The Stovebolt Site Forums The Shop Area The Electrical Bay Blowing fuse - Ideas???

I have a '38 coupe with the 216 stovebolt six. I pulled it out of a barn last December and got it road ready this summer. As part of the restoration I put a new wiring harness from YNZ's in the car & everything's been smooth except for one problem. When I turn the headlights on & drive for a ways the fuse blows. It doesn't pop just as the lights are turned on as it would with a short. It let's me get about a mile from home and then fails. When the fuse blows the lights remain on which is the confusing part of this problem. If the lights are pulling enough amps to blow the fuse, once the fuse is blown they should go out???? When it blows all that happens is the ammeter goes to discharge and if the engine speed falls low enough it will die and not restart. I have double and tripple checked the wiring diagram from YNZ's and everything seems to be hooked up OK & I've not found an obvious short. Ideas???

welcome to the garage Tiny - hope this ain't makin ya bite yer nails!

where is the fuse? if it's not in the headlight circuit [lights still work], then it must be back further, like fusing the whole system, or at least multiple things including the lights, which isn't a good setup - there should be power to a central point [fuse panel] and then fuses on individual circuits so each thing can blow it's own top w/o affecting anything else - it also may just be that the fuse needs to be a higher rating

OTOH, the lights could have a problem causing them to draw waaay too much juice, which seems indicated by the discharge situation - got the wrong bulbs in? could try disconnecting one light at a time to see if it's a problem on one side

more info will prolly get more answers!

Bill

Tiny,

Did you put a headlight relay in with the harness upgrade? Did you change the bulb type lights to sealed beams (12v?). Does your rig still have a generator? When all this happens, how do you restart the car? Save the fingernails, your going to need them!

Stuart

Tiny,

Time to get the fingernails out!!! I just looked at a wiring diagram for a '38 Chevrolet passenger car and the light switch (stock) is pretty bizarre. The switch looks like it powers each front light individually. I don't know how the YNZ harness is wired, but the bottom line is, in a properly wired circuit, anything on the load side of a fuse will cease to function when the fuse is removed from the circuit (blown).

The dramatic discharge on the amp meter is telling you that current is flowing backwards to a ground. Your charging system might have a tough time overcoming this drain.

If you wanted to find the problem the hard way, you could recreate the scenario and wait for the smoke and flames...the problem would show itself!

Stuart

Well, I can think of two reasons that this might happen.

The first is if you didn't replace the headlights with 12v sealed beams, and you have bosch relays. I know, a lot of people tell you that you can still use them. The problem is that the 6 volt lights have a lower resistance, so they tend to suck a lot more current through them. And, the resistance drops when the filament heats up. As a filament's resistance drops, it starts drawing a dramatically higher amount of power. Bosch relays can "lock" in the on position when a lot of current is passed through them.

The second way it can happen is trying to mate a 6 volt positive ground system to a 12 volt negative ground system - with relays. Bosch relays can be triggered by either a 12 volt source or a ground. In your case, you might have mixed the two types of circuits by trying to keep the old headlight switch in the circuit. If wired wrong, this may have accidentally caused the circuit to fail in the ground position - and if it is hooked to the ground trigger side of the relay the lights will stay on after the fuse blows.

Vaughn,

If the light switch "switched" a headlight relay, why would the fuse blow? The pull-in coil in the relay is very small, and would be separate from the headlight load. I think all Chevrolets were negative ground. Doesn't resistance increase in a conductor as it heats up? :confused:


Stuart

Hi y'all & thanks for the feedback. I'm trying to keep the old lady as original as I can afford to do so I've made no changes to the stock wiring such as adding relays, sealed beams or upgrading to 12 volt. I have considered upgrading to 8 volts but thats a whole 'nother thread. The car has one main fuse which resides on the back of the ammeter. The passenger (feed) side of the ammeter is connected directly to the battery via the starter switch on top of the starter. Another terminal that also appears to be on the feed side of the ammeter feeds the light switch which then feeds the tail/instrument panel lights on one circuit, the headlights via the dimmer switch on another circuit, the stop lights/domelight on another circuit and the parklights on another circuit. All of those appear to be unfused as they appear to feed from the battery side of the ammeter. After crossing the fuse the ammeter has a terminal feeding the battery terminal on the voltage regulator (with a jumper going to the "town" horn) and the "hot" side of the ignition switch which then (when switched on) feeds the gas guage, the ignition terminal on the voltage regulator and ,via an armored cable, the coil. The car does have three other fuses which are not shown on the factory wiring diagram. One each on the heater and defroster switches and one on the T&C horn circuit. The horns feed directly off of the battery/starter switch. The heater/defroster switches now feed from the downstream side of the ignition switch. The car came from the factory without a heater so I have no idea where the switches originally would feed from. I didn't want them hot all the time so I put them on the switched side of the ignition switch. Now, unless you are all really sharp with antique auto circuitry you are probably shaking your heads at what I just wrote so post your e-mail addys & I'll scan & send the diagram to those who are interested.

[email protected]

I'll give it a look

Tiny,

Either I am not reading your explanation of the wiring correctly, or you version differs from the schematic I downloaded. I read that all lighting loads come off the fused terminal of the ammeter (ampmeter?).

http://chevy.tocmp.com/chevywiring/diagrams/1938CAR.PDF


Stuart

Tiny,


If you have the schematic, take a peek. Your description above indicates you have the top and bottom connection points reversed on the back of the ammeter. The Batt feed to the Gen isn't suppose to be fused. Only the load...the lights, are fused. Do you have to replace the fuse to get the car restarted?

Stuart

Tiny,

I sure hate to hog this thread, but I am real excited...in my ignorance I actually think I know what the heck is going on. If your car is wired the way you say, it looks like when you turn your lights on, the Gen output increases, blows the fuse that shouldn't be there, which cuts the battery out of the equation. This chain of events means the Gen is supplying power to the ignition system, which slowly fizzles. The lights stay on because they are tied to the battery side of the ammeter.

I am guessing to restart the rig you have to replace the fuse. Did the fuse holder attach to the ammeter as a separate unit, held on with the posts on the back of the meter? Is it in the wrong position?

I wonder if I am WAY out in left field, or even in the ball game.


Stuart

I do have to replace the fuse to restart. The fuse clips are a part of the ammeter, they aren't a separate holder. The only difference I see between my diagram and yours is the power feed to the lightswitch and output to the stoplight/domelight feeds is switched.

My diagram and yours both show that the lighting comes off of the upstream (unfused) side of the ammeter. I haven't tried to take the ammeter apart so I'm assuming the top terminal where the battery input is connected and the bottom terminal connected to the fuse clip on the same side of the ammeter are connected internally.

switched by what? the ignition switch? if they're fused at the ammeter [not ampmeter, Stuart], and have their own switched, they should not go thru the ign sw

Bill

red58,

I have written so much stuff I am not sure what you are referring to..I'm lost.

Tiny,

As I look at the wiring diagram, this is how I interpret it. The back of the ammeter (silent P) has four connections. The top two power the meter...they show current flow, charge/discharge. The bottom two are only studs for the fuse block..an attaching point..insulated from the case.

There is a fuse in that circuit. It will show up as a load on the ammeter whenever any device or lamp on the circuit is energized. If that fuse blows, nothing downstream works. You can see that if it is in the wrong circuit and blows, the coil loses power..isn't this what happens?

If the connections for + and - actually go right to the coil of the meter, the face of the gauge might be an "upside down" style. If the fuse block is factory attached, and the depiction in the schematic is accurate, then the fuse block should be on the bottom, not top. Could the gauge be in upside down in an effort to make it's face look more conventional.

This seems like pretty simple wiring...where are all the engineers when you need them?

I'm still trying to figure out why ammeter doesn't have a fricken P in it.

red58,

Have you looked at the wiring diagram? What are your thoughts.

Stuart.

Hi Bill, I'll send you the wiring diagram if you'll let me have your e-mail addy. That way you'll have the drawing in front of you. Thanks for your interest. Just select the link above to "send e-mail to tiny" and send me a note. I'll reply with the drawing attached.

Shoot! That's why I'm here asking for help because I'm NOT good with wiring. In the morning I'm going to go out and pull the fuse & use my test light to see which terminals are "fused" and which aren't. Anyway the diagram you are looking at is essentially the same the the copy of the GM factory diagram YNZ uses (and I sent you). So if everything is hooked up that way, why does the fuse pop? Bad ground drawing too much current????? I'm so confused............<G>

Tiny,

E-mail me your phone number if you think it would help.


Stuart

Stuart - Yes, in some instances the resistance can decrease. In silicon resistors, the resistance will go up, but in certain types of wire (tungsten) the increase in heat will actually decrease the resistance somewhat. Since there are no headlight relays according to the digram Tiny sent me, that isn't the problem. You also may be right about the ammeter being hooked up backwards.

I can't definitively tell from the diagram what your problem is, but I've got a pretty good guess. This is actually something I've run into before. You may have defective ammeter.

As these early ammeters get old, their construction starts to decay. They do not have a "shunt resistor" design like modern ammeters do - so ALL the current output from the generator goes through the ammeter. The early designs didn't include safety devices like fusable links. Anyway, the resistance of the ammeter will change as it gets older, possibly shorting out. When it shorts out, it tends to fry the wiring harness. The reason you see it happen while the headlights are on is because the generator has to push a lot more current to supply that headlight load - all of which has to go through the ammeter. The ammeter heats up, starts to short out - and more and more current is pushed through the ammeter from the generator trying to keep up. If wired incorrectly, the fuse may blow but the lights will still remain on. If the car shuts off, the current ceases to flow - and the ammeter cools down again (until the lights are turned back on).

I suggest that you try eliminating the ammeter from the circuit for a short while. You can run a test in this configuration (including a seperate fuse for the headlight circuit) to see if that is the problem.

I also suggest that you consider including a fusable link in your wiring harness to keep you from burning it up. I have seen a number of trucks with this type of circuit that were parked simply because the ammeter fried the wiring. No one knew why it had burnt up - and replacing the wiring just fried the new wiring. So the truck was just parked and/or parted out.

It took a while for the engineers at GM to discover the fault in their design - after that they never used that style of circuit again. They used a different ammeter design and incorporated fusable links into their circuits.

I forgot to tell you that when you eliminate the ammeter, you need to jumper the two terminals together otherwise the rest of the circuit won't be energized. It is something that would be obvious to anyone with any circuit analysis background, sometimes I forget that not everyone has that knowledge.

Thanks for the ideas guys. The last time I was under the dash was with the seat out. I tried to get my well fed self under the dash this morning to check the ammeter and couldn't get under far enough to see or do anything plus it was COLD! I now have some leads to follow up so thanks again.