Speedo fix

Calling Jersey John !!!

Why does turning the rear screen heater on then off make the speedo come back to life ???
This works most times on a few GTA so there must be a connection ( 'scuse the pun )

Lee..... there is no connection, but I can see a possible reason, but it is only an opinion based on experience.
When you switch on the rear screen, or any other high current accessory, there is, for a fraction of a second, a dip in voltage which bounces back rapidly, and that jerk in voltage can bridge a minute crack in the print or solder lead-through...... its the same principle as car alarms which detect this voltage drop when even the small current for the interior light can be detected to operate the alarm.
With a voltage as low as 12v, a crack/break in connection can be as little as a few microns, or a bit of surface corrosion, and the voltage will not jump the gap.... give it a jerk, add the "back EMF" serge caused by a relay operating, which can produce an inducted higher voltage than the 12v for a fraction of a second, which is capable of jumping the gap, and remakes the break, albiet temporarily.
You will see some relays have a diode across the coil which suppresses that "back EMF" voltage in sensitive circuits, that type of relay is, of course, polarity conscious, not normally seen in Auto Electrics.
Hope this makes sense.

Thanks ,will investigate further but it seems you have to switch on then off for it to do it .Not 100% sure on that always being the case but will keep an eye on them and see .

but it seems you have to switch on then off for it to do it

Yes, quite possibly Lee.... it is the collapse of the magnetic field in the relay coil that produces the "back EMF" which in turn produces the induced high voltage.
Either the voltage drop (switch on) which jerks the circuit, or the high voltage spike (switch off), which could remake the contact.

Turning the interior fan on and off kicks my speedo back to life.

I am finding the more I use the car the more reliable the speedo is, it now works pretty much all of the time

Good to know, John.

I've certainly found the more you use it the more it seems to work reliably - but then that seems true on all the electrics!

I suffered intermittent speed for the best part of a year. I used to work in electronics and I reckon the problem is the very low current signal used to drive the speedo is sent from the sender on the gearbox to the speedo via screened cable that has joins at the sender, at the big plug socket under the stereo, again at the connector block to the left of the speed, from whence it goes to another connector on the right hand side of the instrument nacelle to connect it to the computer board, then back across to the speedo. All of the connectors are standard automotive types, designed to carry 12 volt and a little current, which helps keep the terminals clean. I used to be able to get mine going by thumping the instrument nacelle, and I found it was worse on hot days when the car was parked up in the sun all day and the interior got hot. I replaced most of the connectors using an mini earphone type plus and socket, which appears to have fixed the problem permanently. I contemplated running a new screened cable all the way and soldering things, but this at least allows easy connection disconnection allowing you to pull things apart if you need to, and the plug/sockets are designed for small audio signals so they are perfect for the job.
If anybody is still having issues, I hope this helps.
Frankly, lots of Euro cars in the 1980s used standard automotive connectors to carry small current supplies which led to lots of problems.

My speedo works everytime now unless I am doing over 120 when it drops to 0 before getting on with the job.

What I did was:

1. Clean the multiplug under the wheel arch with contact cleaner
2. make sure I use the car often

I used to be able to get mine going by thumping the instrument nacelle

Whilst we are all (probably) guilty of this, it's probably not the best thing to be doing.

From a conversation with John, I decided to remove all the pins in the big connector block on the LHS, carefully closed the female ends slightly and refitted. This made the connection between the pins tighter.
Since doing this some months ago, I've had a lot less 'time out' on the speedo dial.

From a conversation with John, I decided to remove all the pins in the big connector block on the LHS, carefully closed the female ends slightly and refitted. This made the connection between the pins tighter.
Since doing this some months ago, I've had a lot less 'time out' on the speedo dial.

When say big connector block on the LHS - is that within the dashboard nacelle or the one behind the radio or elsewhere?
Thanks

I've certainly done the connector in the rear wheel arch - a quick spray of WD40 often helps. But I cleaned it with switch cleaner and then sprayed the lot with silicone spray (like Holt's Damp Start) in an attempt to keep more moisture out of there. Still need the rear-demister switch from time to time...

Yes the block inside the dash. There is a large one on the LHS and then the feed goes over to another smaller narrow connector on the RHS.
I'm not saying it is a defo cure, but for me it has helped a lot. If the speedo drops off, then I use to wiggle the big connector by removing the air vent and sticking my hand in. Speedo would then jump back to life, hence why I suspected the pins werent making a good connection. (I now run the car with the air vents not entirely pushed in - so I can remove them as and when required)
John in Jersey gave some good advice on PCB issues which may also be suspect.

Like you, I'm also in the habit of disconnecting the connector near the rear wheel and spraying switch cleaner. I do this every couple of months to help the cause !

Yes the block inside the dash. There is a large one on the LHS and then the feed goes over to another smaller narrow connector on the RHS.

As Paul says.... undo... then tighten up all the female pins so when reconnected, it is a tight fit. Spray with cleaner.... connect/disconnect a couple of times to scratch off any surface resistance and reconnect. Then move over to the speedo board on the RHS of the unit and spray/clean the pins of the TWO white connectors and spray inside the female part.... look down into the female connectors and see if there is any distorotion in the pin shape.... it will be quite obvious if there is a weak one. These pins can be withdrawn by pressing down the barb which can be seen in the white plastic pin carrier... you can tighten the pins up by carefully bending them then reset the barb, and push the pin back into the plastic..... only do one at a time so as there is no mix up of pins. Do this to both connectors and refit.

Like you, I'm also in the habit of disconnecting the connector near the rear wheel and spraying switch cleaner. I do this every couple of months to help the cause !

This must always be your first port of call for an intermittent speedo. This connector is in the worst possible place for reliable operation .... what johnc has done is excellent ... spray with cleaner then silicone spray.... the only thing extra I would do is connect/disconnect a few times having spray-cleaned to scratch off any surface resistance.
If all this fails then refer to the Members Only Section for Speedo Board overhaul.

Anyone got a pic or description of exactly where I will find the connector near the rear wheel arch and I assume right rear wheel?

Trevor.

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There are three connectors behind my offside rear wheel (mine's a late 1991, so earlier cars may be different) and it is the one closest to the rear wheel which is the connector for the speedo.

Excellent thanks, I'll give it a go.

Trevor.