Lesson Learned About Electrical Problems

JACA · 04-16-2021, 09:00 PM

I've spent the last few weeks getting my coach and toad ready for a long trip (3 months/8,000 miles). As I connected the electrical cable between the coach and toad, and checked all the lights in the Jeep, I noticed that the turn signals, hazard lights, and brake lights did not work. After disconnecting the cable, I noticed that the lights at the rear of my coach were not working. The only lights still working at the rear of the coach were the running lights (headlight on) and reverse lights. After disconnecting the 7 pin connector, and checking the 7 pin socket in the RV, my tester showed that the pins for left/right turn signals and brake were not active. So, I eliminated the possibility that the problem was in the Jeep or in the connector cable.

Then, I spent the next few days carefully looking at every wire connection, bulb, fuse, and relay that might have anything to do with the rear turn signals, hazard, and brake lights. After crawling around under the coach and unwrapping and checking multiple wire connectors and splices leading away from the rear lights, I started pulling and checking fuses. I looked at each fuse to ensure it wasn't blown. Then, I checked all the relays. I could not find the source of the problem.

I finally got exasperated, knowing that electrical problems like this can be like chasing gremlins, especially on a diesel truck that uses a multiplex wiring system. So, I called a local truck repair shop, and made an appointment to drop it off next week. After I explained the problem to the service technician, he said that the diagnosis and repair could take several days to weeks or more, depending on what they find. He also said they would have to bill by the hour, and those electrical problems can get really expensive.

So, today, after dreading the thought of taking my coach to a repair shop and getting a huge repair bill, I decided I would check the fuses one more time. Keep in mind there are LOTS of fuses in different places on a Freightliner chassis. This time, instead of just pulling and looking at the fuses, I decided to use a digital multimeter/ohmmeter on each fuse to make sure it was good. Inside the Power Distribution Module, I found one 15 amp fuse that did not work. Even though the fuse element appeared to be intact, my ohmmeter showed resistance. BTW, this bad fuse was for the CB radio (which I never used, so wouldn't know if the fuse was blown). So, I replaced the fuse.

Then, as I went to turn the truck on, I honestly did not believe that this could have fixed my problem, mainly since this was a radio fuse and had nothing to do with my rear turn signals and brake lights. So, I tested my turn signals, brake lights, and hazards - and lo and behold, now everything works!

Two lessons I learned from this experience:

1. If you have an electrical problem, don't just look at the relevant fuse to see if it looks good - test it with an ohmmeter. (BTW, you don't need to pull the fuses; just touch your wire lead pins to the exposed metal at the top of the fuses).

2. On a multiplex wiring system, don't assume that unrelated electrical lights, devices, systems cannot be on the same circuit. My CB/radio was on the same circuit as my rear turn signals/hazard/brake lights.

**Bama Rambler** · 04-17-2021, 06:23 AM

I always recommend actually testing fuses instead of just looking at them. I've seen many fuses that "Look" good but be blown. Even the old glass tube fuses will fool you.

5picker · 04-17-2021, 07:59 AM

I agree with Bama Rambler, always test with a meter.

I hope your problem is really fixed.
You mentioned the fuse tested with some 'resistance'. A good fuse will have continuity. (your word for resistance?)

You mentioned not having to remove the fuse... if you were checking continuity of the fuse with it still plugged into the fuse block that is incorrect.

You can check voltage through the fuse without removal by touching the black meter lead to ground and then the red to one side of the fuse and then the other. If you have 12v on both sides, the fuse is good..

If you try to check continuity buy touching your meter probes to the tabs on the fuse with it still plugged into the fuse block, you can be fooled by the reading as you'll likely get an erroneous indication from backfeed through the wiring.

Keep in the back of your mind the location of the area in the fuse block where you may have disturbed a connection whilst testing. You may need to revisit that if it quits again.

JACA · 04-17-2021, 08:56 AM

5picker, when I found the bad fuse, I first tested it before pulling it, as I was testing many fuses, one after another. Then I pulled it, and tested the metal tabs, and the ohmmeter did not show "000.0".

I will definitely keep an eye on the lights and fuse, since "something" caused this fuse to fail, and that condition may still exist.

Thanks for your recommendations.

TGR · 04-17-2021, 12:28 PM

That was a very interesting story that will surely drive home the point to "always check the fuse electrically." Thanks for sharing.

Something that surprises me is that the fuse was labeled as the CB fuse. I would expect two fuses, one for the left and one for the right. I wonder if there isn't something more going on.

JACA · 04-17-2021, 12:51 PM

Quote:

Originally Posted by TGR

That was a very interesting story that will surely drive home the point to "always check the fuse electrically." Thanks for sharing.

Something that surprises me is that the fuse was labeled as the CB fuse. I would expect two fuses, one for the left and one for the right. I wonder if there isn't something more going on.

TGR, I wondered the same thing. However, I have to keep in mind that I am not the original owner, and the previous owner apparently installed a number of aftermarket electronic devices that added wiring to the original PDM and main fusebox.

To double-check what I did, I just went back in and removed the fuse from the CB/radio slot, and tried my left/right turn signals, brake lights, etc. They did NOT work - just like the original problem I had. After replacing the fuse, and checking again, all the lights work properly.

I'm certainly not an electrician, but for whatever reason, the replacement of this one fuse fixed my problem.

awellis3 · 04-17-2021, 02:02 PM

Quote:

Originally Posted by JACA

I honestly did not believe that this could have fixed my problem, mainly since this was a radio fuse and had nothing to do with my rear turn signals and brake lights.

In all my years as an auto mechanic, I never saw a 15A radio fuse. Three amps, maybe 5, but never a 15A. Are you calling this a "radio fuse" because of its factory labeling? If so, relabel to read what it really powers. By the way, are your brake, 4-way and turn signal all the same filament(s) on a two-filament bulb? I bet they are and that's their fuse.

**Bama Rambler** · 04-17-2021, 03:21 PM

Since this is a motor home and toad issue, some units use a light converter to power the toad lights. That fuse circuit could have been tapped into to power the converter, so when the fuse blew the converter lost power and therefore the lights quit working.

Ryan-OH · 04-17-2021, 03:49 PM

Talk about frustrating; I've had fuses look and test good but didn't work. When the load was applied the fuse wouldn't hold the amp draw and open but not blow. Now I know why there are hole on top of the blade fuses.

JACA · 04-17-2021, 04:46 PM

Quote:

Originally Posted by awellis3

In all my years as an auto mechanic, I never saw a 15A radio fuse. Three amps, maybe 5, but never a 15A. Are you calling this a "radio fuse" because of its factory labeling? If so, relabel to read what it really powers. By the way, are your brake, 4-way and turn signal all the same filament(s) on a two-filament bulb? I bet they are and that's their fuse.

awellis3, I'm not sure about the bulb filaments, but I assume you're right. Yes, I referred to the fuse as the "radio" only because that's what the factory label said on the fuse panel. I have now labeled it correctly. And, after more diagnosis, I now know the CB/radio is NOT on that fuse at all - it's actually on a fuse that's labeled "Solar Panels". But I don't have any solar panels...

This is the problem with buying a used coach - you don't know exactly what the previous owner(s) may have done or why.

dmctlc · 04-17-2021, 06:58 PM

Quote:

Originally Posted by JACA

awellis3, I'm not sure about the bulb filaments, but I assume you're right. Yes, I referred to the fuse as the "radio" only because that's what the factory label said on the fuse panel. I have now labeled it correctly. And, after more diagnosis, I now know the CB/radio is NOT on that fuse at all - it's actually on a fuse that's labeled "Solar Panels". But I don't have any solar panels...

This is the problem with buying a used coach - you don't know exactly what the previous owner(s) may have done or why.

I agree a lot of what was said in this thread. A lot of good advice for those not that handy with a mutli-meter it drives home why you might want to have one and learn how to use it. If you had ended up not going back and checking it and it ended up in the shop think of the bill they could've generate perhaps by doing the same thing you originally did. Great job but I got to ask. Did you hook up the toad and check that out completely hooked up and running? If not before your trip it might be a good idea. Like mentioned fuses usually don't blow because they're old. Please let us know how you make out. Thanks

Walholler · 04-17-2021, 07:04 PM

Here is another thought on troubleshooting any type of problem, check the simple items first.

JACA · 04-17-2021, 07:35 PM

Quote:

Originally Posted by dmctlc

I agree a lot of what was said in this thread. A lot of good advice for those not that handy with a mutli-meter it drives home why you might want to have one and learn how to use it. If you had ended up not going back and checking it and it ended up in the shop think of the bill they could've generate perhaps by doing the same thing you originally did. Great job but I got to ask. Did you hook up the toad and check that out completely hooked up and running? If not before your trip it might be a good idea. Like mentioned fuses usually don't blow because they're old. Please let us know how you make out. Thanks

Yes, after I replaced the fuse, and everything on the coach worked properly...then I connected the Jeep for towing, and made sure that everything on the Jeep worked properly. I even drove the coach with toad connected for a few miles away from home, and it's all good now.

So, I do agree with you that fuses don't normally blow for no reason, or because they get old. Remember, the fuse in question did not actually "blow" - the fuse element was still intact. Nevertheless, it wasn't working according to the reading on my multimeter.

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