SPEEDO BROKEN - OR SENDER - WHICH ?

[davidharrow]

My Speedo died gradually over the past week or so / 300 miles. It kept dropping suddenly back part way to zero and springing back quickly to the right place. Then it started dropping to zero for a few seconds (odometer stopped as well). Then longer zero periods. Then Monday morning when I pulled away, stayed on zero and 200 miles + later it, and the odometer have not moved.

Question: Is it an electronic sender, or a speedo head problem ? Is there a way to test other than putting in a new speedo and seeing if it works (after buying it I suppose !) ?

Any advise will be much appreciated.

David

 

[bones]

Sounds like the sender, much cheaper to replace as well.

 

[Guest]

Mine broke in a similar manner a few weeks ago, although much quicker (over maybe 20 miles). It was the sender, not too expensive.

Joseph

 

[tripe]

There is a test procedure for the sender. As I recall it involves removing the central rear cover on the tunnel behind the front seats. With the cover off the connector for the sender can be seen (one green wire, one brown). Disconnect the wires. If you then jack up one rear corner (and put the car on an axle stand) turning the road wheel should send a signal through the wires which can be picked up by a suitable test meter.

 

[nathan 1981 930 G50]

Check the connections in the tunnel near the rear seats (the connection is near the gear adjustment linkage) and behind the speedo !

 

[davidharrow]

Thanks for all the advice. Tested wires to sender from between the back foot wells - no pulse.

New sender from OPC - £54.40 + VAT - not too bad at all

Getting the old one out was a PIG - in the end I screwed a big wood screw into the middle (not too far to go right through) and wrenched the centre out. Plastic sides could then be 'collapsed' inwards and everything removed.

New one in was easy.

Thanks for all the advice - Speedo now working fine.

Cheers. David

Job done

 

[nathan 1981 930 G50]

Great news, it makes a change for these cars to be a cheap repair []

Out of interest when you removed the sensor could you then see inside the gearbox/diff ?

 

[bones]

ORIGINAL: nathan 1981 930 G50

Great news, it makes a change for these cars to be a cheap repair []

Out of interest when you removed the sensor could you then see inside the gearbox/diff ?

Afraid not Nathan the sender just plugs in, if you want to test it you will need to know the voltage it is required to produce, I don't know what that is. If it turns out not to be the sender or the clock the other possibility is that the mechanism in the gearbox (a series of rotating magnets emitting pulses) has become damaged, it is possible for one of the magnets to fall off and produce the eratic symptoms you describe. The good new is its not common.

 

[davidharrow]

The sender does not produce a voltage. It is a simple reed switch that is opened and closed by rotating magnets in the gearbox.

And no, you can't see inside once the sender is removed.

David

 

[nathan 1981 930 G50]

Thanks.

 

[bones]

ORIGINAL: davidharrow

The sender does not produce a voltage.    It is a simple reed switch that is opened and closed by rotating magnets in the gearbox.

And no, you can't see inside once the sender is removed.

David

Well not wishing to split hairs.......but by way of clarification, the pulses sent from the gearbox via the sender may not constitute a continuous current but they are pulses of electrical current read by the speedo's pcb, any electrical current requires a voltage to push it along. For the future the sender can be checked for operation by rotating the rear wheel and checking on a meter set to low resistance, one turn of the wheel gives six or eight pulses (can't remember off hand), my gearbox is a '74 sporto with four gears and a transducer to convert the early mechanical drive to pulses for my '78 elecctrical speedo to read.

 

[davidharrow]

Sorry, but the sender does NOT produce any voltage. When disconnected from the wires that go to the speedo a meter will apply a voltage and detect the opening and closing of the reed switch. When connected to the speedo the voltage is supplied by that device and interrupted by the reed switch.

David

 

[bones]

I did say VIA the sender, the pulses come from the roataing magnets , can we put a stop to this I think we're talking about the same thing.

 

[nathan 1981 930 G50]

Roughly how many ohms will the meter register ?

 

[davidharrow]

It will move between open circuit and closed circuit as the wheel rotates - no resistance as such. Between rear seats, well just in front of, open the cover - there is a connector with 2 wires - big connector - disconnect and measure across the wires left in the socket (going rearwards) as you rotate one of the rear wheels with the car jacked up.

Which side the sender is depends on the year - on my '84 it is on the right facing forwards - on many it is on the left - I turned the wheel on the same side as the sender - it is right next to where the drive shaft enters the gearbox casing.

It is a simple reed switch that opens and closes as magnets rotate close to it.

Mine was permanent open circuit, but I guess you could get a permanent closed circuit as a fault, but I suspect it is unlikely.

If I can help further email me and I'll give you my phone number.

David

 

[nathan 1981 930 G50]

That's great, thanks.