Speedo not working.......help fault finding - Alfa Romeo Bulletin Board & Forums

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Old 05-31-2009, 11:07 AM
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Speedo not working.......help fault finding

OK, after 3 penalty point speeding because my speedo doesnt work, I took the plunge & bought a replacement dash pod (as my tacho was faulty too).

Tacho is fixed................ but......

Speedo still doesnt work.

So conclusion is;
a) I have 2 dash pods & the speedo works in neither or
b) I have a fault somewhere further down the line.

I think it is option b.

I have checked the rotating cable, all ok.
I put a multimeter on the output from the pulse generator & found the following;
1 cable has 12v
1 cable is earth
1 cable has a constant 6.7 volts, irrelivant of speed.

now I know that the pulse generator gives out a oulse signal, so the 6.7 volts is probably that signal, but how do I check the "pulse" it is giving with a standard multi meter ?
secondly, I cant seem to locate the wires after they dissappear behind the bulkhead / firewall.

before the firewall, I have a black cable, a pink & white cable, & a white cable.

I look at the 4 connector blocks that go into the dashpod, & there is no pink & white cable. I cant find out which black cable is the one from the pulse generator, as at the pin they are twinned up & always arc to earth.

which cables on the 4 connector blocks belong to the speedo ?

Can anyone help ?

I dont really want to take the dash apart again, untill I know which cables I need to look at.
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Old 05-31-2009, 07:02 PM
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great thread on monopod

Hi Jamie,

There is a great thread on repairing the monopod cluster here

Even if yours is a series 3, the info is much the same. Mine would work at

times and then completely stop. Turned out the times it stopped that I had hit a bump and the white indicator arm had jumped over the stop and was stuck. Once that was fixed then speedo acurracy could be addressed.

Is you odometer working ? if it is than the signal is making it from the speed sensor to your pod.
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Last edited by GuyC; 05-31-2009 at 07:03 PM. Reason: because I still can't add a link properly
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Old 06-01-2009, 04:31 PM
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The pulse generator should probably be sending out a square wave pulse output. I'm sure you could check it with an oscilloscope, but I'm reasonably certain you should be able to see the voltage fluctuating with a DVM if you drive slowly (or just jack up one rear wheel and spin it). If it doesn't vary at all that probably implicates the pulse generator.

On the S4 Spider they moved the pulse generator to down by the exhaust downtubes. It has a heat shield, but it still tends to bake from the heat down there which I suspect results in a high failure rate. Personally I've had two die on me, so if I were a betting man I'd put money on the generator being your problem.
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Old 06-05-2009, 05:15 AM
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I checked the voltage from the Pulse generator & it was 6.7 volts irrelivant of speed. I hade the rear wheels jacked up & let it tick over in all 5 gears.

does this sound like the pulse generator then ?
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Old 06-05-2009, 05:27 AM
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If your using a digital volt meter, just switch to ac, the pulses will read on ac.
cliff
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Old 06-05-2009, 09:10 AM
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Jamie I'm going through the same process at the moment. My speedo kicked in briefly the other day and registered speeds up to 60kph whilst I was actually doing up to 70mph ( I am using a TomTom as a speedo at the moment. Downloaded a program that gives a big display of speed on the TomTom so you can see it easily). The odometer also worked for a while. So I guess there is a pulse trying to get through but it is intermittent.

I have consulted friends with a background in electronics and, as I understand it, the pulses should be fixed voltage but vary in frequency with speed. They should be a square wave and as others have said this could be checked with an oscilloscope. I am tempted to get one from Maplins as they have a handheld one on offer for 78 at the moment.

The diagnostic from the Alfa tech bulletins describes an LED with a resistor to check for pulses. The LED will light up each time a pulse hits it. I have got one made up but haven't used it yet as the pulse generator on the automatic is not very accessible - located on the autobox so need to get under car. I can send you more info. on this if you need it.

With regard to the wires at the back of the pod, I have a diagram that identifies the wires going in to each of the four plugs although it doesn't show how you tell which plug is which. I don't know if it suits your car (its for 1990 - 93). Let me know if you want me to email a copy over.
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Old 06-07-2009, 12:18 PM
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if you could send that to me; jamiemyates@googlemail.com
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Old 06-07-2009, 12:21 PM
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PS; wheres this technical bulitin that mentions the LED ?
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Old 06-07-2009, 01:05 PM
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Hi Jamie,

I have emailed the diagram to you. The diagnostic is on the cardisc CD, listed as one of the Technical Service Bulletins (TSB).
Its well worth getting a copy of this disc. I got mine via eBay,

regards

Nick
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