VDO Head Overhaul

by Dave Jones (published in the Isetta Gazette January 1982)

Has your VDO speedo seized? Then read on.

Signs that the VDO speedo needs overhauling are things like an indicated speed of 30mph as you park neatly between two cars, or an indication that your speed is oscilaating between 30mph and 50mph when you know it isn't. The VDO doesn't seem to wear out, but it does use a special grease that self destructs after 30,000 miles.

Tools you will require:

1. A medium sized screwdriver with fairly sharp point.

2. A small pin hammer.

3. A punch about 1/4" diameter with a flat end. (A 6" nail with its point filed off does fine.)

4. A pair od good, small electricians pliers.

5. A spanner to fit the nut on the back of the speeds.

6. To test afterwards, a 900rpm drill and a short length of flexi drive. Use about 3" of brake cable off of a bicycle, with the nipple filed square.

7. For the overhaul itself; lots of kitchen towel paper, some thin (3 in 1) oil and some 'LM' lithium based grease.

To open the speedo.

A. Loosen the nut on the rear of the instrument.

B. Place face down on a clean surface and, using the scredriver, gently pry the spun over flange holding the glass in place away from the speedo body. Only pry the flange enough to remove the glass and rim to be removed.

C. Remove nut from rear of instrument and push the threaded bush into the speedo body. The works will now fall out of the front, hopefully into your waiting hand. Do try to keep your filthy hands off of the scale pan.

D. Looking at the back of the instrument now that it is out of its case you will see two screws, and at the side you will see another holding the odometer lay shaft in place. Remove the odometer lay shaft and undo the other two screws. The instrument can now be carefully seperated into two halves. The scale end with the aluminium pointer drive cup should be placed on a clean surface for later attention.

E. Using the little hammer gently tap the end of the worm gear sticking out of the side of the drive assembly, cupping the side of the drive assembly in your hand so that the 1/4" ball coming out of the assembly does not fall onto the floor (ours did, and it wasn't fun!). The ball and the worm gear assembly may now be removed.

F. Remove the iron ring from the drive assembly housing and slide out the drive and magnet assembly. Note the position of the flat ring and 'C' washer under the magnet ring.

G. Clean the housing, the shaft and worm of the magnet assembly, the worm gear assembly and the odometer lay shaft with kitchen towel paper and regrease generously with LM grease.

H. Slide the magnet assembly back into place noting the 'C' washer must sit in its recess under the flat ring or you won't be able to re-assemble the rest of the works.

I. Grease the worm gear assembly and pop it back into place. Put the ball back into place and tap it down gently with the punch and hammer until the worm gear assembly has only slight axial play.

J. Place the iron ring around the magnet and put 1 drop (no more) of thin oil in thr little hole in the centre of the magnet assembly where the pointer shaft pivots.

K. Place one drop of thin oil on the bearing just behind the pointer and gently jiggle the aluminium cup to work the oil into the bearing.

L. align the two halves of the instrument so that the odometer drive gear and the worm are on the same side and very carefully seat the two halves together.

M. Insert the two securing screws and do them up carefully, checking that the pointer moves freely as you do so.

N. grease the odometer lay shaft and replace it with its one securing screw.

Testing before final assembly:

Hold the instrument by the threaded shaft and rotate the drive with the 900rpm electric drill and flexi drive. (You have to use the flexi drive otherwise the speedo rattles around so much you can't see if it's working or not. At 900rpm the speedo should read 32mph. Do a couple of miles like this to spread the grease around and to check that all the bits are working correctly. The instrument is now ready for final re-assembly.

Final Assembly:

1. Place the assembly into the case, aligning the lamp hole with the 30mph mark. Check that the hexagonal base in the screwed section fits snugly into the recess in the case. Screw on the nut and tighten.

2. Clean the glass carefully. It's a pig to see that fingerprint on the inside of the glass face after you've put it all back together. Place the bezel into the face ring.

3. Place the assembly into the face ring, face down on the bench, and firmly press downwards while you push the flange back into place with the screwdriver at about 1/4" intervals around the periphery of the speedo. This will secure the glass, but loos tatty.

4. To neaten the spinover, crimp around the edges carefully with the electricians pliers. The crimp will be hidden when the speedo is returned to the facia. Check that you haven't crimped too tightly and pushed the glass circle out of the ring.

Happy bubbling, and here's to the next 30,000 miles!