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Author Topic:   Trailer Lamps Blow Fuse in Tow Vehicle
Ablewis posted 06-12-2010 12:30 PM ET (US)   Profile for Ablewis   Send Email to Ablewis  
It seems electrical problems are my life recently. I am now having a problem with my trailer blowing the "Right Park Light/Right Tail Lamp/license plate Lamp/cluster" 10 amp fuse on my jeep. When this first happened, I thought it was a short on my whaler trailer. However, it just happened when I was using another boat trailer.

I have a multimeter but am not sure where to start. Obviously, I think, the short is in the jeep. However, I'm left wondering why I don't have any problems other then when I add the trailer. Is it the added load that contributes to the short problem?

jimh posted 06-12-2010 06:10 PM ET (US)     Profile for jimh  Send Email to jimh     
Modern automobile designers have reduced the electrical capacity of the vehicle stop lamp and tail lamp circuits in order to save weight, cost, and electrical capacity. The electrical load in modern vehicles is very much greater than in the past.

It may very well happen that you cannot directly connect your trailer lamps to your vehicle. You may have to use a trailer adaptor or convertor. The converter requires a feed from the battery, and it samples the vehicle running lamps, turn signal lamps, and brake lamps. It powers the trailer from its own battery feed, and controls the trailer lamps in synchronism with the vehicle.

A convertor is also required if your vehicle has separate turn lamps and brake lamps.

Jerry Townsend posted 06-12-2010 06:11 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Your problem is in the connector mounted on the vehicle - there may be something loose/broken - that makes contact between the hot and ground when the trailer is plugged in.

Remove, inspect and rewire the trailer receptacle on the vehicle. It may be the ground and power for the RH tail light are close together and perhaps structurally loose - so that cantact can be made. Consider replacing the receptacle on the vehicle. ---- Jerry/Idaho

Ablewis posted 06-12-2010 06:59 PM ET (US)     Profile for Ablewis  Send Email to Ablewis     

You may have the solution. I tinkered with the adapter that plugs into the larger trailer connector. I posted this message because I was on my way to pick up a u-haul trailer and was worried they wouldn't let me use it if the lights weren't functioning correctly (they didn't even check!). However, none of the light functions cuased the fuse to blow. So I think I still have an issue given that I blew the fuse on two different boat trailers. It may simply be a short in the adapter which I will try to replace.

K Albus posted 06-12-2010 08:36 PM ET (US)     Profile for K Albus  Send Email to K Albus     
What year and model is your Jeep? I used to have a 2002 Durango, and one time the converter which Jim H mentions went bad. This resulted in blown fuses in the vehicle. I was driving myself mad looking for a short in the trailer wiring, but there was no short to be found. I took the vehicle to a trailer shop, and the guy knew right away that it was the converter.
Ablewis posted 06-13-2010 11:29 AM ET (US)     Profile for Ablewis  Send Email to Ablewis     

It's a 2005 Jeep Liberty and the trailer harness is original equipment. It might be the power converter that Jim references but I'm going to start with the trailer Wiring Adapter that converts the 7-way round vehicle connector to a standard 4-way connector. I can't explain why I didn't experience the same problem yesterday with yet another trailer but am now assuming it is something loose or broken in this converter.

When I tested the current on the connector yesterday all functions seemed to be working fine. However, I thought I was picking up a very slight current on the right hand turn signal pole when I was using the left turn signal. I had taken off the adapter to then test the connectors on the 7-way connector and it felt like the converter was a little loose. If I were to use the multimeter to check for a short, is it the Ohm feature that I should be using? Or if I was trying to determine if the power converter was faulty, what would I be checking for?

Jerry Townsend posted 06-13-2010 03:39 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Using a multimeter to diagnose a short - use the ohm position - and move the meter's probes to "walk through a circuit". That is, the meter will measure the resistance, in ohms, of the component(s)/circuit between meter's probes.

What you are seeing - no response (zero reading) of the meter means there is no connection/continuity of the component/circuit between the probes. An infinite reading means that there is a direct short within the component/circuit. A reading giving some resistance value means there is some resistance in the component/circuit.

When you start, run a simple test - turn on your meter and set up to measure resistance. With the probes not in contact - the reading shows there is no contact -- and with the probes contacting - the meter reading indicates a direct short.

Now realize that to blow a 10 amp fuse requires a resistance of 1.2 ohms or less. Typically, our electrical equipment might draw somewhere up to 3 amp - or a resistance of 4 ohms - so that anything much above the 4 - 5 ohms suggests a problem may exist. --- Jerry/Idaho

Jerry Townsend posted 06-13-2010 10:25 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Slipped a cog - change the last paragraph reading "... anything much above the 4 - 5 ohms suggests a problem may exist. " ...

to read:

"... much BELOW the 4 - 5 ohms ..." ---- Jerry/Idaho

Ablewis posted 06-30-2010 09:36 PM ET (US)     Profile for Ablewis  Send Email to Ablewis     

It's nice to have a $9 fix once in a while. It was the converter that converts the wiring from 7 pin to four pin. I'm still not completely sure why this didn't create a problem with the one trailer, but a replacement fixed my blown fuse problem.

Tom Hemphill posted 06-30-2010 09:47 PM ET (US)     Profile for Tom Hemphill    
Thank you for following up with what turned out to be a solution to your problem. It may help others in troubleshooting their similar problems.
K Albus posted 06-30-2010 09:48 PM ET (US)     Profile for K Albus  Send Email to K Albus     
Andy, I'm glad to hear it was something simple and cheap.
jimh posted 06-30-2010 10:03 PM ET (US)     Profile for jimh  Send Email to jimh     
I do not think there is much of a conversion taking place in a device that turns a 7-pin standard trailer connection into a flat-4 trailer connection. All that happens is the connector configuration changes, and four circuits of the 7-pin connector are wired to the appropriate four pins of the flat-4 connector. There should not be any electrical changes happening in that device; it just moves the circuits from one connector configuration to another.

See my REFERENCE article on the topic:

The convertor I mentioned previously is one used to adapt vehicles which have the European-style tail lamps with four separate circuits:

--running lamps
--brake lamps
--right turn lamp
--left turn lamp

to the typical trailer lighting configuration which has only three circuits:

--running lamps
--right turn and right brake lamp
--left turn and left brake lamp

Such a convertor usually has a few components in it, and in some cases allows for a separate 12-volt circuit to supply all the current to the trailer in order to not overload the vehicle lighting circuits, which, as I mentioned, are sometimes very limited in their current handling capacity.

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