Testing Electronic Speedo

[Alan 480]

Having a few issues with the 'leccy speedo.

I have tried the wiggle plugs, or just hand on back of dash-board and press, this 'used to fix it' but not recently so:-
whole unit apart, speedo removed , contacts cleaned etc
reassemble etc worked for a while now stopped.

I can easily take the unit out but what can I send it signal-wise to see if the speedo is working on the bench BEFORE i put all together as there is really not enuf wire to have it 'hanging out' and run car with wheel off the deck!
I have a spare flywheel sensor that I could 'jury-rig' to create the pulses (with a magnet on a small motor) and looking at wiring diagram it 'looks' like it only needs a 12V supply and earth with the other two wires going to the pick-up BUT there is 'also' a feed to the ECU in some diagrams

If you look at the Haynes Manual 12.51 it has five wires, but 12.64only has four! (i think 12.51 is correct?)

I also have a spare speedo in loft which I guess I could fit but milage will need to be 'corrected' to match existing unit

[pgmm]

Speedos come in two versions: early 4 pins throught the printed circuit board and later 5 pin small blue connector.

I test my speedos using a spare speed sensor (I'm not sure the flywheel sensor is identical) connected to
on yellow/blue connectors yellow6 and blue2
on yellow/red yellow6 and red3
on grey/green grey 6 and 12

Then when you connect power
on yellow/blue connectors ground on blue1, +12V on blue12
on yellow/red ground on red2, +12V on red12
on grey/green ground on green 2, +12V on green12
you should see (if you have a scope) a 220kHz sine wave on the speedsensor connections. If not, the electronics died.

If you then move iron (magnet is not required - the 480 does not have magnets either) in front of the sensor, the speedo and milage counter should move. I use a spanner in a battery drill that does 1200 rpm - this corresponds to about 12 mph.

Alternative: jack up the left front wheel (the speed sensor is on the left axle, use a garage jack, not the car jack!) and make it run slowly in first gear or D

[Alan 480]

Peter

thanks for the info.

I do have access to a scope but not a spare speedo sensor so I was thinking of jacking up the front wheel when unit was all in car but before assemble EVERY thing!
mine has a small blue plug that connects the speedo unit to the main PCB (93 model)

[pgmm]

The 220 KHz signal (about 0.5V top-top) is only present when the sensor is connected to the (active) speedo electronics. It will be difficult to access the signal in the car, but maybe the connector under the air cleaner box is accessible.

The flywheel sensor has a resistance of 220 Ohms (+/- 60) and the speedo sensor something like 2 Ohms if I'm not mistaken - they are clearly not exchangable.

The small blue plug connects both wires from the sensor to the speedo unit, whereas before one of the wires was connected to GND / shielding and only one wire carries the signal. The new wiring should improve reliability.

[Alan 480]

Still not getting this to work 100%, recently fell over again

final solution might be to run new wires DIRECT into the blue multi plug ie cut into the loom at the grey plug for the sensor and supply an independant 12V and earth

[svenska axel]

Still not getting this to work 100%, recently fell over again

final solution might be to run new wires DIRECT into the blue multi plug ie cut into the loom at the grey plug for the sensor and supply an independant 12V and earth

Still not getting this to work 100%, recently fell over again

final solution might be to run new wires DIRECT into the blue multi plug ie cut into the loom at the grey plug for the sensor and supply an independant 12V and earth

i did this to great effect. get some solder and just bypass the connector from loom to board, only need do the signal wire from the sensor, it struggles to get through more than the 12v supply which does the rest of the board, so if the rev counter and bulbs are working the it has power. another measure i also did re-melt with solder iron where the plugs are soldered to the board, sometimes the joints can become 'dry'. reflow them, can add a bit more solder if you want.

[Alan 480]

OK bit the bullet and bought a 'boy racer' style speedo and added a sensor pick-up onto front N/S drive shaft

when in the back of dash fitting the new speedo I spliced in a two way plug from the original sensor.

now VOLVO speedo works on the passenger footwell, power supply is from the light for the fan switch!

bottom line it does seem to point to an iffy contact between the multi plug and dash-board, the 'new' speedo is fed from the original power supply to the original VOLVO unit so proving that power is not the original fault

[Ulrika480]

Interesting reading.

Any ideas why my rev counter would come and go (all or nothing) but the speedo carry on working ?

[Alan 480]

I 'think' that the only common part there is between the pair of them is the power supply?

there is a signal from the speedo output that goes to the 'engine management' but maybe just to allow fuel consumption to be worked out?

[dagajsek]

Hi.

PLEASE HELP

I've made a R5 conversion of B18FT and have used up all the Volvo loom, cut it, made it like i needed and resoldered it.
Speedo af course does not work.
I have to seperate speedos, none of them works neither in car or on test bench using the pgmm's solution. I also have two seperate sensors. Both have a resistance of around 2ohm, so should work. One is in the car and the other on the bench.

A speedo cluster has a wire which I haven't resoldered to anywhere (Hall sensor-ECU), because as far as i know, I don't need it, in electrical schemes there is no sign of it being connected in B18FT versions.
Am I wrong? I checked up the connectors from volvo, I can see where that wire went, but stil on the other side of the connector, where it should then bo connected to ECU there simply is no wire, so I am pretty sure I am not wrong.
I took apart whole of the speedo and came out with nothing, all seems ok.

But still, the speed dial gets 12+, 12-, it gets readings from sensor... what am i missing? I have no high tech equipment and have oly basic understanding of electronics so please explain me what to do in a way i can understand.

Thanks for help, much appreciated.

[dagajsek]

Ok, got the bench test thingy to work, at least on one speedo and no matter how long I run the machine, there is no movement on the odometer. The speed needle is dancing around and is apparently working.

Stepper motor simply doesn't do anything. How to test it without killing anything?

Any ideas?

[Alan 480]

by dancing around, do you mean it registers a steady figure when drive is 'rotating'
what are you using to give a steady speed?

I think the WHOLE speedo would work/fail, but i guess the odometer could fail separately. it looks a nightmare to swap the odometers from one to other!

[dagajsek]

I've tested it with drilling machine and additional speed sensor.

It works in car also. Still odometer doesn't, but have found some info on capacitors on odometer board that malfuncion and will test it, when I'll have time.

It seems that it's a well known problem.