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MG MGB Technical - Speedometer Cleaning

I may have inquired about some of this before, so please forgive - but I still haven't figured this one out.

It has been suggested that a possible reason the speedo in my 72B reads so high, is related to my lubing my cable too far up.

The speedo worked fine up until the cable snapped down by the transmission. I replaced cable and speedo worked fine. Then it was suggested to lube the cable better to avoid it drying out in the future. So out came the speedo again, I pulled inner cable out of the housing, and greased up the inner cable end to end, and re-installed. Soon after the malfunction started.

The needle does return to 0 at stop, but bounces up and down around the actual speed in the <30 mph range about +/- 5 mph. Once up around 40+ mph it reads a good 10 mph higher than actual, and is not a steady needle.

So, I've taken inner cable back out, wiped it off down 10" or more and re-installed, but this hasn't changed anything.

Someone suggested that the excess grease from earlier may have worked to inside of speedo and now the inside is too greasy/dirty/messy to read accurately.

Does this make sense to anyone?
Does this sound like something that can be easily cleaned?

If cleanable, just what am I trying to clean and what cleaners/solvents, etc., are safe or not safe to use?

Thanks.
R.W Anderson

Hi,

Somehow I don't think that over greasing the cable could influence the speedo to read high. For the grease to get to the parts to do this would require a lot of grease in the speedo, as the cup and magnet assembly are positioned such that any grease finding its way into the works would be slung away by the cup, which is driven by the cable.
To read high there must be some extra interaction between the cup and the magnet, or the return spring is faulty.

Below is a link to speedo repair, which also gives a good description on how the speedo works.

home.comcast.net/~rhodes/speedo.pdf

Herb
Herb Adler

Hi again,

What I wrote above is wrong. I did it from memory. Having quoted the link to speedo repair I read it again and found that the cup and magnet are actually reversed to how I thought they were. With the correct alignment it could be possible that a bit of grease could get into the works and cause your troubles.
Anyway the article is very good.

Herb
Herb Adler

A speedometer cable breaks only when it is prevented from turning. Usually this is caused by a problem inside the speedometer. To see if this is the case, turn the speedometer by hand using the end of an old cable. Or I guess you could use the inner part of your current cable. If the speedometer needs attention, it will be apparent when you try to turn it by hand. A good speedometer will turn smoothly and effortlessly with no binding. Any resistance, especially an intermittent resistance, means the speedometer will need to be disassembled, cleaned and reassembled with fresh lubricant.

If you don't want to disassemble the works, don't bother trying to clean or lube. You really must take the works apart to get them clean. If you are mechanically inclined, speedometer work may be within your grasp. Practice speedometers are not terribly expensive on Ebay if you want to establish your skill level ahead of time. The attached photo may give an idea of what you're getting into.

Cleaning can be done with lint-free cloths or paper towels and WD-40. Disassembly takes patience, magnifying glasses and small screwdrivers. It's helpful to photograph the works from every angle before starting. You probably don't need to take apart the numbered barrels, although you will need to remove their assemblies to get at the main bearing. Test the main bearing before assembly and remove any high spots with 600-grit sandpaper. Remove all traces of grit after sanding. Use a light grease such as Lubriplate on all bearing surfaces.

Until you decide what to do, I'd recommend leaving the speedometer cable unfastened from the speedometer. The other piece that may break is the right-angle drive connected to the transmission. I know from recent experience that they cost about $60 these days. -G.

Glenn G

I'd have said it broken strands from continual flexing back and fore digging in and finally breaking all of them was far more likely, otherwise the majority of cable replacements would be accompanied by a speedo rebuild.
Paul Hunt 2010

Paul, in 100% of my experience, cables were broken because of problems in the speedometers. Granted, my experience of broken cables is limited to two speedometers on different cars, but 100% is a clear majority of accompanying speedo rebuilds. Anyway, it is easy to test for speedometer problems. -G.
Glenn G

All of this is good advise, but I need to re-emphasize that the speedo was working fine, and VERY accurate - no bounce, no shaky needle, nothing, just nice and steady, right up until cable broke, about 1' or so from near transmission end.

I replaced the housing and the inner cable.

Upon install all worked fine again.

But then I had to go and open it all back up again, as it was suggested I needed to better lube the inner cable. I used synthetic grease in the palm of my hand and ran it up and down the length of the cable. Reinserted inner cable into housing and reconnected cable to speedo.

Since then the speedo bounces wildly, and at 50+ mph it reads 10 mph high, and relatively high lower and higher speeds.

I've had cable out since then several times to double check the connections, bends in cable, etc.

So speedo does spin freely when out of car. It "appears" fine, but I cannot explain why it now bounces and reads high.

I will add, and this may be critical. At one point, after all the numerous cable disconnects, I could hear and feel a clicking/thumping in the speedo. If you touched the glass face you could feel what you could hear. I eventually found 1 strand of wire on speedo end of cable that was not as tightly bound as the rest, I carefully removed this strand down to the plastic sleeve at the speedo tip of the inner cable.

No more clicking, no more feel of clicking in speedo. It just keeps reading 10 mph high and it bounces.

I'm about ready to simply ship it off for cleaning and service, but the price for service appears to excede the price of simply buying a new one. I'd buy another from ebay, but that is always a gamble. I'm on my 3rd or 4th speedo (original didn't work at all, bought wrong gear size once, etc.). This one worked so well, up until cable broke and worked fine after replacing cable, but only for a few hundred miles.

I have NO right angle drives. Cable screws straight into back of speedo, and screws straight into side of transmission. Yes, after speedo - cable runs straight out from speedo through firewall and ever so gently turns along firewall to run down along right side of car to transmission.

I think it is the "it worked fine after new cable install, but didn't later" that is the frustrating part. Buying a new one would be simple, but I'm stuck on trying to figure it all out first.

Thanks for your patience.

BobA

R.W Anderson

After fighting with 3 speedometers, with mixed results, I finally sent mine off to be cleaned and recalibrated. I even had one that was NOS with only 7 miles on it, but time takes its toll on everything. Entropy. RAY
rjm RAY

One of my speedometers broke two cables before I found the problem. I had no idea why the first cable broke, so I simply installed a new cable. When that one broke, I took apart the speedometer and discovered a tiny spring inside was broken. The function of that spring was to keep a lever from fouling the works when the car was reversing. The speedometer worked fine, but the cable couldn't survive being held still by the speedometer while the transmission end was turning in reverse.

While I still think the speedometer is the likely culprit, there is another reason why a new cable might have a strand far enough out of place to cause problems. Possibly the inner cable is too long, and it is binding when tightened at both ends. Or you may be missing a washer or distance piece that helps establish the correct clearances for the cable ends. You might check a parts diagram from a Victoria British or Moss catalog and see if you are missing any parts. -G.
Glenn G

Bob, I just re-read your post and note that you replaced the inner cable. Did you not replace the inner and outer cable at once? Cables are usually sold as a unit, and they should not be mixed, as the end clearances may vary enough to cause problems. -G.
Glenn G

Housing and inner cable were both replaced new, from a local repair shop that stocks Moss parts.

Comparing original housing and cable, it was noted that the new set was a touch (1-2") shorter than original. Measuring new set assured me it was the proper length for OD for my car as listed in Moss catalog.

My discussion above reflects that I repeatedly removed the new inner cable from the new outer housing.

I've not removed the new housing since original replacement was done.

So I'm stuck with finding a shop to repair my speedo I think.
R.W Anderson

"The function of that spring was to keep a lever from fouling the works when the car was reversing"

Explain? The eccentric cam turns backwards when the car reverses, but the effect of that is still to cause a back and fore motion of the odometer ratchet so the odometer still counts forwards (unlike Christine). Haven't seen anything like you describe in my speedo.
Paul Hunt 2010

Despite what their catalogue says, Moss only seem to supply the longer 60 inch cable ... or at least the one they sent me was 60 inch despite the part number referring to the 57 inch one.

I actually have/had the same problem as Bob ... I had the speedo repaired and it was fine ... then the cable broke ... I replaced the cable and the speedo became jumpy. It is worse at some speeds than others ... the low point of the oscillation is actually close to the true speed - it's as if the something is kicking the needle high.
Geoff Everitt

If the needle is kicking high that implies the spinning disc on the end of the cable is catching on the fixed disc attached to the needle. If the inner is jumping I'd expect that to average out. You may be able to verify this by putting a drill (in reverse) on the back of the speedo.
Paul Hunt 2010

I would check the inner cable for length at the speedo end. Try installing a small fiber washer to hold the cable a bit farther out from the speedometer. Possible the inner cable is pushing hard on the speedometer shaft and making it hit the drum.
Sandy
SANDY SANDERS

Thanks for that ideas Sandy ... I'll try something along those lines in the next few days.
Geoff Everitt

Paul - I haven't seen that spring in an MG speedometer, either. I was speaking of a Smiths speedometer from an early Volvo P1800. I would like to explain further, but that was a long time ago, and the speedometer is very difficult to open. The point is that things inside speedometers can break cables.

Given the circumstances in Bob's case, I'm more inclined to think that the cable is binding. -G.
Glenn G

It's over 30 years ago I had this problem. I changed the cable and had the speedo rebuilt, but it was the 90 degree drive from the gearbox.
c cummins

I said-

"Thanks for that idea Sandy ... I'll try something along those lines in the next few days."

I got some washers with the right hole size and trimmed off the outside to make them fit, so I have effectively made the cable some 2-3mm shorter. I don't think anything has changed. When my SatNav shows 72mph, the speedo oscillates from 70-78 mph ... pretty much as I stated above *the low point of the oscillation is actually close to the true speed*. I can live with that.
Geoff Everitt

This thread was discussed between 16/04/2010 and 25/04/2010

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