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Author Topic:   Trailer question
Mako posted 06-02-2000 10:08 PM ET (US)   Profile for Mako   Send Email to Mako  
Everytime I put my truck into reverse, it blows the fuse to my dashboard gauges. I've got a '93 Toyota 22RE and a Reese wiring harness. Anyone else experience this problem? Any suggestions? I'm running out of fuses. And yes, I've intalled 'bigger' fuses already to no avail.
jimh posted 06-03-2000 12:56 AM ET (US)     Profile for jimh  Send Email to jimh     
DC electrical problems are sometimes harder to solve than one would think.

Does the fuse blow:
--only then the trailer wiring is attached
--even if the trailer wiring is not connected

This will help localize where the fault is.

You mention that you have increased the capacity of the fuse and it still blows; does the fuse blow immediately, as if there were a direct short across it? Often you can tell something about the current that blew the fuse by the appearance of the fuse element. If is is just sagging and broken, a slight over-current has blown it. If it is blackened and completely gone, a very high current overload (perhaps a dead short of the battery) has blown it.


Mako posted 06-03-2000 01:08 PM ET (US)     Profile for Mako  Send Email to Mako     
The fuse blows only when I have a trailer attached, whether it's my own trailer or a U-Haul. Everything works well until I need reverse. The 10A fuse is slightly blackened and the 15A is neatly severed.
dfmcintyre posted 06-03-2000 01:59 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
Mako -

It sounds like the backup light circuit (which may be on the dashboard gauge circuit, check your owners manual) at your harness is crosswired.

Have you troubleshot with a circuit tester?


Mako posted 06-04-2000 05:27 PM ET (US)     Profile for Mako  Send Email to Mako     
An excellent point, Don. I'll try bypassing the wiring harness with a direct splice to the reverse lights. That way, the trailer hookup won't be a factor. I'll let you know after I take the boat out next Thursday.
dfmcintyre posted 06-04-2000 08:44 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
Mako -

In the meantime, pickup from an automotive supply store, a basic circuit tester. In my electric kit, I've got one of those thingies that look like a screwdriver with a sharp point instead of a driver bit, with a lightbulb on the inside of the handle and a length of wire coming out of the handle to an alligator clip. Handy little thing to have. Pickup one and try this:

Clip the clip to the frame, and check each connection at the plug. You should have a ground (which usually is the one connector thats molded different then the rest). That one should not light.

Running lights - that one will light while the lights are one, you'll need to have at least the parking lights on.

Left and Right turn signals - just make sure that only one is blinking

Brake lights - have someone press on the pedal, or activate your four way flasher... same circuit.

Most trailer plugs don't have a reverse wired into it, as having a trailer with backup lights is very rare.

If I was a guessing man, I'd say that the backup circuit is feeding into your dashlights, that might be on the same circuit as one of the above.

Best - Don

jimh posted 06-04-2000 09:57 PM ET (US)     Profile for jimh  Send Email to jimh     
--everything works until you hit reverse, and
--the fuse blows only when the trailer is connected,

Then: the wire in the harness that is going to the trailer that expects to be connected to the back-up lights and or the brake reverse release is carrying too much current for the fuse.

There could be two likely causes of this:
--there is a solenoid on the trailer brakes that draws too much current and is blowing the fuse, or,

--there is an error in the wiring and this wire is grounded to the trailer chassis, causing the circuit to be a direct short and blowing the fuse.

For better trouble shooting it is best to use a very accurate digital ohmmeter that can tell the difference between a cold bulb filament with a resistance of only an ohm or two and a direct short to chassis which may have a resistance of about an ohm from the wiring, etc.

The lamp resistance increases when it warms up as the current flows through it, so the hot resistance of the lamp is higher and does not blow the fuse.


triblet posted 06-05-2000 11:05 AM ET (US)     Profile for triblet  Send Email to triblet     
Cause two is unlikely because it happened
with two different trailers.

Cause three: the plug on your truck is
wired such that the backup light line
goes to the pin that the trailers are
expecting to have ground on. This shorts
back to the truck through the trailer hitch.

Quick test. Plug in the trailer without
hitching. What happens?

Do the other trailer lights work?

And what's fishy is that the U-haul blew
it. U-hauls normally have a four pin
connector (left, right, brake, ground)
and don't care about backup lights.

Chuck Tribolet

triblet posted 06-05-2000 11:09 AM ET (US)     Profile for triblet  Send Email to triblet     
And go back to the original fuse size.

Toyota picked the fuse size based on what
the wiring can handle. Automotive wiring
these days is sized as small as possible to
keep weight and cost down.

Chuck Tribolet

Walt Steffens posted 06-05-2000 06:05 PM ET (US)     Profile for Walt Steffens  Send Email to Walt Steffens     
Mako: Definately do what Triblet said and go back to the original fuse size. You are risking at the least an electrical fire and at worse case loss of your vechicl as electrictal fires start most often in hidden places.

You say you have a Reese wire harness, is it the big round (1.5" dia)5 connector unit used for travel trailers? Or a flat four connector type? Was it spiced in or was it installed by spitting an existing harness plug in the rear area of the truck and installing a "T" type plug ?

If it was spliced in I would guess that someone tapped the back up light curcut into the trailer ground terminal thinking it was a vechile ground (probibly a white wire) and everything is grounding back through the ball of the hitch untill you hit the back up light switch on the transmission. If it is a plug in to the existing wire harness you probibly got the wrong harness pig tail.
Best. WLS

Mako posted 06-06-2000 08:45 PM ET (US)     Profile for Mako  Send Email to Mako     
Okay, I've gotta check the forum more often... The connector is a four-pin flat with a plug-in harness. The trailer is only for a 13', so there's no brakes (same deal on the U-Haul--could this be the problem?). I haven't been able to hook it up lately, so I don't know if it blows when it's not hitched. The trailer lights work even after the fuse blows, and the turn signals blink on the correct sides. I'll go back to the 10A, but I need to go to AutoZone first!
dfmcintyre posted 06-07-2000 06:02 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     

First go to AutoZone and buy:

1 - Some fuzes of correct size
2 - probe type testor that I mentioned

It does not matter if the trailer has brakes or not. It WILL have brake lights however.

The four pin connector is normal. Many (not necessarily yours, it might depend on model) vehicles are set up as follows:

white wire is ground.
Brown wire is running lights
yellow wire is left stop and turn
green wire is right stop and turn

With the probe, you can at easily check three of the connections.


Mako posted 06-08-2000 12:53 AM ET (US)     Profile for Mako  Send Email to Mako     
Okay, here's what I ended up doing. First I went back to the stock fuse size.
For some reason, there's 5 wires going in to the four-pin plug. On the Reese harness, the power to the reverse light is split into both the vehicle harness and the trailer harness. Since my trailer has no brakes to override in reverse, power is supplied to SOMETHING on the trailer that blows the fuse (a short circuit, I presume). Mind you, I almost failed physics because I couldn't grasp electricity. So I cut the reverse power to the trailer harness. The dash was still alive on the truck after using reverse. I then hooked up the trailer and put the truck into reverse and the dash was STILL okay. Problem solved, but I don't really understand why...
jimh posted 06-11-2000 04:52 PM ET (US)     Profile for jimh  Send Email to jimh     
Does you car have the Euro-style turn signals, that is, a separate yellow indicator in the tail-lights for turns?

Most American cars use the same lamp for brake-indicator and turn-indicator, as does the trailer.

The problem comes when trying to attach a trailer with only three circuits, i.e.,

1) running lights,
2) left-turn/brakes,
3) right-turn/brakes,

to a car with four circuits:

1) running lights
2) brake lights
3) left-turn
4) right turn

That is the whole raison d'etre of the "adaptor". It most likely contains just a few diodes (which cost pennies) to steer the voltages to the proper lamps while keeping them isolated.

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