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Trailer tow package and trailer wiring questions

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Trailer tow package and trailer wiring questions

 
  #1  
Old 09-25-2013, 04:50 PM
AeroZona
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Question Trailer tow package and trailer wiring questions

I have 1994 Aerostar XLT with what I believe is equipped with a trailer tow package and wanted to know a couple things if anyone knows.

There is some wiring coming out from the bottom left driver's side and ends with one plug right there and then there are more wires that go from there to the center rear with 2 more plugs there.

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I'd like to try and find out what the plugs do and if there's a harness available to plug into the factory plugs? I went to a couple trailer companies like Big Tex and then U-Haul and they all said they just cut/splice into the wiring on the vehicle which I told them I didn't want and thought wasn't necessary.

I did go to the Ford dealer but they weren't much help. They did give me a part # which I think is a harness to plug into the factory plugs but I haven't seen any pics or description of what it does. They gave me F39Z-15A416-AA and goes from $89.32-$120.00 Too much money for that, but...

So, for now, I've been looking/testing the small round 4 pin factory plug in the bottom center. It has green, yellow, brown, & black wires going into it. I do get stop & turn signals but no running lights from this plug. Checking further, I'm not getting anything for running lights (brown wire) but if I take my test light and ground a pin on the small 2 pin plug, then running lights but very dim. Something maybe I'm missing?

So, just looking for some help & information if anyone knows or has done what I'm trying to do here.

I'll post some pics on the install of the hitch if anyone's interested.

Thanks!
 
  #2  
Old 09-25-2013, 05:36 PM
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93nighthawk
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Here is the catalog pic:

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And like you were told, SRP is $127.60 and it is currently obsolete.

I do know that aftermarket companies like curt and hopkins do make direct plug in kits.
 
  #3  
Old 09-26-2013, 08:53 AM
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Originally Posted by AeroZona View Post
... if anyone's interested...
Yes, interested. Good topic.
I have to do this same job by next season. The ultimate would be pinout documentation on current trailer side standards and van side As-Is. The starting point for me is the factory schematic, but it (the schem) is not exactly easy on the eyes. And I anticipate a full rewire just to bring up reliability, so a translating harness is less of a concern for me. I'll be climbing around in there soon.

Luv your screen name.
 
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Old 09-26-2013, 11:16 PM
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Looks like Asavage posted a good pic of what wire goes where here:

https://www.ford-trucks.com/forums/2...er-wiring.html
 
  #5  
Old 09-26-2013, 11:19 PM
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This is my understanding of the three tow wiring systems that the Aero has used. I've had one of each over the years. They fall into three eras: 1986-91, 1992-96, and 1997. I'm am not positive about the break years, but it's a reasonable guess.

Early on, Ford called the tow pkg wiring the Trailer Tow Relay Module (TTRM).

1986-1991
On my '89, the TTRM was located in the left rear jack area. Because Aeros up till 1996 all use separate turn and brake lights in back (amber turn, red brake), the TTRM combines the turn/brake signals into combined signal for the trailer. It's a nifty setup, and it is only activated when the factory trailer adapter harness is plugged in (more about this later). When active, one can clearly hear the relays click back there, when you brake or use the turn signals. The relays isolate the load of the trailer's turn signal lights from the analog flasher that the early van's used, so the speed of the flash does not change when you plug in the trailer. Back in the day, this was a serious problem when towing.

Below is a pic of my '89's TTRM. The twisted-together yellow wires is where the factory fuseholder used to be (This is how I got it back after I'd sold it and the guy butchered a bunch of wiring all over the van). It gives you an idea what it looks like on the early (pre-'92) models, at least when it's unscrewed and flipped upside down as show below:

Click HERE for larger


Over in this thread I detail how the Ford factory trailer adapter harness works, and where to get matching connectors (what donor vehicles) to build your own. This information is applicable to the later Aeros too (except 1997).

This is the wiring diagram for the early ('86-91?) factory trailer tow module (TTRM):

Click HERE for larger




1992-1996
The newer factory tow package module (1992-1996?) is located behind a rectangular panel on the left side of the van, left of the second-row seating. IIRC, it's behind this panel:

Click HERE for larger


Located above the left rear wheel arch:

Click HERE for larger


View from when I was adding the TTRM to my '93 that didn't have it from the factory:
Click HERE for larger


It does not use relays for the left & right trailer feeds as previously; instead, it uses a fancier Multifunction switch that combines left/right/brake for the trailer feed and passes it through the TTRM unchanged, but retains relays for running lights and backup lights. This era of Aeros used a solid-state flasher relay that does not change speed with lights load.

The pic below is of a 1992? TTRM that I bought at a junkyard, and installed in my '93. Its harness is so long because the TTRM itself (the fuse panel and relays) are quite a ways forward of the taillights.

Click HERE for larger


Click HERE for larger


This is the wiring diagram for the middle ('92-96?) factory trailer tow module (TTRM):

Click HERE for larger


Note1: the MF Switch diagram does not appear to be drawn correctly (it's definitely got wrong indications in other sections of the manual). In particular, it does not show that the MF Switch sends out separate left/right signals to the van's lights, but combines them for the trailer feed. The combining is not accomplished in the TTRM for this era.

Note2: There are three relays on my TTRM, but only two shown in the Ford wiring diagram. I never did trace out what the third relay was used for in mine.



What's not obvious, but important:
One the early system ('86-91), all connectors under the bumper are dead, until the factory trailer adapter harness is plugged in. The relays' grounds are enabled by a ground loopback in the adapter harness. There is a single fuse buried in that TTRM, and it's not fun to get to (remove jack, unscrew TTRM, flip over). I've blown a few over the years, and I finally extended the fuse wiring to install a fuse holder out where I could get to it without removing the jack!

On the mid-era system, the left & right signals are passed through the TTRM but have their own (hard to find, hard to replace) fuses. Make yourself familiar with how to get to them. So, the left & right signals are always enabled at the rear of the van at the tow harness. However, the backup relay and running lights relay both use the ground loopback of the earlier system, so you must provide a ground to get the running lights and backup lights on your trailer to work.

A couple of adapter harnesses I've had over the years:

(The factory harness does not come with the receptacle shown below; that's just the way I either bought it or wired it, I forget which.)

Click HERE for larger



The connectors on that harness (see thread linked above for a guide to obtain these connectors from other donor JY vehicles, to build your own adapter harness, if you dislike cutting off your van's connectors.)

Click HERE for larger



A trailer adapter harness unwrapped, showing the ground loopback:

Click HERE for larger



1997 only

In 1997, Ford dropped the amber rear turn signal lens and went retro, using a combined turn/brake system like almost all domestic vehicles used until the '80s. I do not have a factory tow wiring setup on my '97, but I added my own homemade adapter in this thread, and it's a pretty comprehensive post. The factory setup has the running lights relay and the backup lamp relay, and while I'd like the latter, I haven't found a suitable donor vehicle. The TTRM does not appear to be a separate module in 1997; from the diagram below, it looks like those two relays are floating somewhere. The trailer adapter harness ground loopback design is retained in this era (if you have factory tow wiring).

This is the wiring diagram for the late (1997-only) factory trailer tow module (TTRM):

Click HERE for larger


As can be seen, the wiring is much simplified; no fancy MF Switch to send separate turn/brake to the rear and combined turn/brake to the trailer. The electronic flasher relay doesn't care about the extra trailer lights load. It's very straightforward, because the trailer's lights mirror the van's lights.

================================================== =
@AeroZona: If you have those two connectors under the back of the van, you do have the factory tow package.

With that as a prelude, so to speak, the reason that you have no running lights is that you need to enable the TTRM's running light relay via the ground loopback.

Connect the BK wire in one connector to the BK wire in the other (NOT the BK/Pink: that's for the reverse lights). If I read the tea leaves right, that should enable the relays in the TTRM, and you should get running lights on BR/W.

The easy thing to do is to cut off those van-side connectors, permanently solder the ground loopback, and wire a four-prong flat connector to the remaining three wires (and the fourth wire in a flat-four connector is white: connect it to the ground loopback too). There's no downside on the mid-era rigs. On the early era rigs, you have to listen to the noisy relays back there, even when you're not towing, but I never took my adapter harness off on my early, and got used to the extra clicking.

If you prefer a factory-connector type install, build your own adapter harness using the wiring guides above and the connector sourcing info in my first linked thread.

Hope this helps (it sure took long enough to type! But I had all the pics from the past eleven years).
 
  #6  
Old 09-27-2013, 09:16 AM
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Originally Posted by asavage View Post
...from the past eleven years.
And now you've written the best post ever on the topic.
 
  #7  
Old 09-27-2013, 09:42 AM
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Originally Posted by 93nighthawk View Post
I do know that aftermarket companies like curt and hopkins do make direct plug in kits.
Are the plug-in adapter kits for use with the factory tow wiring? None of the kits I've seen are; they are for adding trailer wiring to rigs without factory tow wiring. They'll still work on a factory tow wiring-equipped rig, but then they don't use the running lights relay.

The Aero headlight switch is overloaded enough that I think moving the trailer's running lights load off the HL switch to the factory tow wiring relay is a good idea -- if you have factory tow wiring, or (as on the mid-era Aeros) can easily add it from a donor vehicle. Other than having to remove the interior panel on my '93 to screw in the tow module, I don't recall it being especially hard to add.

Headlight switch replacement guide for '92-on is in this thread.
 
 
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