stratcat442 Posted July 8, 2011 Share Posted July 8, 2011 Hello All, Having a problem when MK sets in the heat of day. Voltage at main and oil light circuit drops to 3.5volts. Any and all help is much appreciated. Quote Link to comment Share on other sites More sharing options...
0 fv1609 Posted July 8, 2011 Share Posted July 8, 2011 Hello All, Having a problem when MK sets in the heat of day. Voltage at main and oil light circuit drops to 3.5volts. Any and all help is much appreciated. Sorry I don't understand, can you gives us some more details please? Quote Link to comment Share on other sites More sharing options...
0 stratcat442 Posted July 10, 2011 Author Share Posted July 10, 2011 The lead marked INS that feeds the main & oil lights, and starter switch has 24Volts when unplugged at the switchboard panel. When plugged in to switchboard the voltage drops to 3.5v when the condition occurs. Have checked main grounds. Batteries are new and good. I think it's in the switchboard, possibly the starter switch itself but was hoping someone else has experienced this and can confirm. Thank you for you reply Quote Link to comment Share on other sites More sharing options...
0 fv1609 Posted July 11, 2011 Share Posted July 11, 2011 INS is supplied from a 10A circuit breaker in the Distribution Box No.1 (together with another one for 30A that supplies the main vehicle lights) I would put an ammeter between supply INS & the switchboard INS it supplies & see if an excessive current is being drawn. If it is more than 10A then the thermal breaker will operate. Once cooled down it will reactivate itself. It is does not totally break the circuit in the modern sense, when a fault exists nearly 200 ohms is introduced in series into the circuit. Or you may have a fault in the circuit breaker itself, which fairly unusual Whilst you are at it, if you haven't done this for a while, I would unplug (one at a time) each end of all those bullet connections make sure they are clean & the female inner tube clean & tight, then plug in very firmly. I had an intermittent on one this year, it was very misleading as it looked as if everything was plugged in correctly. But because of the waterproofing lip on the female it held the bullet plug in place although it was barely touching electrically. A clean & a really hard push solved it. It was caused by part of the female lip having rolled under & prevented full entry of the bullet but held it in place & would not seem to go in any further until I had rolled the bent lip out of the way. Quote Link to comment Share on other sites More sharing options...
0 stratcat442 Posted July 11, 2011 Author Share Posted July 11, 2011 When I unplug supply INS from switchboard it returns to 24v at supply INS. When I plug back in to switchboard that's when it drops to 3.5v. I'm actually able to turn ignition switch on and jumper 24v to SOL lead at switchboard and start the MK. From INS at ignition switch through indicator lights to starter switch is where voltage drops. Are you familiar with the internals of the switchboard? How is it grounded? Remember too that I am experiencing this problem when it sets outside in the heat of the day and temps rise in the cockpit. Quote Link to comment Share on other sites More sharing options...
0 stratcat442 Posted July 11, 2011 Author Share Posted July 11, 2011 I meant from INS in to switchboard to indicator lights, not INS to igntion switch. Quote Link to comment Share on other sites More sharing options...
0 fv1609 Posted July 11, 2011 Share Posted July 11, 2011 When you start it with the 24+ jumper to SOL I assume the ignition & oil lights come on in the normal way? By doing this you are in effect bypassing the 10A circuit breaker, which I feel is the main suspect, especially now you have shown the system works with it out of circuit. You may feel that the circuit breaker is working because you can measure 24v on it with your meter. The problem is the circuit breaker is a bit of a misnomer in that it doesn't entirely break the circuit. When the "circuit is broken" because of the coil inside it there is nearly 200 ohms connected in the circuit. When it is unplugged because your voltmeter draws an extremely small current then 24v can be read on the meter. Once a load starts to draw meaningful current the resistance of the coil causes a voltage drop. Obviously the greater the current drawn the greater the voltage drop. So by dropping down to 3.5v suggests a current of 0.1 amps is being drawn. Given that this "circuit breaker" is a thermal device would fit in with the fact that this problem is most obvious in the heat of the day. Part of the ignition switch is grounded to the case of the switchboard. If you were thinking along the lines that there is some shorting out to earth that is reducing the voltage, this would be unlikely if it is all working as per my question at the beginning. Sadly I am only too familiar with the insides of these switchboards both Mk 1 & Mk 6 which I imagine is what you have. Mks 2-5 seem to have been entirely different units. The internal construction of the Mk 6 is significantly different from the Mk 1 & incorporates not just a different light switch configuration but the ignition switch is of a superior design. None the less they are only too prone to failure, I think I have about 5 defective ones here! Quote Link to comment Share on other sites More sharing options...
0 stratcat442 Posted July 11, 2011 Author Share Posted July 11, 2011 Good info. Thank you. I didn't know the circuit breakers didn't produce a complete disconnect. I pulled the distribution box and found corroded breakers. Distribution box number is F.V.157930. Do you have an extra one laying around? Or a line on one? Quote Link to comment Share on other sites More sharing options...
0 fv1609 Posted July 11, 2011 Share Posted July 11, 2011 A shame its all messed up but at least it is diagnostically satisfying. Although I have a nice one here, it is my reserve one I'm keeping I'm afraid. In your half of the world might be worth trying Pat. He always tries to carry a good stock of spares. http://www.khakicorpsimports.com/ Over here these are really your only hope: http://www.marcusglenn.com/ http://www.milweb.net/go/banister/ http://www.champ-sparesukltd.com/ There are always private sellers on ebay of course Depending on the packaging & when it was packaged or repackaged it might display different numbers. So although it is quite correct to look for FV157930 it could turn up in another guise. NSN 6110-99-803-5839 Pre NATO codification it was LV6/MT4/CB/235/1 RAF as 16E22920 Although it was used in various vehicles for some reason it assumed another identity in Thornycrofts as LV7/TY/FV157930 If all else fails or it is too expensive you might consider dismantling it & giving it a good clean. I have instructions for resetting the gaps for both breakers. Quote Link to comment Share on other sites More sharing options...
0 stratcat442 Posted July 11, 2011 Author Share Posted July 11, 2011 Can you go ahead and set me up with the info on point gap setting for the breakers? Going to give a good cleaning and see what happens. Thank you again Quote Link to comment Share on other sites More sharing options...
0 fv1609 Posted July 11, 2011 Share Posted July 11, 2011 Yes I think that it is worth a try. I'll upload the complete section describing their function. Quote Link to comment Share on other sites More sharing options...
0 fv1609 Posted July 11, 2011 Share Posted July 11, 2011 Quote Link to comment Share on other sites More sharing options...
Question
stratcat442
Hello All,
Having a problem when MK sets in the heat of day. Voltage at main and oil light circuit drops to 3.5volts. Any and all help is much appreciated.
Link to comment
Share on other sites
11 answers to this question
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.