Erratic speedo

[Michael Aldred]

Hello fellow 7ers,

This is my first post, though I have been reading the forum since June last year. I bought a 1937 Austin 7 AAL Open Road Tourer in May 2019 which had been in storage in South Australia for a few years. I had it shipped to Melbourne and got my mechanic friend Barry to look her (Tinkerbelle) over. On his advice I replaced the kingpins and track rod ends and brake cables. A few other things needed to be fixed such as the indicators and fabricating a new drive shaft for the windscreen wiper. I obtained a roadworthy certificate in January and the car is now registered in Victoria. This week Barry and I checked the tracking and found that it was toeing out by about half an inch. That could account for the twitching of the steering which is now much improved since we corrected this. Now I have turned my attention to the speedometer which is very erratic. If I drive at a steady speed of, say, 30mph the needle flicks up to 45 to 50mph every few seconds. Using a sat nav the 30mph is probably the correct speed. I have disconnected the speedo cable and cleaned it. When I reconnect it to the speedo gauge and put a battery dril onto the gearbox end the same error occurs so I assume that the problem is in the gauge rather than the gearbox or cable. I would be grateful for any suggestions as to the nature of the problem and how to fix it.

Michael

[Chris KC]

Whenever I have had a flickering speedo reading in the past, replacing the cable has fixed it. Are you able to try another cable - borrow one from a friend perhaps?

[daveg]

I had this problem and found that it was caused by the oddometer part of the speedo.  If you take the back off the speedo
you should see a ratchet arm that acts on a cog on the end of the oddometer dials. As a test hook the ratchet arm away from the 
cog and see if the problem is solved.

[Derek Sheldon]

What’s a speedo ?. No seriously mine has never been smooth, much as you describe. I have a bicycle speedo fitted to two of my cars for accuracy. It has the added bonus that it can be set to Kilometres when I go abroad

[Bob Culver]

Does the speedo turn easily by hand? i have found after long standing the diecast swells and seriously stiffens the drive. Causes the cable to wind up and flick. Curable with care.

[Tony Press]

Hi Michael,

When this annoyance passes you must join the Melbourne Austin 7 Club.

http://www.austin7club.org/

We have a brilliant retired instrument maker who will fix it in a jiffy.

Keep well.

Tony.

[Michael Aldred]

Thank you all for your responses. 

I think I will try a new speedometer cable anyway as the existing one did have a bit of a kink where it passes through into the passenger compartment. 

As the instrument panel is now pulled out I will follow Dave’s suggestion and look at the odometer mechanism. I did find the following website http://www.magnetic-speedometer-repair.c...ometer.php which gave me some confidence is opening up the mechanism, but I am much more attracted to Tony’s idea of letting an expert look at it. 

I have been a member of the Melbourne club since July of last year and have used the excellent spares department extensively in my repairs so far, but because of family matters I have had to return to the UK twice in the past six months for a month at a time which has hampered getting the car to the stage it is now at. 

I will report back in due course. 

Michael

[Jamie]

Michael,

If it helps to reassure you, I have just resurrected a seized magnetic speedometer using pretty much the same guide as the one which you have found. I took my time and, working slowly and methodically, successfully cleaned and repaired a seized device. The hardest part was refitting the return spring on the oddometer. In the end, I used a piece of cotton to drop the spring into place and to then extend it to hook it over the securing tang.

Regards,

Jamie.

[Michael Aldred]

Thank you to daveg for his advice. Disengaging the ratchet arm which engages on the cog at the end of the odometer wheels does indeed resolve the erratic movement. So my next question is what do I do about it? I assume the ratchet arm is there for a purpose and needs to be applied to the cog.

Michael

[Jamie]

If the arm is the same as on my speedometer, it advances the odometer. It is operated by an eccentric attached to the gears which rotate when the speedometer is spinning. A photograph would make this clearer if you have one.

Regards Jamie