TO FUSE OR NOT TO FUSE ? QUESTION FROM INCOMPETENT ELECTRICIAN

[PT81PlymouthPickup]

When I wired my truck many years ago, I didn't know what I was doing (still don't!) but I struggled through it the best I could prior any internet help. Anyhow, everything worked fine. I installed an original style harness from Harnesses Unlimited. A couple years after that, I installed a heater, signal lights, radio, and converted to 12v system including a circa 1970 Mopar alternator. I was able to get everything to work well and still does. Lately however, I've been worried about the lack of fuses in several circuits. When I made the upgrades, I did cobble together a fuse block to protect signal lights, brake lights, and radio. The rest of the system (headlights, heater, horn, charging system) is unprotected. Looking at some diagrams I had used for the charging system, they don't show any fuse? I'm guessing the fusible links used during that era protected it? There is a 30 amp fuse on the back of the amp gauge? I'm considering adding some inline fuses, but not sure where to place them or if it's worth the effort? Anybody dealt with this and how did you do it?

[Tooljunkie]

I used a 10 circuit fuse block. Not enough fuses in my opinion.. 12 maybe. 
i used fuse links for the charge circuit as they will take loads better than a fuse. Could use a 70’s dodge truck as a reference guide. 

[1949 Wraith]

Are you using the original light switch and does it have a fuse on it like my '42?

Looks like you have a blank spot on your fuse panel for the heater if you want and you could put an inline fuse at the join of the horn wire.

 

Since it is a 6 volt era wire kit all the wires will have no issue with the 12 volts, but I put some relays to help protect the light switches when I converted my '51 to 12 volts.

[PT81PlymouthPickup]

31 minutes ago, 1949 Wraith said:

Are you using the original light switch and does it have a fuse on it like my '42?

Looks like you have a blank spot on your fuse panel for the heater if you want and you could put an inline fuse at the join of the horn wire.

 

Since it is a 6 volt era wire kit all the wires will have no issue with the 12 volts, but I put some relays to help protect the light switches when I converted my '51 to 12 volts.

Thank you Wraith! Yes, I have an original switch installed. I did not remember there were fused light switches a for our era (39-47) trucks. That would solve my concerns without splicing into the harness, which I really don't want to do. I just went through a box of old electrical stuff I had saved. To my surprise found a switch like you described. Probably one I picked from scrap yard? Looks like it will work with little effort. Not sure what to do with the extra wire for the light in my switch? Do you think 42 was the first year to use the fused switch? I had a good laugh when I examined the old fused switch. Someone had replaced the fuse with a piece of steel brake line. Lols! Any idea of what amp fuse was used? Might you or anybody reading this have a part number for the switch with fuse? Or know where I might purchase an NOS one? Sorry for all the questions.

 

After examining more closely I discovered I forgot that I had already installed an inline fuse for the heater. A fused switch will solve the light circuit and adding a fuse for the horn is simple enough.

[1949 Wraith]

Does your '39 have a jewel in the light switch for high beam indication? That is what my '38 sedan has.

[PT81PlymouthPickup]

3 hours ago, 1949 Wraith said:

Does your '39 have a jewel in the light switch for high beam indication? That is what my '38 sedan has.

No jewel, but it has the bulb which works. lights up when I pull headlights on and brighter when I engage high beams. Is that how yours works? I can only tell it's working by removing the socket and holding it to a ground. Obviously I have the wrong combination of switch to knob. The switch's stem is hollow to allow light to pass. I really don't know what switch or knob was in there originally? The harness I purchased for the 1939 had the socket for the bulb. I don't care about the high beam indicator. I'm more concerned with protecting the wiring. The old switch that I found yesterday looks identical to a switch listed on eBay for a 1940 Mopar truck. NORS made by Cole Hersee?

https://www.ebay.com/itm/144202974371?hash=item21932ac4a3:g:09YAAOSw7JVhQmIR 

Not sure if they were quality? I haven't been able to find a Mopar part number for 1940 Dodge or Plymouth pickups.

[1949 Wraith]

Your light knob is the same as my '42. The switch with the bullb in it is the same as amy '38 Dodge sedan the stem of the switch is hollow and the bulb lights up and a jewel on the knob lights up for high beam.

 

My '41 Chrysler coupe and '42 Fargo have a jewel in the dash above the speedo for high beam indicator.

 

Your used switch looks like the right one. Take it apart by prying the tabs on top and clean it up they are very robust.

[PT81PlymouthPickup]

10 hours ago, 1949 Wraith said:

Your light knob is the same as my '42. The switch with the bullb in it is the same as amy '38 Dodge sedan the stem of the switch is hollow and the bulb lights up and a jewel on the knob lights up for high beam.

 

My '41 Chrysler coupe and '42 Fargo have a jewel in the dash above the speedo for high beam indicator.

 

Your used switch looks like the right one. Take it apart by prying the tabs on top and clean it up they are very robust.

Yes, I think my used switch may or may not have been original to my truck, but it certainly looks like it would be the easiest way to protect the headlight wiring. I too thought about taking the switch apart, but it's very corroded. All of the original plating is gone and rusted. If I can find an NOS one would be preferable. I'm not sure how to identify the applications for this switch and corresponding part number? The aftermarket one I found looks right, but I'm always suspect about aftermarket stuff quality.

[1949 Wraith]

Take the spare one with the fuse on it apart. They have brass contacts inside and are very nicely made. Brass grinding wheel will clean the outside. Nothing toi loose if you are not going to use it.

 

The one on E-bay looks good. But it might be missing the top of the fuse holder, but should be the same as your spare,

[PT81PlymouthPickup]

I am totally confused. Senility is creeping in.   I wired this truck about 25 years ago. I can't remember much about it now.   Anyhow, after checking some parts books, the switch listed for the 1939 PT81 was 591181 which is the one I have installed. It does not have a fuse. I just read in another Mopar manual that the a fuse on the back of the amp gauge protects the lighting circuits? I'm going to pull that one tonight and see what stops working. I'm guessing if a short occurs anywhere in the lighting circuit, all the lights will fail? Would be preferable to have stoplights and signals on separate circuit? If the fuse at the ammeter does in fact protect the lighting circuits, at minimum I can easily add a fuse for the horn and possibly a fusible link for the charging circuit. I'm wondering if the fuse at the ammeter also protects the charging system?

 

I consider myself fair at mechanic endeavors, but this electrical stuff hurts my head!   I'm on a mission to figure this out! This quest all started by me wanting to document and list the location of, and amp ratings of fuses. When I kick the bucket, at least my son or the next owner might have a clue?  Lols!

[lostviking]

In the 46 trucks, there is a 30A fuse on the rear of the light switch. Everything is pretty much fed through leads coming from this switch, so it protects (the fuse) everything. BUT, when it goes, so does everything.

 

If you are not building a points show truck, I would add a fuse box, and run every lead coming from where ever the central point of feed for your truck, in my case the light switch, through an appropriate sized fuse for that circuit. So, if you lose anything but the ignition, you can still get home, or to somewhere safe.

 

LV

Edited June 11, 2022 by lostviking

[wings515]

It's been a while since I was on this site and I was very surprised to see your PT81.  It is a duplicate of mine with respect to the color combination and I live about 100 miles North of you near Scranton.  As to fusing, I did install a fuse block and separated the circuits with 4 or 5 different fuses.  As you stated this was a while ago and I do not recall my initial separation idea.  I guess I should document what each fuse does, maybe later this week.  In any case my light switch did NOT have a lighted knob but did have a fuse mounting on the back.  This fuse was used to protect all the lighting circuits but the horn and heater blower were not in the protected circuits.  I also replaced the vacuum wiper motors with electrics so I know one of the fused locations on the block is for these motors.

I kept the 6 volt positive ground and found an alternator to replace the original generator.

I am investigating the Pertronix electronic ignition but in addition to the module, you have to change the spark plug wires to suppressor type.  The module for 6 volt Positive ground is $130 plus around $40-$50 for wires.  I am trying to justify the expense.

If you want me to diagram how I divided the circuits I'll spend the time to investigate further.  

Just remembered I also changed the headlight bulbs to Halogen.  Thinking these might have a higher current load than the originals, I added a relay for these controlled by the headlight switch.

BTW, I am a retired EE so electrical issues are no problem for me.

Regards,

Dan Kahn

[PT81PlymouthPickup]

19 hours ago, wings515 said:

It's been a while since I was on this site and I was very surprised to see your PT81.  It is a duplicate of mine with respect to the color combination and I live about 100 miles North of you near Scranton.  As to fusing, I did install a fuse block and separated the circuits with 4 or 5 different fuses.  As you stated this was a while ago and I do not recall my initial separation idea.  I guess I should document what each fuse does, maybe later this week.  In any case my light switch did NOT have a lighted knob but did have a fuse mounting on the back.  This fuse was used to protect all the lighting circuits but the horn and heater blower were not in the protected circuits.  I also replaced the vacuum wiper motors with electrics so I know one of the fused locations on the block is for these motors.

I kept the 6 volt positive ground and found an alternator to replace the original generator.

I am investigating the Pertronix electronic ignition but in addition to the module, you have to change the spark plug wires to suppressor type.  The module for 6 volt Positive ground is $130 plus around $40-$50 for wires.  I am trying to justify the expense.

If you want me to diagram how I divided the circuits I'll spend the time to investigate further.  

Just remembered I also changed the headlight bulbs to Halogen.  Thinking these might have a higher current load than the originals, I added a relay for these controlled by the headlight switch.

BTW, I am a retired EE so electrical issues are no problem for me.

Regards,

Dan Kahn

Hi Dan, Thanks for your response and offer! But no need to diagram your circuits. From my recent quasi research the 39 PT81s originally should have a 30amp lighting fuse on the back of the amp gauge and a high beam indicator in the switch knob. Strange that yours is different?  The stem of the light switch is hollow, allowing illumination from the bulb to project to a jewel in the knob. My switch has the bulb, but I don't have the jeweled knob. In fact, I've never seen one. Only one of three terminals on the amp gauge is fused. I just discovered I had powered my light switch from a not-fused terminal. A mistake I made many years ago. I've been driving it trouble free for many years with totally unfused lights, and horn. (Fire waiting to happen?) I did at a later date add a radio, heater, and signal lights which I did install individual fusing for each of those. So, my current thinking is to switch the lead powering my light switch to the fused terminal on the amp gauge, and add a inline fuse for the horn. Like you, I also switched out my generator for an alternator. I installed a 1970's era Mopar charging system with an electronic regulator. I did however switch to 12v negative ground system. It too has worked perfectly for many years, but I'm thinking now that I should add a fusible link to protect this circuit?  Does that make sense? And as for the headlamps, also like you, I installed halogen light bulbs. I have no idea about current load, but they have been working fine without any relays. I'm currently trying to figure out how the high / low beam switch works. Since the headlamps are only one filament bulbs, I'm guessing there must be some sort of resistor in the dimmer switch to reduce the full current to the lamps for low beam? 

 

I installed a Pertronix module in my truck a couple years ago. It works well, but if you want to do it right, the install was not quite as easy as they would have you believe. I ended up pulling the distributor and modifying the housing by adding a hole and grommet to get the wire through in a better spot for proper strain relief.

[PT81PlymouthPickup]

After examining more closely, I think I may have a solution?  In my diagram below the amp gauge on the left is how I had wired it years ago I believe incorrectly. The one on the right is how I think it should be wired according to the Mopar wiring diagram. But, it originally had just a cut-out relay on the generator. So, if I hook it up like the gauge on the right I will be running the newer charging system thru the fused circuit. My harness has the generator wire, horn, and headlamp switch all on one connector. I could change just the alternator wire from the fused terminal and connect it to a non fused one. Just not sure best way to go? I can't imagine it would be smart to leave it this way. If a short occurred in the lighting circuit then the alternator would not charge. Advice welcomed!