The Craftmans Shop > New from Old
It's BIG, Yellow and digs holes! JCB 3CX Project 8 is joining the Tractor Shed
awemawson:
A very curious cable tracing problem
For some time - in fact since I got this machine, I've been trying to solve a wiring issue in the cab. Obvious symptom - interior light not working but in fact nor are the various sockets for beacons. Circuit shows that unswitched 12v is supplied to fuse A7 and the output of this fuse goes into the loom as cable 303 then splits as 300A, 300B etc to feed 12v to the cab items. The loom goes by a very contorted route which I've only just found !
It turns out that it emerges on the RH chassis leg behind the rear wheel. Rear wheel off (oh boy they are heavy!) and clipping my trusty 'Tempo' tone tracer to fuse A7 output shows that there is a break in this run behind the wheel - excellent - sorted :ddb: or so I thought :bang:
Picking the very tough woven sheath apart I revealed a place in cable 300 where the copper had corroded and broken - no external damage to the sheath (before I wrecked it!) :scratch:
So carefully cutting the cable back to non corroded copper I crimped in an extra length and thought things would be sorted - oh no !
awemawson:
A quick test of the light - no power :bang:
Clipping my tone tester to the light power input in the cab and testing to the output of Fuse A7 a very strong signal - VERY peculiar. Testing with my Fluke multimeter absolutely NO continuity what so ever even in the 10's of meg ohms range.
The wire is supposedly isolated at the cab light and and at the fuse A7 output end (no fuse inserted) - has absolutely no conductivity using a multi-meter but gives a very strong tone on the tone tester - both with the source at the fuse and the source at the light. No shorts to earth - so how this can happen is a complete mystery to me .
I have cut my crimped repair and fed 12v directly from the battery to the end that goes up into the cab and sure enough the cab light can be switched on, proving that the fault is in the run from the break behind the rear wheel up through the side console and on to the fuse block. I've been trying unsuccessfully to unscrew the fuse block to get behind for and inspection - retained by four long screws into thin (1.5 mm) sheet metal - two have come out but two just turn - threads stripped I suspect. Access is far from ideal being beside and to the right of the drivers seat. Another likely place for problems is where the sheath goes though the cab floor but access is even worse !
I would welcome any suggestions.
russ57:
I've never used a tone tracer, but since it basically sends an ac signal(? ), could capacitive coupling at a break cause that?
If there is one 'mystery' break, there could be more.
-russ
awemawson:
Yes Russ it injects a signal of about 1khz - warbling to make it more distinctive - and the detector picks up this signal amplifies it and drives a small speaker. The detector has a threshold detector that if it reckons it's 'On Target' brings on a red light.
awemawson:
Russ I'm sure that you are right about capacitive coupling (cross talk in telecoms). I grounded the fuse A7 output terminal, fed the signal into the break where I cut my crimp and can follow the signal as far as where the loom goes up into the cab.
Any further tracing is going to need me to open up the side console where the fuse box is which I really didn't want to do it was a pain last time and no reason for it to be better now !
(post #30 here https://www.madmodder.net/index.php/topic,13429.msg161541/topicseen.html#msg161541 )
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