MAN 8.136 FAE 4x4 REVERSING LIGHT
Your ex-NATO 8.136 (or 8.150) is partly wired for a reversing light, but to actually get one fitted and working poses several problems.
The main problem, aside from completing the wiring, is getting hold of a suitable reversing switch. MAN no-longer stock the correct reversing switch (only an option on the 4x4s) and the switch (MAN #81.25507.0015) listed for the ‘commercial’ 8.136 (the MAN G90 trucks) doesn’t fit the VW 007.1 gearbox on the 4x4 truck. It has an M18x1.5 thread, whereas the 4x4 gearbox thread is, unusually, M12x1.25 (M12 fine). There is only one switch – for a Hyundai/Mitsubishi - with this thread listed in all the ‘after-market’ switch manufacturers’ catalogues that I can find: it is the wrong length and also has normally-closed contacts (You need normally-open contacts).
Finding and fitting a suitable reversing switch.
Since the original switch is no-longer available, there are two options. (1): tap out the gearbox to take eg. an M14x1.5 thread. There are then several reversing switches of the right length that would fit – e.g. FAE 40500 or equivalent (FACET 7.6001, Intermotor 54100, etc), or (2): modify a switch of the right length by cutting an M12x1.25 thread on it. Option (1) is generally inadvisable as there is clearly a danger of dropping swarf into the gearbox and it’s difficult to do it anyway with the gearbox still in the truck. Option (2) is relatively easy.
I have found two switches that work well when modified in this way. Both have normally-open contacts. The first, FAE 40380 (FACET 7.6011) has an M12x1.5 coarse thread. The required M12x1.25 fine thread can be simply cut into the existing coarse thread, but be very careful to keep the die square to the axis. This is best done in a lathe using, e.g. a tailstock die-holder. Because of the different pitch, some of the original threads are, of course, cut away when you do this and it does look a bit untidy, but there are plenty of full threads left and the switch screws in and tightens without difficulty. A sealing washer about 1.5 mm thick is needed to limit the switch travel to about 1.5 to 2 mm. This switch takes a 19 mm AF spanner and has flying leads to connect it to the 8.136 loom.
A tidier option, that nevertheless certainly does require use of a lathe, is to use an FAE 40500 (or equivalent), turn the M14 thread off completely to leave a nominally 12 mm diameter shank so that a full M12x1.25 thread can then be cut on it. This switch has 6.3 mm Lucar spade terminals that can connect directly to the truck loom. It is slightly shorter than the other switch, so doesn’t need a washer. Fit it with a little sealant on the thread so that it tightens directly against the gearbox flange. This gives it about 1.6 mm travel. A 22mm spanner is needed – except for the ‘equivalent’ Intermotor 54100 that has a 19 mm hexagon.
The supply (black/violet) to the switch from Fuse 9 (shared with the horn) and the output (black/white) to the reversing lights are both available in one of the unused flexible trunking conduits folded on top of the gearbox. The switch connections will not be via the excellent waterproof plugs and sockets used on the rest of the truck, but can nevertheless be waterproofed with tape or (for the FAE 40500) a rubber sleeve that fits over the switch.
Wiring.
The black/white output from the switch goes back to terminal 17 of socket A on the back of the fuse box (at the left end of the Central Electrical Unit under the dashboard ). It is connected internally across to terminal 18 on socket F (at the other end of the unit) which supplies all the rear lights. (Note, by the way, that these plugs can be fitted the wrong way up, so mark them if you remove them – or nothing will then work!) From there, a single black wire goes through the main rear-end trunking to the big 36/34-pin plug on the distributor box at the rear.
Here there are further difficulties. According to the readily-available circuit diagrams for the ‘commercial’ 8.136 (G90) trucks produced alongside the 4x4 FAE, the reversing-light wire from the fuse-box should be violet on later 1987 G90s (though it was black before that) and should go to pin W of the big distributor box plug. It should then be connected internally to pin 2 of socket A (rear light cluster, right) and pin 1 of socket D (rear light cluster left) and pin 7 of socket C. Unfortunately, the 8-pin plugs fitted to the 4x4 are not fully populated and these connections to these pins are missing. There is no reversing-light supply on these pins anyway – and there are no reversing-light wires to the rear light clusters or the 12-pin trailer plug either.
The (VW-MAN) FAE wiring at the distributor box differs from the G90 wiring. (I do not know if the later, post-1987 MAN-built FAE wiring is the same as that on the G90 – that would make life easier on these vehicles). The FAE distribution box (marked TRS 002-34) nevertheless has all the same internal connections as those shown on the G90 circuit diagrams – it may be identical). The problem arises because the black reversing-light wire from the fuse box goes to pin K on the big distributor-box plug instead of pin W (as on the G90) and has no connections to sockets A and D that supply the rear light clusters. Instead, it is connected internally to pin 2 of the unused socket E at the bottom on the front of the distributor box. There is no plug fitted to this socket, but, at least, there is a potential supply there.
One possibility would be to re-route the black wire to pin W on the big plug, as on the G90. This would then provide a reversing light supply on pins A2 and D1, allowing the use of G90 plugs and tail-lights, as above. This is not an easy task and should not be lightly undertaken. The terminals K and W in the plug need to be swapped (or re-soldered). They are not easily removable and there is a danger that either the plug or the terminals could be damaged in the process.
The other option is to use the connection available at pin E2, currently blanked off. One of the plugs in sockets B or C would fit there, but plug B supplies the 12-pin trailer socket. Plug C is a possibility if the 24V 2-pin output socket on the rear cross-member is not needed, but it unfortunately it doesn’t have a connection for pin 2! The only option seems to be to purchase a new plug that will fit – or get one from a scrap G90.: plugs A, C and D on a G90 should all have connections to pins 2 and 8 (the earth on all lugs).
In our case, we had no need of the 12-pin trailer plug, so we used the plug from socket B that supplied it. The black/white wire (even the right colour code!) connects to pin 2. All other wires except the two brown earth wires were snipped off close to the plug and insulated with shrink wrap to avoid short circuits.
The live and earth connections wires were extended with 1 mm2 wires to each reversing light, attached using shrink-wrapped soldered connections. These were led in the original 10 mm flexible trunking to a waterproof T connection (Durite 0-330-60) bolted to the rear cross member (using one of the trailer socket bolts) and thence, still in 10 mm trunking, separately to the two 48W LED reversing lights mounted beside the original round tail-light clusters. Job done!
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