Does anyone know why when the trailer battery charge relay is removed it will not allow the engine to turn over. The security lock comes on and the vehicle will not start until the relay is put back in. I had the engine running and removed the relay and the engine shut off right away. What happens if the relay would fail while driving down the highway. Will the engine shut off? If so that would not be a good situation.

 

I drove my Expedition home from the dealer (new) without the tailer charge relay or fuse, but haven't tried it with the fuse and without the relay.

Green, 30a fuse; Black relay.





-- Chuck

 

you sure you aren't pulling the Fuel Pump relay? it's under there as well. There is also a PCM relay in there.


None of the Fords come with the trailer relay installed. You have to install it later on if you want the function.

 

Chuck/
I too drove my expedition new home from the dealer without this relay installed. It was just this past weekend when I was comming home from a camping trip that I realized that the trailer battery was not charging. As a temporary fix I used the rear defrost realy to get the battery charge circuit to work (This is the same relay my Ford dealer told me to get. It will work but it is not the right one). When I got home I switched it back and discovered that I could not start the engine with out a relay in the battery charge slot. I did find the relay in a kit that came with my expedition. if you get a chance can you see if you have the same issue if you remove the battery charge realy.
thanks for the reply.

 

Tylus,

I am pulling the relay that is in the picture above. I beieve it is #56 according to the owners manual, but don't quote me.
thanks for the reply

 

Looks like you have a problem that's unrelated to this relay.

0718 EST. Pulled relay. Truck fired right up. As expected. I didn't diddle with the other relays or fuses.

It's relay 56.

-- Chuck

By the way the trailer battery charge line won't charge your trailer battery in other than the technical sense that a trickle charge will charge it. Measured amp flow after a few minutes is about an amp. All batteries quickly acquire a surface charge and battery charging is by voltage differential regardless of the amp rating of your alternator or charger.

This is always a surprise to folks until they realize the alternator is only designed to power the lights, etc. and recharge the starting battery.

Starting your truck uses only a small amount of power compared to what you used in a camper.

Here's a totally made up example but demonstrates this:

Say it takes 6 seconds to start your truck. Also assume the starter draws 600 amps during this process. It doesn't need to crank for 6 seconds and the amp draw is much lower but these numbers make the math easy this time of the morning!

600 amps for 6 seconds is 3600 amp-seconds. See where this is going?

There are -- you guessed it! -- 3600 seconds in an hour. Yep, it takes at most one (1) amp hour of battery power to start your truck. Your camper lights, pump, etc. will pull 60 amp hours in a short weekend.

The one amp charge rate is just what the starting battery needs to get that 1 amp hour back in it for the next start, and that's how the voltage regulator is set. Your camper battery, at the end of a long, voltage dropping wire, will take days of driving to charge off the truck. Just too many amp hours to put back and too little voltage to do it very fast.

Trailer battery "charge" line will keep a 12v refrigerator (common in very small campers) running on the road, but only puts a small amount back in the battery. If you don't have a 12v refrig this connection is just nice to have, not a requirement.

-- cfs

 

Thats really curious about the very low charge rate (amps) through the aux battery charge ckt. Its been a while since I pulled a TT, but the old Aux charge soleniod for that circuit would charge at whatever rate the Alt would put out for the battery voltage. The Alt was capable of putting out a charge rate that would charge a really low Aux battery. The solenoid was a continuous Duty metal case and would provide isolation when the vehicle was shut off.
Do the new vehicles have some current limiting device in the Aux circuit? (I dont have a new EVTM---hopefully soon will)

 

Batteries absorb current based on voltage differential. The truck's voltage regulator is set to trickle charge the starting battery. Maybe 14.3 volts. That voltage drops at the end of the long, comparatively thin wire to the rear bumper and thru your trailer's umbilical cord.

A really low trailer battery will have a high voltage differential and absorb more current. But not for long! After a few minutes the surface charge will approach charging voltage and the current will slow to a crawl. I've measured the amp hours into my battery via an amp hour meter. 10 hour drive? Maybe 10 amp hours. Same results with two different trailers and two different trucks. It's the nature of the beast.

This isn't a defect, it's how the system is designed to work (and the nature of flooded batteries).

This same effect is present on boats that use automobile alternators and batteries for starting and "house" battery. Motoring all day long brings the starting battery back (it only used an amp hour to start the engines) but the house battery is often still low. There are special marine alternators with 3-stage chargers built in available. These will provide much higher voltages -- and consequently higher current.

Anyway, the relay in question only switches the Aux 12v connector at the rear of the truck.

-- Chuck