Electrical Surge with OD off

[SkunkedAgain]

Two weeks ago while towing my boat up north, I noticed my headlights getting brighter at times. Then I noticed that the dummy gauge for the battery was flickering when the headlights flickered brighter.

I pulled over and checked the battery and alternator. Results for both were normal. I continued towing and the problem persisted until I turned the overdrive back on.

I took my truck into my trusted shop yesterday but they couldn't find anything wrong, except to suggest that maybe it's an early sign of the alternator going bad. My assumption was that it was some kind of electrical short circuit. I think that I need to get the problem to replicate again but haven't been able to do it. I might need to hook up the boat and turn on the lights again.

Has anyone had a similar situation or a good guess at the cause?

[SkunkedAgain]

Nobody responded to this one but so I assume it's a not-so-common problem.

However, I've isolated the problem to only happening when my boat trailer is hooked up to my truck. Does anyone think that this could be a trailer wiring issue? I'd rather track it down myself but think that this has to do with the truck than the boat trailer.

[fishing tech]

Maybe a poor ground.

[Shack]

Yep, a short to ground concern somewheres on the trailer is my suggestion.

[Whoaru99]

A short in the trailer wiring seems a likely culprit, although I tend to think a short bad enough to dim the lights would also be bad enough to blow a fuse somewhere along the line.

[SkunkedAgain]

Thanks everyone. I'll test everything this weekend.

Although I still can't conceptualize why the short would only manifest itself when overdrive is turned off.

[Macgyver55]

One feasible scenario is that you have a poor ground (not a short) on the trailer and the lights are dimming and brightening as the load on the charging system changes. The fact that it is more pronounced when the overdrive is off, might be explained by the fact that the RPMs are higher than when overdrive is on, the alternator is more likely to be charging at its full potential and the voltage regulator is responding to the changing load by increasing or decreasing output as it changes.

My personal opinion is that a short that is bad enough to dim lights should/would have blown a fuse by now.