Where To Install Fuses and Relays From Tow Ready Wiring Harness # 118243 on 1999 Ford F-250 SD

Updated 07/07/2020 | Published 06/05/2008

Question:

1999 Ford F250 Superduty with factory 4 wire flat plug. I am upgrading to a 7 way connector with part 118243. Looking at the distribution boxes there are 2 side by side in the engine compartment, the smaller one only holds 2 relays while the big one has combination of relays and fuses. But when I plugged in new harness there is no power to the middle pole back up lights. The owners manual says relay 38 but with no location diagram? Is there another relay box that I am missing? I am only using the plug for pulling a boat trailer so need the regular 4 pole functions with the addition of back up light to deactivate the brakes.

asked by: Gene

Expert Reply:

On your 1999 Ford F-250 Super Duty the two boxes in the engine compartment are the power distribution box (larger box) and the trailer tow relay block (smaller box) which is attached to it. Open the covers to access the relays and fuses.

You will need to look in your owners manual and locate the Trailer Tow Electric Brake and Trailer Tow Battery Charge fuse locations and insert the 30 amp maxi-fuses (provided) into them.

Also in the owners manual locate the Trailer Tow Backup Lamp Relay and the Trailer Tow Battery Charge Relay locations and insert the relays (provided) into the trailer tow relay block. Put the covers back on the boxes.

With all of the fuses and relays installed you should have full power to your 7-Way connector. I am including a link to the installation instructions on the Tow Ready wiring harness.

expert reply by: Mike L

Ron

5/9/2020

the 99 super duty has the transfer case relays mounted next to the fuse box. the tow relay box unless ordered with tow package 7 way does not have the additional relay box. if the relay box you have has a pig tail that plugs into the factory harness near the ecm / under truck harness connectors it will be a y connector . my early 99 drove me nuts until i visited a junk yard and got the additional relay box.sadly the manuals dont tell you this

Chris R.

5/12/2020

Thank you for this information!

Taylor B.

7/3/2020

I’m having same issue. I have a early 99. There’s a small 2 relay box next to main fuse block under the hood. Is this the block for tow charge relay? It’s got two openings. Can’t tell which is which..

Chris R.

7/7/2020

The owner's manual for your F-250 should state where exactly that fuse location is - and the underside of the relay box should hopefully have a diagram/map as well. I wasn't able to track this information down myself.