tim hunt Posted July 25, 2020 Report Share Posted July 25, 2020 Something tells me I should have listened to this wise advice. The ammeter in my 4A was never correctly zeroed and always showed a charge of some 2 or 3 amps when on open circuit. I learned recently that a small screw on the rear of the instrument, 'sealed' with a blueish varnish would allow the zero the be altered. Yesterday I fitted a replacement column light switch and whilst I had the speedo and tacho out anyway I thought I would remove the ammeter as well and have go at resetting the needle. I fiddled with it for a while and eventually achieved a very pleasing result, as in the attached. I put a small drop of Araldite on the head of the screw to hold it in place - in retrospect was that where I went wrong? I reassembled the dashboard and everything checked out fine with the exception of the ammeter, which refused to move. I checked the connections to starter solenoid, control box and overdrive relay and all showed continuity. I then removed the ammeter and found a satisfactorily very low resistance across the terminals. I then connected it in series with a 21watt bulb and connected the circuit to a small 12volt battery, the ammeter needle didn't move, nor did it when I reversed the connections. The needle doesn't appear to be stuck in place since if I tap sharply on the case it will move a little either way. Before, very embarrassed, I send the instrument away for expert attention can anyone with previous experience give advice as to what the problem might be and tell me how to dismantle it and check for the fault? I couldn't find any specific guidance on the Internet. Tim Quote Link to post Share on other sites
tthomson Posted July 25, 2020 Report Share Posted July 25, 2020 Hi Tim, The rear of the ammeter has some tabs that hold the bezel in place. These may need loosening (but not fully). Then the bezel can be rotated to a position where it pulls away from the rest of the instrument. Note that there should be a rubber seal between the glass and the bezel and this may prevent it turning easily, and will probably need to be replaced on re-assembly. Once the bezel is removed, it should be relatively easy to figure out how to remove the rear shroud. Most importantly - Make sure you make notes of everything you do and where things are located. It sounds like the Araldite has got onto the pivot making it resistant to turning. cleaning it up might prove difficult, and it may be better to ask if anyone has a spare ammeter that they could be persuaded to part with. Good luck TT Quote Link to post Share on other sites
Mick Forey Posted July 25, 2020 Report Share Posted July 25, 2020 Tim, the small screw on the back adjusts the end float on the bearing, not the zero. It is easy to remove the ammeter, strip it on a bench, clean off all the Araldite, adjust the small screw to get the needle to move freely without play, then carefully bend the needle to set the zero, reassemble and install. It is a very simple gauge to sort out. Mick Quote Link to post Share on other sites
stillp Posted July 25, 2020 Report Share Posted July 25, 2020 TRy putting a bit more current through it Tim, a headlamp bulb or 2 x 21W flashers in parallel. Pete Quote Link to post Share on other sites
peejay4A Posted July 25, 2020 Report Share Posted July 25, 2020 When I had an ammeter with a permanent offset I zeroed it by sticking a small neodymium magnet to the back of the case and moving it about to zero the pointer. It probably messed up the linearity of the deflection but that hardly matters and no need to dismantle. Quote Link to post Share on other sites
RogerH Posted July 25, 2020 Report Share Posted July 25, 2020 14 minutes ago, peejay4A said: When I had an ammeter with a permanent offset I zeroed it by sticking a small neodymium magnet to the back of the case and moving it about to zero the pointer. It probably messed up the linearity of the deflection but that hardly matters and no need to dismantle. Like they do on ships with binnacles (I think) Roger Quote Link to post Share on other sites
peejay4A Posted July 25, 2020 Report Share Posted July 25, 2020 I believe so. To correct for surrounding ironwork. Those car compasses had compensating magnets too. Quote Link to post Share on other sites
tim hunt Posted July 25, 2020 Author Report Share Posted July 25, 2020 Thanks to all for the timely suggestions. It looks like a fun morning in the garage is beckoning! Tim Quote Link to post Share on other sites
tim hunt Posted July 27, 2020 Author Report Share Posted July 27, 2020 Pleased to report that after carefully scraping away the cured Araldite with a small screwdriver I freed the needle and got the instrument working again. The bezel came off quite easily and the rubber seal was undamaged and reusable. I dismantled the gauge and gingerly bent the needle a bit. Its zero is better than it was but not quite perfect. I am now going to leave well alone having learned my lesson. After all, I put up with it as it was for nearly 50 years and since when driving one's viewing position is not directly head on allowance has to be made for parallax anyway. The actual zero position would only ever concern a concours judge and my car will never be subject to such scrutiny. Tim Quote Link to post Share on other sites
Lebro Posted July 27, 2020 Report Share Posted July 27, 2020 Good result. Bob. Quote Link to post Share on other sites
stillp Posted July 27, 2020 Report Share Posted July 27, 2020 +1. Thanks for letting us know. Pete Quote Link to post Share on other sites
tthomson Posted July 27, 2020 Report Share Posted July 27, 2020 +1. Good that you were able to fix it. Thanks for keeping us informed. TT Quote Link to post Share on other sites
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