Wiring diagrams are a easy tool of finding faults in a electrical system but I have noticed that many people are reluctant to turning to wiring diagrams because they look to be complicated or difficult to read.
Let's have a closer look at wiring diagrams and how to read them.
Before we take any information from the manuals we must first ensure that we have the correct airline effectivity number. Different aircraft have different systems installed, different options, different wiring, some have been modified to a different status than others etc etc etc.
We use effectivity numbers to identify systems, if a specific wire is in a range of aircraft that range of aircraft's effectivity numbers is displayed on the drawing of that specific wire. For the purpose of this demonstration we'll take effectivity number 001
U can see that the effective aircraft for this drawing are 001 up to 015 and 025 up to 099. If we were working on aircraft effectivity number 016 this schematic should NOT be used.
Now we have to identify the system that has a fault, let's say that the spoiler-7 sensor excitation power is not received by the sensor.
U can see the spoiler-7 sensor drawn as a coil in the left upper box. The M930 code is the equipment number, this identifies this sensor in the aircraft, the equipment number can be used to find the partnumber of this sensor.
The D1697 number is the connector equipment number.
Note that the top of the connector D1697 has a breakmark on it, this indicates that not the entire connector is displayed on this drawing. If we want to know what is connected to this connector we can go to the hookup list and find all the wires going into this connector but more about this later.
The Wing STA 335 number gives the location in the aircraft where this sensor is installed.
U can find the STA (station) diagrams in chapter 6 of the maintenance manual. each number of the STA represents one inch so STA 335 is one inch next to STA 334 ;-)
The circle around the wires indicates that the wire is shielded by a outer conductor wich in this case is connected to a ground point called GD2656-S.
The lower right corner shows two wires with a 'figure 8' drawn around it, this means that the two wires are twisted.
These are the wire numbers, these numbers can be used in the wire list to find the length of the wire and the type of the wire that is needed.
The wires coming from the spoiler sensor go up to a disconnect station. Logically this is the Wing disconnect station. When the wing was installed, these connectors were connected last, this is also where the wing is electrically disconnected from the aircraft if the wing needs to come off for some reason.
Note that the connector has two parts, the connector and the receptacle. The equipment number for these parts are D5983P for the connector and D5983J for the receptacle, once again the partnumbers can be found in the equipment list.
A little further down the line in these two wires are 2 factory splices installed in a avionics rack leading to other sensors that need excitation voltage.
If the excitation voltage is not received at the spoiler-7 sensor but the other sensors do receive the excitation voltage then splices are the usual suspect. A wire could have broken of here, burned away due to high resistance or simply fell out of the splice due to a 'bad splice crimp'.
For the purpose of this story let's assume that a continuity check was carried out on the wiring and there was no electrical circuit between both of the pins at the sensor and the avionics rack.
We have therefore confirmed that the fault is between the rack and the sensor, ruling out the splices. In a troubleshooting scenario we usually assume that dual faults do not exist. Some times a stroke of bad luck puts us completely on the wrong track when multiple faults DO exist in the circuit but most of the time it is only one fault causing a malfunction.
Now we can discriminate the cable for the fault by disconnecting the connector D5983P in the middle and measure at that point so that we know if the problem is in the wing or in the body of the aircraft.
We open the connector and for this example we find this:
(ignore the fact that this connector has sockets, for this demonstration the connector has pins and the sockets are in the receptacle)
This connector is seriously corroded, internally the wires can be desintegrated and the contacts could have been broken. We need to replace the connector, the receptacle, the sockets and the pins.
On the equipment list we find the equipment numbers D5983P and D5983J. Both these numbers are mentioned in the equipment list twice, in the top right u can see the effectivity numbers. We have established that we are working on effectivity 001 so for the receptacle we take this one.
We do the same for the connector, for effectivity 001 this is the partnumber we need.
Now that we have the partnumber we can go to the Standard Wire Practices Manual.
We need to look up the partnumber so we go to the cross reference index and select 'B'.
Then we scroll to the partnumber wich closest resembles our partnumber.
Note that the other BACC45 connectors also refer to 20-61-11 so in this chapter we will find information on all these connectors.
Now we go to chapter 20-61-11 and we find that this chapter describes MIL-C-26500 front release connectors.
We can get some general information about the connector wich can be found on page 14 in this chapter.
In this section we can find the breakdown of the connector and receptacle.
Our partnumbers are for the connector: BACC45FT16A24P6 and for the receptacle: BACC45FN16A24S6 so the connector is a Bayonet coupled connector size 16 with a straight single leg clamp backshell, insert configuration 24 pins with code 6 keying (keying prohibits other keying numbers to fit on this connector, a attempt of making it foolproof).
In this section we can also find alternative partnumbers for this connector and receptacle if this partnumber is not available or not in stock.
Now we should make a selection of pins and sockets, special pins and sockets have to be used in some systems like egt (exhaust gas temperature) measuring for the accuracy of the electrical measurement of the thermocouples, these use alumel, chromel or constantan materials on specific wires. In our case we need to have standard pins and sockets so we need to look at table XXIII, XXIV or XXV
These tables contain partnumber information and color code information about the pins and sockets to be used. If u need to confirm the use of a pin or socket by the color code then u can find them in the tables.
Let's look at the boeing parts list for these contacts.
Now wich size to pick. Wire conductor thickness is measured in AWG (american wire gauge), the higher the number the thinner the conductor.
The pins and sockets also have a certain diameter, our connector and receptacle has certain size holes for wich we need to find the correct diameter pins and sockets, for this we turn to the 'insert configuration' section of the chapter.
We can see that there are supposed to be 24 holes in the connector and the receptacle in wich contacts can be placed. The size of the contacts is 20 and we can find a drawing of the connector in figure 59, this figure can be important if the contact numbers are lettered instead of numbered and the letters worn off.
Here's an example of where a hookup list comes in really handy. There is a small alphabet, a capitol letter alphabet and there are double letters. To make it extra easy, the capitol letters I, O and Q do not exist in connectors because they would look too similar to other letters (I looks like J and O looks like Q) making it too hard too read. For this same reason, the small letters l and o aren't used.
U can check the 'hookup list' in the wiring diagram to check the contact count.
For example about the effectivities, our 001 effectivity together with 001-013, 025-027, 125-199, 225, 014-017, 028,099 and 226,299 has contact 6 connected to wire W0367-009-22 and for all other effectivities this contact is unused.
Also u can see that some wires in the connector are gauge 20 and some wires are gauge 22.
Before we can crimp new contacts onto the wires of the damaged connector we must first disconnect the damaged connector, for this we need an extract tool. We find the partnumbers for this tool in the 'contact removal section'.
I select the size 20 front release extract tool and remove the contacts from the damaged connector.
Ignore the fact that this connector is obviously not the damaged connector but for the purpose of this demonstration I show the frontrelease extract tool.
We need to strip the wire to exact specifications for the contact that we are going to use.
This information we'll find in the 'wire preperation' section of the chapter.
Gauge 24 is the thinnest conductor gauge for this connector, gauge 22 is one size up, 20 one size up from 22 and so forth.
Logic tells us that we can't insert a thicker gauge wire in a thinner gauge crim barrel, i.e. we can't put a gauge 18 wire in a gauge 20 crimp barrel because it just won't fit.
If it so happens that the conductor thickness is two sizes smaller than the crimp barrel, in some cases the conductor may be folded back to ensure proper crimping of the contact.
We find the correct strip length wich in this case should be 0.19 inch.
Now that we know that the contacts size is gauge 20 (got this information from the insert configuration list, this must be the same as the engaging end because this determines the size of the actual contact) and we have both gauge 20 conductors and gauge 22 conductors we can return to the p/n list and select our contacts. I select the localised gold ones for now.
Tthe gauge 22 thickness wire will fit in the gauge 20 crimp barrel but we need to crimp that contact with deeper indents than the gauge 20 wire because the gauge 22 wire is thinner.
We can now place an order of BACC47CN1S and BACC47CP1S contacts for our new connector and receptacle.
Now we have the connector, the receptacle, the pins, the sockets, we know which wire numbers are to be installed in wich contact (hookup list) and we know the cable preperation. Now we just need to know the correct crimptools and the inserts and extracts.
We can now select the crimptools for the given conductor thickness and crimp barrel thickness. We have seen that there are gauge 20 AND gauge 22 wires going into the connector and receptacle so we need to set different settings on the crimptools.
We need to crimp the gauge 22 wire and the gauge 20 wire and as u can see in the table, the setting for the gauge 22 wire for the 1-01 tool is 3 and the setting for the gauge 20 wire for the 1-01 tool is 4. This means that the gauge 22 wire has deeper indentations than the gauge 20 wire because the gauge 20 wire is thicker than the gauge 22 wire.
For the purpose of this demonstration let's look at how to crimp the gauge 22 wire.
U can see that there are 2 tools recommended for crimping this contact, the M22520/1-01 and the M22520/2-01, the 1-01 tool is the larger one.
If we use this tool, the turret should be 1-02 for the location on the contact where the crimp should come, the color setting should be red for the diameter of the pin and the setting should be 3 for the indentation depth.
The 2-01 is the smaller one. Here a 2-02 locator should be used for the size of the contact and the location of the crimp and the setting should be 6 for the depth of the indentation.
The stripped wire can now be inserted into the contacts and we need to check the inspection hole and the length of the stripped conductor.
I start with the pins.
I use the 2-01 tool (the smaller one) with the 2-02 locator and the setting according to the crimptool table (setting 6 for the gauge 22 wire, setting 7 for the gauge 20 wire).
After the crimp is made, we check the mechanical strength of the contact to make sure that the crimp is not a 'bad crimp'.
Like the removal tools we needed to disconnect the damaged connector we need a insertion tool for connecting the new contacts into the new connector and receptacle. Partnumbers for these can be found in the 'contact insertion section'.
We select a size 20 insertion tool with a wire outer diameter of less than 0.06 inch.
And we can connect the new contacts to the new connector.
Unused holes in the connector should be plugged off to stop moisture from getting into the connector.
Now that the new contacts, a new connector and a new receptacle are fitted we need to tighten the backshell of the connector. From the connector partnumber breakdown we could see that we need a single leg clamp backshell for this connector. p/n of this backshell is: BACC10GH
For some connectors a certain backshell is selected by a letter code in the connector partnumber and in some cases we can find a table with backshell partnumbers in the chapter on the connector.
Before the wires are inserted into the connector logic would suggest that the wires should first feed through the backshell.
Once the wires are through the backshell and the contacts are properly inserted into the connector we can tighten the backshell.
To tighten the backshell we need to firmly hold the connectors core section and then screw on the backshell, we can do this by connecting the connector to the receptacle to hold the core section in place or we can use a 'backshell removal and installation tool' like this.
We put the tool onto the connector and tighten the backshell.
There is a range of these tools available for different types and sizes of connectors.
Now the cables need to be tied with wire lacing to finish off the job.
We install the receptacle with 4 small receptacle screws and we connect the connector to it.
We have now installed a new connector and receptacle including all the contacts. We still need to test all the systems that have wiring going over this connector, we can find these systems by looking in the hookup list and finding the references listed there.
After these tests we can consider this job done and the system serviceable.