench and just make sure that things work before/after fiddling with stuff rather than have to keep putting a unit back into the car, going for a drive etc.

Nige005 · #4 (**Post Link**) 04-12-11

A quick way to see if the Speedo is working is to connect the handle of a normal metal working File to one of the Sender wire connections and then scrape the other connection wire along the File itself. Doing this on my Alfetta the Speedo jumped up and down in time with the scraping, reached about 40mph.

Obviously you'll need the 12 Volt supply to the Speedo Head.

Sounds daft but it does work, the File acts as a rough ( very ) Pulse Gen.

anglospider · #5 (**Post Link**) 04-12-11

Thanks Nige. Did you notice if that was enough to shift the odometer reading too?
I guess it would need quite a few passes to get the equivalent of a tenth of a mile but I will give it a go.

When you say two wires, though, it sounds a bit different to this speedo. There is only one sender wire that goes to it so I'm not sure how I would make the file work. A 12 volt feed to one end of the file, and then another wire with one end to the speedo, the other end run over the file so it gets 12 volt pulses?

Nige005 · #6 (**Post Link**) 04-12-11

Originally Posted by anglospider

Thanks Nige. Did you notice if that was enough to shift the odometer reading too?
I guess it would need quite a few passes to get the equivalent of a tenth of a mile but I will give it a go.

When you say two wires, though, it sounds a bit different to this speedo. There is only one sender wire that goes to it so I'm not sure how I would make the file work. A 12 volt feed to one end of the file, and then another wire with one end to the speedo, the other end run over the file so it gets 12 volt pulses?

No, I didn't do it enough to see if the odo worked too but I can't see why it wouldn't as this is the only input from the Sender.

I know you have seen the Post about fixing my Speedo, if you want a refresher than have a look in the Link, Post # 7 on really.

http://www.alfaowner.com/Forum/the-c...ic-speedo.html (Alfetta GTV6 Electronic Speedo.)

As to the wiring there are two wires from the Sender and iirc one of them picks up a switched 12 Volt on it's way to the Head, this also goes back to the Sender. Can't remember if there is a separate 12 Volt to the Head, can look if you like but not sure it would help you a lot.

Don't think your idea of using the " File Method " would work but I'm still trying to get my head around it, time to dig out the diagrams I think !!! I think I have a pic of it so I'll stick it up here in a minute.

It could be that one wire is the Pulse and the other is the 12 Volt feed to the head which is also the 12 Volt to the Sender and Head, if you see what I mean.

Nige005 · #7 (**Post Link**) 04-12-11

Here are you diagrams, you'll need to " blow " them up to make sense of them !

21 is the head, 52 is the PG, time to get the magnifying glass out !

Hope they help.

anglospider · #8 (**Post Link**) 04-12-11

Thanks Nige, I'll print and have a look.

There are two wires to the sender, one is a 12 volt but I thought that was to provide power TO the sender (it earths through the screw thread to the transmission).

Nige005 · #9 (**Post Link**) 05-12-11

Originally Posted by anglospider

Thanks Nige, I'll print and have a look.

There are two wires to the sender, one is a 12 volt but I thought that was to provide power TO the sender (it earths through the screw thread to the transmission).

It sounds much the same as the Alfetta one then so with any luck the File " trick " should work.

The quickest way to test the odo though would be to use the Sender itself driven by an electric drill, easy if you have a spare........ On the Alfetta the Sender can be removed very quickly, if it's the same on the Spider this has to be the best way to test the Head unit.

Hope you have fun working on this. I didn't enjoy testing the whole system on mine but once I'd isolated the fault to the Sender I enjoyed the challenge of getting it working again, having realized that the unit was made of the fabled unobtanium......

anglospider · #10 (**Post Link**) 05-12-11

On the automatic spider the sender can only be removed by dropping the transmission and I don't have a spare to hand (they are unobtanium too. I could only find one supplier, in the US, and paid $215 for the one in the car.I was told there were no more available after that..)

Anyway, I will let you know how I get on.

Nige005 · #11 (**Post Link**) 06-12-11

Originally Posted by anglospider

On the automatic spider the sender can only be removed by dropping the transmission and I don't have a spare to hand (they are unobtanium too. I could only find one supplier, in the US, and paid $215 for the one in the car.I was told there were no more available after that..)

Anyway, I will let you know how I get on.

Ouch, sounds a bit of a pain.

Hope you can figure it all out and get a result.

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