Questions on factory trailer wiring...

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Questions on factory trailer wiring...

Postby NorthGaAire » Wed Jun 12, 2019 12:55 pm

My 78 Cherokee has a factory trailer hitch and 7 pin wiring with the black box.

A PO added another 4pin flat cable (spliced off the original wiring harness).

I can hear the relays working as the vehicle lights are operated - but not all of the lights work through the 7 pin trailer connector.
All of the vehicle lights do work.

I have studied the 79 TSM electrical/trailer wiring section. Is there maintenance that has to been done inside the relay box?

I would like to keep the factory wiring. Is there a reason not to? or is there a better solution to get the 7 pin connector up and working?

1978 Cherokee 4dr, 360v8, 4spd, SOA conversion

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Re: Questions on factory trailer wiring...

Postby tgreese » Wed Jun 12, 2019 1:35 pm

I'd think you could use the 7-pin connector if you can find a mate to the socket on the Jeep. Standard part, I think. I'd also guess that the flat connector was added to tow a utility trailer or such, by a PO who didn't want to mess with the heavy-duty harness. That 7-pin connector is supposed to power trailer brakes and backup lights, which wouldn't be needed for a light-duty trailer.

Electrical problems are almost impossible to solve without the vehicle in front of you. Pretty much you have to trace it out with your meter or test light and locate the fault.

If ypu look on the Tom Collins page, there is a very nice diagram for the 1978 7-pin harness (class 3 & 4) showing both a schematic and a pictorial diagram of the harness. Note that the schematic is on page 2, even though page 3 is labeled "class 3 & 4." Note also that there is a circuit breaker and relays, which could have failed by now. The control unit at the back of the Jeep takes power from the electric tailgate window circuit.

If one of the relays or the circuit breaker are faulty, it would be easy to replace them with a work-alike part. I don't think you'll find the exact part for replacement though. Maybe. If it were me, I'd make a new box with new relays and circuit breakers. Maybe $25 in parts, not counting the wire. Possible you could reuse the box.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
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Re: Questions on factory trailer wiring...

Postby csuengr » Wed Jun 12, 2019 2:54 pm

When I put a 7 pin on my 77', I just spliced the marker/indicator lights and backup lights into the harness that runs under the rear of the Jeep. I put a fused and relayed wire for 12V and just wired in the brake controller.
1977 Cherokee S, Ford 5.0, 5 speed, BW 1356, 33 x 10.50 BFG's. No longer my DD.
2007 Mercury Milan, 2.3L, 5-speed, now my DD. 29 mpg average.

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Re: Questions on factory trailer wiring...

Postby mx71 » Thu Jun 13, 2019 8:54 am

If you decide to give up what you have, I'm using this with no relays. I would like to wire in the brake controller, but I haven't yet. I spliced in the rest of the wires that aren't included on a 4 pin. ... UTF8&psc=1

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Re: Questions on factory trailer wiring...

Postby NorthGaAire » Mon Jun 17, 2019 2:41 pm

Thank you guys.
1978 Cherokee 4dr, 360v8, 4spd, SOA conversion

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