Talk Morgan Forums Morgan Sports Cars Classic Morgans (pre-CX) 1998 +4 4 Seater - Speedo

Hi Zach,

I will have to try the driving for 20 miles to check the ODO and see if that works. I have done this roughly in the past and the ODO does appear to work. The speedo on the other hand appears to be getting worse with long periods when driving showing at 0 miles an hour, or 100 occasionaly it shows the correct speed according to the GPS speedo.

I am not sure if you or any one else knows about the resistor which is on the block that attaches to the back of the speedo.

I have read somewhere that this fails and that can cause issues, but I have not read anywhere how to test this or what to replace it with if it is faulty.

I would be very interested in seeing the VDO test rig.

Regards

Laurie

Hi Laurie

There is a component connected between pin 8 (sensor signal in) and pin 3 (earth) on the plug that fits into the back of my instrument. Unfortunately I don't know what it is because it is contained within a piece of sleeving. It could well be a resistor and if that is the case it would be a 'pull down' resistor but this depends upon the type of sensor fitted. My sensor is fitted in the vehicle's gearbox, I suspect your sensor is near the front wheel hub and they may be different electrically. I have not heard of failures of this component.

I followed the cable from my sensor and it arrives at a three pin connector, hanging in the cables on the drivers side (see photo). The steering column shaft is just under my hand. The three pins carry earth, plus 12 volts with the ignition on and the third pin is the signal back from the sensor. I jacked one of the rear wheels up, monitored the voltage on the signal pin with a volt meter, got my wife to spin the rear wheel and watched the signal volts changing from high to low as the wheel rotated. You might be able to do a similar test if you can find your sensor connector but in your case it will probably be one of the front wheels. Obviously the ignition needs to be on during this test.

I have posted the details of the VDO speedo test rig within another post, see here

https://www.talkmorgan.com/ubbthrea...er-not-working-on-2008-plus-4#Post817814

I've check both hubs and there is no sensor on there but there is the below wire that is heading to the back of the gearbox, although I can't see where it goes.



The wires that come out of this are below



Which go int this block



There seem to be a lot of wires coming out of that block, it is really difficult to trace then under the dash as its a bit of a dogs dinner under there.
I'm not sure what happens to the rest of them but below is what is at the back of the speedo.




I'm not sure if I am better off knowing this or not.
Is all the cabling on the MOGs of this age a bit of a mess?

Yep ! Especially if a previous DIY owner has added accessories

ps, that light purple with white tracer needs sorting where you have exposed inner copper

Hi Laurie

My car is fitted with a Ford Zetec 1.8 engine and a Ford MT75 Gearbox. I think I read somewhere your car is fitted with a Rover Engine and I don't know anything about them or what gearbox is fitted. I am absolutely sure that other people on this Forum would be able to tell you.

Turning to your photographs, the fourth photo showing the speedo connector looks very similar to mine. I am pretty sure that the yellow wire is the signal in from the sensor and goes to Pin 8. The component in the sleeving goes from the yellow wire to two black wires, which I think are Pin 3. That would be the ground connection to the instrument.

You can check the pin numbers. Hold the speedometer the correct way up i.e. odometer at the bottom, look at the rectangular socket at the back of the instrument, there are two rows of four pins. The pin numbering starts at the bottom right hand corner which is Pin 1 to the bottom left hand corner which is Pin 4, then from the top right hand corner Pin 5 to the top left hand corner Pin 8.

Your third picture shows a connector with a number of wires. If this is the speed sensor connector the sensor would need three of those wires and I am going to guess that the other wires are associated with the reverse light switch, which is probably also in the gearbox. It wouldn't be a bad idea, with the car battery disconnected, to disconnect and reconnect this connector a few times to try and clean the pins.

The chances are that the yellow wire in this connector is the one that carries the sensor signal. You can check this. Switch your multimeter to its lowest ohms range, connect the two meter leads together and the meter should read somewhere between O and 0.9 ohms, make a note of the reading. To be safe disconnect the car battery negative terminal. You may need help but hold one of the meter leads on the yellow wire of the speedo connector and with the other meter lead back probe (you might need to use a safety pin) the yellow wire in the connector in photograph 3. If the meter reading is the same as you saw when you put the meter lead together then it is pretty certain that these two yellow wires are connected. If this is the case you could then normalise the system by plugging the speedo back into its connector, reconnect the car battery negative terminal and then, with your multimeter set to measure DC voltage say 20 volt range connect the negative meter lead to ground and the positive to the yellow wire in the connector in photo 3. With one of the cars back wheels jacked up and the ignition turned on, spin the wheel and this voltage should be changing up and down as the wheel rotates. Hope all this makes sense.

Best of luck

Regards Tony