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Tracing wires - advice needed

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John Hilton29/03/2016 22:25:58
81 forum posts
19 photos

I wonder if members might be able to advise me on a tricky problem please... I have an old edm machine, made c 1990. One of the switches has failed, but I can't work out which one as all the wiring between contactors, power supplies, logic boards and relays are all black and not numbered. There are about 50 wires in a bundle in one section, all the same colour! I know which contactor is not activating, and can prove it will work with power. So I know where the wires start, but can't prove the ends with a simple continuity tester as it is all pretty well inter connected so I just get false readings.

Is there any other way of isolating a circuit like this, perhaps sending a pulse or signal down the wire?

Any suggestions gratefully received. John

Emgee29/03/2016 23:32:00
1091 forum posts
199 photos


Sounds as if you have connected a supply to the contactor coil direct to confirm it works when power is applied.

1 side of the coil will be common to other contactor coils so you should be able to establish which 1 that is. The other side of the coil supply wire should be diconnected from the coil to prevent bad readings, ensure the power is isolated and check for continuity between this disconnected wire and other connections in the cabinet.

If this fails you may have to split the bundle and physically trace the wire to it's source. If there is an interlocked circuit it could well come from 1 of the other contactors normally open contacts.


John Hilton30/03/2016 07:30:26
81 forum posts
19 photos

Thanks Emgee. The contactor drives the pump and has the usual soft start coil added. This contactor also provides the input to the next contactor and so on for 5 contactors.

I hadn't thought about isolating them - good suggestion.

I will post a photo - just for fun- showing just one bundle of wires - these go to some switches including "Start" - although I think the contactor in question may be powered via a relay - there is a huge board with loads of them on it.

Thanks. John

John Hilton30/03/2016 07:34:17
81 forum posts
19 photos

Emgee30/03/2016 09:19:34
1091 forum posts
199 photos


I can't view your picture but it may be due to my system.

If the Start switch/button mentioned is the "process start" that would be the best place to begin tracing the control circuit, should all fall into place as you work through the order of running. Perhaps there is an external control preventing the pump operating, low fluid level, high fluid level, blocked filter, high/low fluid temp, timer control.


Emgee30/03/2016 10:30:37
1091 forum posts
199 photos


So basic didn't mention it before but just to be sure have you checked all circuit fuses are OK, any MCB's are in the ON position, no overloads need re-setting, machine head not on top/bottom limit, Emergency stop in?

You may be able to get an electrical schematic from EDM for your machine, faults are then much easier to spot.


frank brown30/03/2016 20:11:44
436 forum posts
5 photos

If the problem is sorting out where " this black wire goes" and the answer is over 20 different possibilities. Then I would isolate the kit and connect a low voltage power supply to your wire and to the chassis. Then with a suitable low wattage bulb or a suitable meter with one side connected to the chassis, just go from connection to connection until continuity is found. You should be able to check out 20+ terminal per minute. Do check all the terminals as the wire might be looped to a couple of different places. A bell transformer and a 6V bulb would be ideal as any leakage through a contactor coil would be obvious ( very, very dim bulb).

There are pulse type cable tracing stuff around but they are not suitable for this sort of kit, only in the wall or ground or on individual PCB tracks, not bunched wires.


martin perman30/03/2016 20:33:33
1575 forum posts
66 photos

When you say the wires are not numbered do you mean by tags as some cables come with numbers printed into the sheath usually about six inches along their length.

Martin P

stevetee30/03/2016 20:51:13
123 forum posts
13 photos

You can build a good cheap little tester for low voltage circuits quite easily A+ lead a-ve lead and 2 lttle bulbs like maybe 12v 2w bulbs in series. At the centre tap ( as it were) between the 2 bulbs have a litle sewing needle or flying lead so you can touch various parts easily , a bit like this.

+ve-------------------0---|---0---------------- _ve

At rest both bulbs glow dimly , touch a contact with the probe and if it's live the first bulb goes off and if it's earthy the second bulb goes off. If its dead both bulbs continue to glow dimly. When the one bulb goes off the other bulb will glow more brightly. Very handy little tool for car wiring etc. I don't need to emphasise LOW VOLTAGE only.

Simon Williams 330/03/2016 20:52:09
385 forum posts
65 photos

Hi John, as a control panel builder in a previous life a "bundle of black wires all black and not numbered" sounds like the stuff of nightmares.

First check along the wires - are they laser marked within their length? Or is that too easy! Some industrial machinery - particularly if it was mass produced - had pre-formed and pre numbered wire looms assembled into it, worth checking carefully that there really isn't some ident system other than the usual coloured ferrules each termination.

Whether there is or no, you are really committed to un-bundling the wires and following individual cores. Messy, time consuming, confusing and difficult to put it all back together neatly. The search and find (battery and bulb) approach will tell you quite a lot, but it can be confusing to differentiate between a cross connection and a "real" connection.

Identifying the power into the circuit and the common return/neutral is a good start, after that think of the control circuit as rungs of a ladder - as they would be drawn in a plc diagram - there can only be a limited number of rungs, and mostly they will be interlocks or contacts in series. Easy to trace if time-consuming. Tag the ones you've sussed with sticky tape and numbers, just as you would expect to see idents if they'd been fitted.

And regular cups of tea. It's going to be a long job! Essentially you are reproducing the manufacturer's cct schematic, so having one of the same machine (even a similar machine) is a good start.

And when you've got all that done, now find the fault. Easier said than done, particularly if the innards of the machine are not familiar! But if it is just a switch that has failed, you can check them off one by one with your faithful battery and bulb, but you are going to have to disconnect the rest of the control circuitry to be absolutely sure exactly what you are testing step by step.

Good luck, Simon

Edited By Simon Williams 3 on 30/03/2016 20:53:27

Chris Evans 630/03/2016 22:14:46
1417 forum posts

Give us a clue to make of machine I spent half of my 50 years tool making driving EDM machines. I know a few people and may have access to a few manuals.

John Hilton31/03/2016 07:27:40
81 forum posts
19 photos

Thanks all for your advice and suggestions.

Sadly none of the wires are marked or numbered, and almost all are black!

Chris, The machine is a Mecrode Minor. I have scoured the web to find anything on these, without success. If you know any means of getting to the "logic" of the machine that would be really helpful.

I see my picture didn't post well. I will have a go at another one on a different computer.

Sleeves rolled up, then, and time to separate millions of wires.


KWIL31/03/2016 09:41:19
3069 forum posts
56 photos

Try these people, they do repairs


Chris Evans 631/03/2016 09:45:34
1417 forum posts

John, I have not seen a Mecrode Minor but did work at one place that ran and still have early 80s Mecrode machines. I retired nearly three years ago but am due to pop in very soon to use a surface grinder for a job I am doing. I will look for a manual and the card from the guy who looked after them for us he was always helpful over the phone. The two machines there where tempremental things both would sometimes refuse to restart the dielectric pump after running for a few hours then start after a break of about 5 mins. What is your location ?

John Hilton31/03/2016 10:30:46
81 forum posts
19 photos


John Hilton31/03/2016 10:42:00
81 forum posts
19 photos


I am in Taunton - Somerset.

Any reference material would be a great help. If not a million miles away I would gladly come and see any similar machine, or a schema or manual if not practical.

I noticed two problems with my machine that may cause irregular running.

1. The cooling fans are not up to the job, and the power boards (RC pulse and a "darlington triple" tend to melt down.

2. The electrolyte level monitoring system is poor and tends to jam - especially at the lowest level. As a work around I overrode those levels manually and just controlled it from the valves.

Stevetee - Thanks - This looks like an excellent suggestion - I will make one for the testing:

Simon - I fear you are correct, sleeves rolled up and wires everywhere. I had an uncle in the Navy once whose job was to rewire Nuclear subs in the '70's. On a small scale I can better sympathise with the bad language now!

You will see I have posted some pictures now.


Michael Gilligan31/03/2016 10:56:42
13104 forum posts
570 photos


I see from your pictures that it's modern enough that they used plastic cable ties.

... On the bright side: at least you won't be cutting that lovely lacing that they did way-back.


Clive Hartland31/03/2016 11:49:58
2434 forum posts
40 photos

Some years back i had to fault trace a similar situation and I got round it by pricking though the cables with a sharp pin point and was able to follow the circuit that way. It does no harm unless you are in a wet environment. Of course mark the cables as you go as you can get a bit lost around corners etc.


Chris Evans 631/03/2016 20:29:05
1417 forum posts

John I am up in the West Midlands but the engineer I mentioned is/was Gloucester based. Not sure if he is still working as we are all getting older ! The pictures of your machine look modern compared to the ones I used.

Martin Emanuel13/02/2019 22:30:50
1 forum posts
Hello, i have the exact same machine and I can not get hold of an operation manual, can anyone please help me? Based in stoke on trent, happy to travel.

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