For a couple of years now my speedo, odometer and trip meter have worked sporadically. I've had all combinations of failures - favourite of the speedo was to register 30-40mph when the car was at rest after starting the engine, until travelling, when it would drop to zero for the rest of the journey!
Recently for a few months, things have "settled down" to the trip and ode working as they should, but the speedo needle still firmly refuses to budge from zero at all.
One may assume (?) that the back axle end of the proceedings must be in order or the mileage wouldn't be working?
Therefore I am wondering if anyone could hazard a guess as to what is wrong with the speedo? The car is only used twice a week - parked outside - and damp does get in behind the glass in this wintery weather until the heating dries it out. Could this failure of the speedo needle be a mechanical matter needing a simple clean up internally?

 

I had the same problem on my volvo just recently. I went to a junkyard found a good used sender unit connector wire and replaced it. Now my speedometer works fine. The problem (with mine) was that the safety wire (it holds the connector & speedometer sensor together) had cut through the insulation on both of the wires (right at the top of the connector). Good luck.

 

Many thanks for your reply.
Was the problem similar on yours i.e the milometer and trip working ok but the speedo not?

 

Yes. I've had that combination of speedometer failure before I replaced that cable. But I only put up with the sporadic operation of my speedometer for less than one week before I fixed it. I'm not exactly sure what would happen if you let that go on for a few years.

Still it isn't that bad. Worst case scenario is you go to a local breaker yard, get another working speedometer(and a sender unit connector wire), come home, and rebuild your own speedometer (and replace your sender unit connector wire). After a couple of hours, your speedometer should be working fine once again. I've always have kept a spare speedometer on hand just for this reason. But I haven't needed to use it yet. Good luck.

 

Ok - many thanks for your reply - when I can get the back of the car off the ground I'll take a closer look!!

 

Still trying to work out where the prob lies. Today at mot / service got the mech to take a look at the speed sensor under the back of the car and he reckons it looks ok - no leaks and is well attached. I have already had the instrument cluster out and cleaned the terminals connecting the speedo.
Having trawled this website I see mention of "instrument cluster grounds in the right footwell" being a possible cause of a fluctuating speedo. Being thick, can anyone describe to me where this "grounds location" is exactly / how to uncover it in order that I can take a look.
I had an occasion a couple of years ago when the heating system sprung a leak and soaked the carpet in the driver's footwell. Although I had the repair and clean up done at a garage, I am now wondering whether this leak could have caused a bad grounding leading to my sporadic speedo / milometer?

 

Update: Have had the dash out again and this time extracted the speedo unit from the main cluster (having first cleaned terminals, screws and rerun all the soldered joints on the main cluster circuit board). The capacitors (?) that are hidden by the speedo unit don't show signs of any leakage.
What I am trying to determine is exactly how many of the wire connections to the main cluster refer to the speedo?
(None of the digit drums of either the trip or mileometer have any damage to the teeth and all move / rock freely).
Once reassembled, the speedo (once again) worked for about a minute before packing up (the trip and mileometer are still in one of their "frozen" periods).
In general, I am still left with the same occasional problem - the trip / mileo work normally sporadically and the speedo only for a short while at startup.
I would still appreciate if anyone can tell me where to look for the "grounding bar" as I am now wondering whther this is the problem area.

 

P.s - ref my query of how many leads go to the speedo - On the cluster circuit board there is a "2 prong" male brass connector (close to the speedo fixing screws) protruding from the circuit board (this is in fact U shaped and fitted to the speedo's own circuit board) - does it matter to which of these the female lead / plug is fitted - is there in fact a second female lead lost somewhere in the bowels of the dash that should be fitted to the second of the two prongs?
I notice elsewhere on this site in a photo regarding disassembly of the speedo for capacitor replacement purposes that one of these lugs is marked with a biro "x" which indicates to me that there seems to be a preference and a requirement for only 1 lead to be attached??

 

Hi all!
Me again. aka "PITA" !!
I was wondering if someone can pick up again on this thread as, with the new MOT rules, I am going to be up the proverbial creek owing to this speedo prob.
Update - the speedo needle has moved once (to the correct speed) in the past 3-4 months for about a mile. The mileometer however is more active and occasionally springs into life for a few hours, sometimes days, then goes back to sleep again for a few weeks!
On my RHD J reg 940s estate, the two wires I can see coming from the back axle into the car by the fuel filler pipe are brown and white. I understand from others posts that from here they go, via the small black plastic sealed box, internally to a connection / "grounding bar" behind the R/H driver's footwell side panel. From here they go up to the speedo.
Is it possible that there is corrosion at this "grounding bar" point due to a heater leak of a few years ago, or is the connection at this point sealed and not likely to be a possible cause of the fluctuating speedo / mileometer?
Also, having no way to get the car off the ground at the mo, would it be possible to check the sensor by simply tapping into the brown and white wires by the "small black box" and checking for any voltage by applying a meter at that point?
If so, what settings should I use on the meter?
Sincere apologies again for my mechanical ignorance!!

P.s I see elsewhere that there is mention of ABS being connected to this speedo sensor - dunno whether it applies to my model but... the ABS light does not come on unannounced and the only time ABS came into operation, it worked and then reset itself.

 

Most likely the cause is the capacitors--they don't have be visibly leaking to be faulty. The symptoms you describe are textbook for bad capacitors. I have had many cars with this problem and have three (two '91 940s and a 92 740) right now with the same issues. 1991s and 92s Yazaki Meters are especially prone. I don't think, here in the US at least, we got any VDO's, at least I haven't seen any VDO speedos since late '80's models. The problem with diagnosing this correctly is that the capacitors act up intermittently based on conditions such as temperature, moisture, etc. and the various combinations make for various scenarios. The problem is inside the speedometers themselves, and since they all work sometimes, the issue is not a broken gears or faulty wires, etc. The biggest mystery to me is how the capacitors interrelate, i.e. which does what in effort to pinpoint the bad one, but if one goes through the trouble of doing one, then may be all should be renewed...

 

Many thanks for your reply. Have taken note of your comments. I have had a go at the much mentioned idea of connecting a couple of wires to the sensor cable - in this instance from the plug connector inside the tamperproof box in the back of the car, rather than under the car as I have no way of getting the back lifted at the mo, but can raise no sign of a pulse on the speedo needle by shorting the 2 wires together. Added to the fact that I believe on a 1992 940S the rear ABS is governed by the rear speedo sensor and my ABS seems to be behaving itself, it's all starting to point towards the speedo itself. I'm still not sure what to find re the "grounding bar" elsewhere on this site mentioned by the (Uk RHD) driver's right foot kick panel - any description of this or photos would be a real help as I am not sure what corrosion a previous heater leak could have caused in this area when the carpet got soaked?

 

Update: Car managed to get through the MOT - think they don't worry as long as the mileage has changed!
As per past history, it's always a case of: if the mileometer works, the speedo doesn't, and, after a spell of a few months where the mileometer worked (albeit sporadically), we are now in a "speedo" phase where the speedo works most of the time. For the past 2 weeks it has worked on every journey with only one occasion where after the first few miles, the needle dropped into slumber mode for the rest of the trip. However, as indicated, the mileometer is now consequently in its hibernation period again!
My first step will be to replace the sensor end of the connection which should at least cut by a third the probable location of this intermittent failure.

 

The problem is the speedometer itself, more exactly the CAPACITORS. They fail intermittently and in a various combinations which results in the erratic behavior you describe. You can dance around the issue all you want avoiding the facts but that won't change anything. '92 models are particularly prone to this failure.

 

my 92 740 (virtually the same as the 92 940) had the bad caps, I replaced all the caps on the speed, quite carefully, and its WORSE than it was before ((

apparently you can't replace a 91/92 with a 93+ speedo unless you A) changet he whole instrument panel and B) change the fuel level sender in the gas tank :-o

 

No "virtually", it is the same as a 940; but replacing the capacitors is a fiddly work and any minute imperfection in the process may result in the speedo continuing to have problems, (or be worse)...

A lot of these DIY "fixes" we have for cars unfortunately sometimes do not arrive at the desired goal...

 

We (an engineering friend and I) did an extremely 'good' job of the cap repair, carefully and thoroughly cleaning the PCBA and around the chips after removing the caps, we used proper 60-40 rosin lead-tin real solder (hate the modern lead-free solders), etc. I used all made-in-japan rubycon and nichicon caps, from digikey.com with the same or better voltage and temp specs, we checked the capacitance of the new caps with a fluke before installing, etc etc.

 

Well firstly gents - many thanks for your responses (although at my age I fear I am a little too old for any kind of dancing).
Perhaps I should have been a little more specific in my post - the reason I mentioned replacing the sensor is simply because although I have no current way to get right under the car, I noticed that the 2 wires coming from the outer plastic / rubber tubing and into the top of the sensor had both completely lost their insulation over the years and were bare. I have temporarily separated them. Not being to well read on the ins and outs of a cars workings, but trying to be logical, in my mind there are 3 main areas, all of which I need to either check, or replace. Firstly the sensor and its wires leading into the back of the car, then the wires running from the connection box there to the front of the car, and lastly onwards to the speedo itself. Once the first part has been replaced I figured it would be 1 down, 2 to go.

 

oh, I noticed you asked about the ground earlier. its just forward of the door hinges, one on each side. remove the kick panel under the dash, then remove the plastic panel thats just forward of the door, and voila, you should see the two main cabin grounds. most everything is grounded on one side, the speedo and ABS and cruise control are on the other side (probably to keep them as far from noise sources as possible). The ECU is also there on the right side. On a 7/9, a lot of stuff is grounded to a rail thats in the fuse/relay panel, it in turn is grounded to one of these two side grounds.