Volvo 740 Turbo Estate Speedometer Fault

[Oilyman]

Firstly this is my first post so I apologise if its in the wrong place or covered elsewhere. Problem is that my speedometer has stopped working and the MOT is due next month. We have tested the sensor at the back by shorting and that makes no difference to the speedo movement so we are veering towards a problem in the dashboard. Any ideas would be appreciated as apart from this fault the car has been a fantastic runner and passes MOT every year, we don't want to lose it. We would prefer to fix ourselves but would consider having it repaired but need to find somebody reliable - our current mechanic does not deal with electronics, my Partner has rebuilt cars when younger - Marinas mainly and is experienced with electrics but we really need a starting point as to what to look for. Any advice would be much appreciated. Thanks in advance.

[Ian21401]

Do a search in this section using parameters “speedometer fault”. There are numerous threads with a massive amount of info.
When I had a speedo. fault it was eventually cured by replacing the capacitors in the speedo head and cleaning all the contacts on the PCB, including the parts of the PCB under the small screws.

[Laird Scooby]

Originally Posted by Oilyman

Firstly this is my first post so I apologise if its in the wrong place or covered elsewhere. Problem is that my speedometer has stopped working and the MOT is due next month. We have tested the sensor at the back by shorting and that makes no difference to the speedo movement so we are veering towards a problem in the dashboard. Any ideas would be appreciated as apart from this fault the car has been a fantastic runner and passes MOT every year, we don't want to lose it. We would prefer to fix ourselves but would consider having it repaired but need to find somebody reliable - our current mechanic does not deal with electronics, my Partner has rebuilt cars when younger - Marinas mainly and is experienced with electrics but we really need a starting point as to what to look for. Any advice would be much appreciated. Thanks in advance.

Welcome aboard! Does your car have ABS? If so, is the ABS warning light on while driving?

If not, the speedo sensor and wiring to the cabin is ok - the fault must lie in the instrument cluster.

First port of call in that case would be to remove the cluster, clean all the contacts on the back of the cluster and in the multiplugs, use a thin screwdriver to "tease" the sprung loop of copper out a bit more to ensure a good contact.

Try that, if it doesn't fix it then you'll either have to try changing the capacitors or find another speedo.

If the odometer works and is still counting the miles, it is definitely the instrument itself, the capacitors might help but if not, new speedo.

Be warned, the Yazaki and VDO instruments aren't interchangeable, either individually or in complete clusters. You will have to replace like for like.

Post a photo of the speedo face and we can identify them for you.

[Ian21401]

Thanks for your post Dave. I was expecting you to contribute with your 700 series knowledge.
I intended to come back to this after I had completed my present task.

Oilyman, Have a look at my old threads, “Intermittent Speedo/Odometer Fault” and “Help Please re Speedometer Fault”. Especially the link to “brickboard” in the latter thread.
Good luck.

[bob12]

Good advice above ^^^^.

As said a pic of the front (and back) would assist. As you have a 740 the speedo and cluster is more than likely of VDO manufacture.

My first question is are both the speedo needle and the mileometer/odo not working. I ask that because the spedo unit splits the signal from the sensor on the rear diff. and, if one or the other is still working it definitely excludes a problem with the sensor or the wiring to the cluster/speedo unit.

Bob

[Oilyman]

Thanks all - very quick reply at the moment as other half just in. I will take a photograph and post but am sure that its the Yazaki one. The ABS light wasn't on but had come on and off intermittently in the past. The odometer and tripometer are not working either. We were hoping it was the sensor but we did try to short it like we read about but nothing moved on the speedometer and the fuses looked ok. Thanks so much for all your help so far. We brought the Volvo for £500 years ago - basically for nothing when factoring the new radio and how much tax was left and I love it to bits - we have a Vauxhall Carlton as well and its always the electrics let you down and the thing we find hardest to fix.

[Laird Scooby]

As a general rule, shorting out a sensor that provides a signal for speed (of anything) is a good way of simulating failure.

On the other hand, for things that measure a quantity of something such as fuel level, temperature, pressure etc usually shorting the sensor out results in an FSD - Full Scale Deflection of the instrument concerned.

That's not a hard and fast rule though. If you short out the fuel level sensor on your 740, the gauge won't move. Use a 270 ohm resistor and it'll read nearly full.
Conversely, on your Carlton, shorting the fuel gauge sender wire to earth will result in the gauge flying over to full. If it doesn't it's suffering the commmon Vauxhall problem of a poor tank earth - easily fixed by cleaning with abrasive paper/wire wheel in a Dremel or simila on the terminals for the earth connection to the tank.

From everything you've said so far, the fault is either in the connections to the instrument cluster or the instrument itself.



That's a VDO if memory serves correctly, i'm sure if it isn't Bob will correct me!

The Yazaki ones have the odo and triup meters in opposite places and little "bars" between the digits on the tripmeter, a bit like this one :



The bottom one now has another 1000+ miles on it.

If cleaning the connections etc don't help, if you're any good with a soldering iron you can replace the capacitors, if that doesn't fix it then it's a case of trawling fleabay for a good secondhanf cluster but they're not cheap these days. You may be lucky and find a good used one on here in the For Sale section.

Purely out of interest, what year and model is your Carlton?