Helen B Posted December 11, 2023 Share Posted December 11, 2023 Hi folks, I wonder if you can help... We have a 1986 Bedford MJ, which is building air pressure fine and easily reaches 7-8. The brakes appear to be working fine (albeit only driving around the yard), however the warning buzzer and light won't go off. We've tried various things in an attempt to fix it, including buying and fitting new switches, which don't make any difference. There's no water or rust in the tanks. This has been going on for ages now and becoming very frustrating! Has anyone any ideas what it could possibly be? Many thanks in advance Helen Quote Link to comment Share on other sites More sharing options...
john1950 Posted December 11, 2023 Share Posted December 11, 2023 Is It filling all of the tanks? Quote Link to comment Share on other sites More sharing options...
Helen B Posted December 12, 2023 Author Share Posted December 12, 2023 Not sure? How would you know? Quote Link to comment Share on other sites More sharing options...
john1950 Posted December 12, 2023 Share Posted December 12, 2023 Find the tank drains and see if any air comes out. Quote Link to comment Share on other sites More sharing options...
Helen B Posted December 12, 2023 Author Share Posted December 12, 2023 We've drained the tanks empty and filled them up. Undone the drain valves and checked there's no water or crap in the tank. There's no obvious rust etc Quote Link to comment Share on other sites More sharing options...
john1950 Posted December 12, 2023 Share Posted December 12, 2023 (edited) Have you done a pressure test on the tanks to check what actual pressure is in each one. Just a point when working with air at pressure it only takes 5 psi to breach your skin and cause an air bubble in the blood stream which can be fatal. So be careful. Always wear gloves. Have you tried lifting the air a small amount? Edited December 12, 2023 by john1950 Quote Link to comment Share on other sites More sharing options...
Mk3iain Posted December 12, 2023 Share Posted December 12, 2023 Hi, Do the brake lights come on when operated ? Be sure you have changed the pressure switches for the low pressure warning and not the brake light. (daft question but you never know). With the system pressure at zero remove the switch or switches and run the engine to purge any crap that is near the switches blocking their use. If possible a direct reading pressure guage fitted at the switch union will confirm the switch is seeing the correct pressure. If all is ok, maybe looking at a wiring problem, do you have the wiring diagrams ? Quote Link to comment Share on other sites More sharing options...
Helen B Posted December 12, 2023 Author Share Posted December 12, 2023 Hi John Yes, I think Phil has already done that, but I'll check. Sure he's lifted it to 8 or 9 and it sill happens. Really stuck on this one! Quote Link to comment Share on other sites More sharing options...
Helen B Posted December 12, 2023 Author Share Posted December 12, 2023 Hi Iain Yes, the brake lights work and it is the low pressure switches that we've bought new and changed. Phil's checked for water, rust, muck etc and they're fine and have emptied the tanks. Phil has looked at the wiring and there's a loose wire. He's not here now, so would need him to confirm. i Know there doesn't seem anything obvious. Been on this for weeks now - its a real mystery! . Quote Link to comment Share on other sites More sharing options...
john1950 Posted December 12, 2023 Share Posted December 12, 2023 This is beginning to look like an electrical fault, a cable has gone down to earth, Quote Link to comment Share on other sites More sharing options...
Helen B Posted December 13, 2023 Author Share Posted December 13, 2023 Thanks John, we think you're right, as it doesn't seem to be anything else! Quote Link to comment Share on other sites More sharing options...
john1950 Posted December 13, 2023 Share Posted December 13, 2023 If you take the wires off the switches one at a time do you still have the problem? Quote Link to comment Share on other sites More sharing options...
Helen B Posted December 13, 2023 Author Share Posted December 13, 2023 Yes. We’ve tried every combination, including one old one & a new one, swapping them about, one connected, then the other etc. There is one switch which doesn’t produce the buzzer or the light and another combination on the other that puts both on and ring switch off! There’s a couple of loose wires in the dash that we’re not sure what they’re for. It’s not helping as some bright spark has painted all the pipes & wires green! Phils having a go today hopefully on some of your suggestions. thank you 😊 Quote Link to comment Share on other sites More sharing options...
john1950 Posted December 13, 2023 Share Posted December 13, 2023 Has yours got the small air tank on the N/S front chassis? If so does it have a pressure switch? Quote Link to comment Share on other sites More sharing options...
john1950 Posted December 13, 2023 Share Posted December 13, 2023 If you disconnect one switch and the buzzer and the light go out, I would reason that either that tank is not getting up to pressure to open the switch or the switch is faulty. You need to do a check on the tanks to see what pressure they actually achieve. Quote Link to comment Share on other sites More sharing options...
Helen B Posted December 13, 2023 Author Share Posted December 13, 2023 I believe if you take the 2 wires off the switches the buzzer and light goes off, which made us think we needed new switches, but the same happens with them. It has got the small tank you mentioned with an automatic drain valve which Phil has replaced as the seal had gone on that. As I say we're running out of ideas. Unfortunately we don't have any gauges / fittings to connect in to the tank and Phil's not had chance to take a look to see if he can work something up. Thanks for your helps so far 🙂 Quote Link to comment Share on other sites More sharing options...
Sam White Posted December 16, 2023 Share Posted December 16, 2023 Likelihood if all the switches are good and the gauges are showing the correct pressure you have a wiring issue. The 'healthy' signal is not getting back to the dash. It's a case of getting out a multimeter and perhaps an extra bit of wire to help with the long loom run back to the cab to identify the culprit. Also by identifying the 2 contacts that are not talking, yiu can pull a new cable point to point rather than trace an old loom for damage, likely causing more problems to fix! Quote Link to comment Share on other sites More sharing options...
Helen B Posted December 18, 2023 Author Share Posted December 18, 2023 Thanks Sam. That’s helpful. We’ve definitely concluded that it’s the wiring! Quote Link to comment Share on other sites More sharing options...
Mk3iain Posted December 18, 2023 Share Posted December 18, 2023 If there is no obvious visible damage to the wires I would focus on junction boxes and places where there are connections. It just takes a whisker of wire to cause a short or a loose connection to give an open circuit. Quote Link to comment Share on other sites More sharing options...
Helen B Posted December 18, 2023 Author Share Posted December 18, 2023 Yes, thank you. Will have to check everything over 👍 Quote Link to comment Share on other sites More sharing options...
Mk3iain Posted December 18, 2023 Share Posted December 18, 2023 I had the same problem with my Militant a few years ago and it took me ages to sort it. I went over the whole system time and again. Then one day I checked the back of the buzzer unit after removing it and just noticed a stray wire thread that occasionally touched the metal of the cab. Nuts! I think some parts of the system are the same or similar (it was built in 1970) but the buzzer unit is an old type. Still it is so easy to miss and so annoying. Quote Link to comment Share on other sites More sharing options...
Helen B Posted December 19, 2023 Author Share Posted December 19, 2023 It is nuts! We’ve been at this for weeks now on and off. Sure Phil has done this, but will mention it. Thanks again Quote Link to comment Share on other sites More sharing options...
Richard Farrant Posted December 19, 2023 Share Posted December 19, 2023 Has the pressure switch been removed and an air guage fitted to ascertain if this is an air problem, and not an electrical fault? Quote Link to comment Share on other sites More sharing options...
john1950 Posted December 19, 2023 Share Posted December 19, 2023 When you are building the air pressure up, the gauge should start to climb, get part way up falter and drop back a bit as the distributor valve starts to fill the secondary tanks them resume its climb until it gets to full pressure. Blaming the warning lights and buzzer straight away is a bit like shooting the messenger sometimes. Quote Link to comment Share on other sites More sharing options...
Helen B Posted December 19, 2023 Author Share Posted December 19, 2023 Thanks John. The testing so far shows it’s building pressure correctly. However I’ll relay this to Phil. We’ve come to the conclusion it’s more than likely an electrical fault. Quote Link to comment Share on other sites More sharing options...
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