The only previous thread I could find that sounded like my problem was this one, for a 940: https://volvoforums.com/forum/volvo-...up-down-52919/

I have a '91 Volvo 740 that generally runs great, but every once in a while (like less than 1% of the time), the speedometer will start to bounce around crazily, like between 30 and 60 mph if I am doing 30. It is usually solved by turning off the car and restarting. I was hoping it was just a loose wire or something, but based on that previous thread, sounds like it may be bad diodes or something? It looks like the lower dash panel can be unscrewed; is this something I should try to tackle myself? Or should I not even worry since it hardly ever happens?

In this video I am traveling at the lower end of the bouncing range, around 30 mph. Luckily I had my camera handy when it started to happen.
YouTube - ‪Orbs in the speedometer‬‏

 

Bad diodes in speedo head. Need new speedo or competent repair. The Brickboard.com has a good repair write up for DIY. Or you can live with it but it will get worse with time.

 

OK thanks, I'll look for that write up. It almost never happens, so I'm not too concerned yet. It hasn't happened in weeks, I just happened to run across the video on my camera.

 

I would 1st check the sending unit located in the rear differential. It just may be a partially broken wire or a bad contact. The unit sits in the rear end housing and gets pretty dirty back there. Take a look .

 

Will a failing speedometer cause a check engine light to come on? The speedometer is part of the ignition system. Please advice

 

no, not related.

 

Quote:

Originally Posted by wfd

Will a failing speedometer cause a check engine light to come on? The speedometer is part of the ignition system. Please advice

When it finally craps out, it will set a 311 code and you will have idling problems. As long as it's intermittent, probably not. Just got done with my own '91 740T speedo adventures, about 260 bucks worth. Capacitors go bad and leak all over the circuit board. Hard to find someone willing to fix it; even harder to find someone that actually knows how. My speedo story began exactly like your video.

 

I haven't further looked into this but is the speedo independent of the other gauges in the instrument panel? Can it be replaced separately?

Thank you Lev and Joseph for your help

 

I haven't further looked into this but is the speedo independent of the other gauges in the instrument panel? Can it be replaced separately?

Thank you Lev and Joseph for your help

Oh yeah, I replaced mine with one from the wreck yard several times and they would work for a while, then crap out just like the one I had originally. You have to remember, they're all 20 years old. Bought me some time, though, and I got a lot more knowledgable about clusters.

One thing I learned that you might find useful: The cluster has a fuse that is part of the circuit board, not in the fuse box. Because the circuit board is on the outside of the cluster where you can see it rather than the inside, as it is on 240 models, it is extremely easy to blow this fuse and end up with no working instruments, lights etc. - dead cluster. This is not a problem if you first disconnect the battery before taking it out, as the manual tells you to. I skipped that step and learned the hard way. Volvo makes a replacement fuse that basically bridges the part of the circuit board that gets burnt through, but it's a special order item that the parts guy probably won't even know about. It's about 8 bucks for one fuse. I'll let you imagine how much time I wasted in the process of learning this..

 

Two years and 30k miles later, it's still an intermittent issue, but becoming much more frequent. Since I just had to spend $500 on a crash sensor for the airbag, I'm hoping to put this repair off a bit longer.

 

1991/1992 7/9's have crappy speedos. sadly they aren't interchangable with later ones, unless you change the whole instrument panel, and then your fuel gauge reads wrong unless you change the fuel sending unit. arrrrgh.

I have a 1992, I've done the capacitor replacement on my speedo and its /still/ intermittently sticking at 45mph and not reading higher. for now I'm using my GPS as my speedo ;-p

 

I believe the sending unit is mounted in the rear axle . I would check the wiring and the connector back there. The salt and mud have a tendency to destroy the wire/connector. Check and see if it is intact and clean it.

 

I did the capacitor repair and it didn't work. I suspect I didn't get some of my solder joints fully reconnected. There are multiple layers of the circuit board to get through. I enjoyed the exercise; bought the caps for $.50 each from Digikey, a solder sucker which I didn't have and a new set of small screwdrivers just for the hell of it, then followed the Brickboard procedure. My speedo works more often now but still only about 30% of the time. Some day if I get real bored, I might reheat all the joints I did and flow a little more solder in there. If you're feeling adventurous, I would try the repair. C.