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Ammeter repairs

Author: Bob Humphrey

Following on for your editor's plea for material, here are my ideas at repairing a Lucas Ammeter as fitted to my 1932 RN saloon. I do not profess to be an expert or that this method is the best and only way to get inside. I could not find a replacement and couldn't face a repair by someone else, so what follows is how I repaired the meter.

For me, the ammeter is an essential instrument showing the state of charge going into the battery via the dynamo, or discharge via the lights etc when the dynamo can't keep up. The problem with my ammeter was that it was stuck at the 10 amp discharge mark all the time.

Before starting work, disconnect the battery from the car, then remove the instrument cluster by unscrewing the 2 x 0BA screws. Locate the 2 cables that are attached to the two terminal studs either side of the ammeter. (ignore the centre terminal stud). Before removing the wires, mark each cable (left and Right) with a bit of tape. Unscrew the terminal screws completely and remove the wires.

Ammeter repairsWith a small flat screwdriver, undo the countersunk screws that are inside the terminal studs and remove along with the stud itself. Get ready to catch the ammeter as it falls out (or not). Tease out the ammeter from the panel if it is still in place.

Now work can begin. Access to the insides is a little difficult and for the brave hearted. The ammeter is made up of two cases (front and back) which are soldered together but fortunately just a couple of blobs. The proper way is to unsolder them, but you will need a very small gas blow lamp and a solder sucker but you can 'carve' most of the solder away with a sharp Stanley knife. Once this has been done, 'break' the remaining solder by gently prising the joint apart with a sharp small screwdriver – gently does it.

Ammeter repairsNow you have the solder joints broken, grip the terminals in the vice and with a large screwdriver, gently prise the two cases apart. Take great care that each side is prised up evenly so as not to damage the case. As the case comes apart, be ready to catch the glass and the leather seal between the glass and the bezel.

Ammeter repairsNow you have the front case off, recover the glass and leather seal which may be stuck to either the glass or bezel or both. At this stage leave the bezel in place. Clean the glass in preparation for putting it all back together again.

Now we need to remove the bezel itself. It is kept in place by three or four little brass tongues which are part of the main case. These are very brittle due to their age and may snap off, but that is not a disaster. Gently bend the two tongues out at the bottom of the meter (leaving the top two untouched). Looking at the photo, you can see one of the tongues sticking out (at the top of my finger nail). Gently lift the bottom edge of the bezel up using a sharp knife. Now slide the bezel away from the case gently easing the pointer through its slot.

Ammeter repairsThis is what the inside looks like. All the electrical current used or made by the car (except the starter motor) passes through the black coiled copper wire attached to the two terminal studs. This in turn generates a magnetic field and depending upon the direction of the current, the pointer either swings to the right or left. Generally speaking, the only things that can go wrong is that the coil is burnt and shorting out or that the pointer has come out of its bearings. A burnt coil is a bit terminal (pardon the pun), but you may be able to save it by prising the coils apart and then painting a little epoxy resin onto the coils to re-insulate them. If like mine, the pointer didn't move, its a gimbal problem. The gimbals are made up of two parts, a fixed part which is just under the pointer, and an adjusting screw on the back of the case. Use a pin to clean around the screw and put some WD40 (other products are available). Let it soak in for 15 mins or so.

Ammeter repairsWith a small screwdriver, see if you can move the screw a quarter of a turn or so. Some ammeters have had the screw soldered in, so there is not much chance of loosening the screw!

Depending upon your luck, tighten this adjusting screw ½ of a turn and spring the pointer gimbal back into place, further tighten the screw if you think it is necessary. If the screw was soldered in place, move the pointer out of its normal position and with a pair of long nosed pliers, tweak the fixed part of the gimbal, closing the gap between them. "Spring" the pointer back into its gimbal and see if it is free to move across the full scale from -20 -0 - +20 amps. (20 amp charge, that would be nice!)

Re-assembly is much the same as disassembly, perhaps replace the thin strip of black leather between the glass and bezel. (Pinch one of your wife's black 'evening gloves' ?? as a source of material)

Before you finally solder the case back together, test it out in the car or on the bench to see the needle swing in both directions. If you are going to solder the case back together, only use the smallest dab of solder. You may prefer to use a spot of you favourite epoxy glue instead but remember, it has to fit back into the hole in the dashboard.

Good luck!

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