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Dash lights & tail light fuse blowing, what to check?

4K views 39 replies 10 participants last post by  jdsaengine 
#1 ·
I have a 32 hot rod that I purchased, so I did not do the wiring. After 4yr of ownership the 20A fuse for the dash lights and rear taillights now blow whenever I turn on the headlight switch (headlights still work) What would be a good starting point for finding the problem, and how to check for the problem or what piece of electrical diagnostic equipment would I need. Right now all I have is a gauge for checking voltage. Thanks for any help, just hate to tear the car apart looking for a wire touching metal, but that may be the case.
 
#3 ·
Your best bet is to just start looking for pinched or worn through wires. If it were me i would look at the wires to the tail lights first, they are the longest runs and are exposed to the elements.
If you cant find anything you need to isolate the wiring to at least eliminate some things. Without seeing the car it is difficult to say what the elimination process would be but i would cut the tail light feeds close to the source and check them both to see if one is shorted to ground. Once you find which wire is shorted you can either trace it to find the short or just run a new one. The most common place for shorts is where the wire goes through the body or frame. Unless you have some real fancy equipment you will need to find the issue visually.
 
#5 ·
do you know what head light switch was used.. or can you disconnect the battery and remove it.. photograph it.. post it..


a question while you are thinking about this..

when you rotate the headlight switch knob.. did the dash board illumination lights change brightness indicating you have a big wire wound resistor built into the headlight switch..

if so.. you might be able to unplug the head light switch and check the individual circuits from there .. it all depends on how it was wired up..

this is a late 60s GM headlight switch.. i am just showing that the headlight power and headlight output are usually separate from the tail light and dash bulbs. there are dozens of possible headlight switches that might have been used. each one slightly different. but if you look carefully.. disconnecting the instrument wire will let you know if the tail light circuit is blowing..

you can use your ohm meter.. to check the wiring.. there have been failures in the resistor block that spins of it shorting out in the switch also..
 

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#6 ·
Thanks guys for quick response

first. Only gauge lights and tail lights are out. But. . .tail lights work when on emerg. flasher and when braking.

the headlight switch is under the dash and a pull down type then you can rotate it to dim the dash lights. this switch turns on the headlights fine right now.

So. . .would some sort of short at the gauge cluster be more likely. getting the cluster out would be a big PITA, but other than the switch it self being a problem.

I'll get a pic of the light switch for reference.
 
#8 ·
It's possible that something foreign could have found it's way onto the headlight switch shorting a couple of contacts together. The first place I'd look is the dash lights and the switch.
I had a buddy a few years ago that had a 69 Ford Ranger and the dash lights would come on, dimly, when he applied the brakes, so strange things can happen.
 
#9 ·
The switch would have to have a short to ground somewhere and that is much less likely than a shorted wire. As a general rule the headlight switch does not have a ground wire attached to it and does not require a ground. It only switches the hot leg to the appropriate circuits.
As I said originally try to separate the wiring that goes to the tail lights from the dash lights. This should make it fairly easy to narrow down where the issue is.
 
#10 · (Edited)
here is a pic of the headlight switch as promised. As we all talk this thru, I'm thinking more a short or grounded wire behind the dash cluster. For a long while now when ever I turned on the lights, all the gauge needles would jump over about an 1/8". First time it happened I about had heart failure, one min. I had 180F water temp, then turn on the lights and I have 240F temp. So maybe I have had a minor long term problem turn into a major problem
 

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#14 ·
Each gauge has a power ground signal and light terminal. If there are loose wires on teh gauges it could make them act weird. if the power or ground wire is loose it may have finally moved enough to short out. I would take a hard look at teh back of that gauge cluster
 
#15 ·
can you take a few more pictures... looking straight at the headlight switch connector.. and then looking straight in at the switch with the connector unplugged.. so i can try to identify the switch..

you may also want to set your phone up to take a picture when you say cheese.. and stick it up into the dashboard.. and start snapping random pictures in an attempt to examine the wiring..

take a picture of the fuse box also... see if there is any name or part number to possibly identify the brand and model of the wiring harness. you might want to look thru anything you got with the car.. it might have a wiring diagram in there. or at least the brand and model of the wiring harness.

you will probably want to start at the tail lights and take the lens out and extract the socket to check the wiring..

you may want to do the same to the fronts.
 
#16 ·
Me, I'd just get a new HL switch since they're cheap enough and easy to install (and rule out). Doesn't hurt to have a spare one around.

Got a schematic of the harness in the car? You could start tracing wires starting with the HL switch.

Here is a Kwik Wire harness booklet, it will give you a good idea of what goes where and help narrow things down a bit.

http://kwikwire.com/content/Kwik Wire - Instruction Book.pdf
 
#17 ·
Take out the front park light bulbs and the rear tail light bulbs. The brakes and signals work on the brighter side of the rear bulbs but the low works off that switch. Replace the fuse and pull the light switch to see if the fuse blows. That circuit literally doesn't normally go anywhere except the 4 corners of the car.
 
#19 ·
thanks all for the info. I will start figuring this out over the weekend. Probably remove taillight bulbs or disconnect wiring to the bulbs first since its easier, but I also believe its behind the gauges, Its just major hassle to remove the cluster with a CD player located below it, but at least then I might see any grounding issues with the needles jumping.

In the past I've always built my own cars (first car a T bucket in 1971) so I knew where every nut and bolt and wire was. Purchased this one about 4yr ago that was built about 15yr ago, and while it was professional built, its still sorta like an easter egg hunt to figure things out on her.
 
#20 ·
Let me help you with your easter egg hunt...

to narrow down which circuit is causing the issue.. i took your picture and folded it in my mind.. added what is probably the proper circuit names..

you will want to photograph the connector from the top. .. then unplug it..

do you know how to depin the series 56 connectors used.. by pushing the wire and terminal inwards slightly while depressing the latching tab on the flat side of the terminal then pulling it out..

i attached my modification of your image..

the red wire at the top of your picture i labeled Tailight power into switch is probably the fused power circuit that blows.. but that is not the cause..

do you see the White wire i labeled instrument cluster. that probably goes to the instrument cluster.. in many OEM wiring setups.. that does not feed the cluster directly that feeds a 3 or 5 amp cluster fuse in the fuse box. then up to the cluster.. this prevents a dashboard short from taking out your tail and parking lights.

there are 3 wires above the white wire.. a dark blue i labeled tail or front parking..

and a pair of tan wires into one terminal.. also labeled tail or front parking.

its going to be one of these circuits that is causing your tail light fuse to burn out..

if you don't have a hand held digital volt meter... you may want one of these.. Digital Multimeter - Save on this AC/DC Digital Multimeter

you will want a pair of these also. 36 in. Low Voltage Test Leads so you can connect your test probes to ground without having to hold them. frees up at least one hand.

you can set it to 200 Ohms scale and measure each of the wires directly to ground.. this will tell you which one has the dead short..

post the results by wire color..


white...

blue..

tan pair..

the tail light filaments in 1157 bulbs are around 2.0 ohms ..
the Brake and turn filaments are around 0.5 ohms..

since you are working with the tail light circuit.. you are going to be working with the 2 ohms... now.. my brain does not work well in math.. with multiple 2.0 ohm bulbs in the circuit.. will there be only 2 ohms or will there be less.. they are in parallel.. so i would expect only 2 ohms.. this would be on the TAN pair of wires or the dark blue wire..


since i don't know what bulbs are used in your instrument cluster backlighting circuit.. i don't know what kind of resistance you are going to get and i don't have a single 194 or 168 bulb near me this morning to measure. expect over 2.0 ohms. on the white wire to ground.. if you have less.. you may have a short on that circuit..

please print this text so you can follow it under the dashboard..
 

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#21 ·
Wow, that is VERY good info, thanks

I do have a multi tester, but never really used it for OHM readings more for continuity to track a bundle of wires, so this will be interesting to see if I can discover a wire short with it. Maybe I'll increase my wiring knowledge this weekend if nothing else.

thanks,

Jim S.
 
#25 ·
OK, WayneP712, I did some diagnostics this morning. removed the headlight switch, checked the ohm readings on the spade connectors of the plug, this is what I got.

Blue wire - tail or front parking 1.6 ohms
double tan wire - tail or front parking 0.9 ohms
white wire - instrument cluster 6 ohms

granted what wire goes to what is an educated guess, but the higher ohm on the white wire would be the problem ???

I pulled the tail light bulbs out of their socket, then tried the switch and fuse blew again quickly, so I guess that means those are not the problem ??

while I can see the gauge cluster backside and verified that all spades were plugged in to the various gauges so none are loose and flopping around to a ground, I'm going to try and remove the cluster to do a better check .

and to New Interiors, nope did not do anything to the car. Probably the last time I even drove in the dark and activated the lights was last summer, so its probably been a year since I used the switch.
 
#26 ·
disconnect the double tan wire from the switch connector body.. plug the rest back in and see what happens..

this will also allow you to see what works.. leaving you the issue of finding the short in the double tan wire circuit instead of 3 different possible wires..

i am taking that you know how to remove the connector from the cavity.

you will need a tiny flat screw driver.. you have to push the wire and terminal farther into the connector body so you free the latch. then insert the flat tool.. or tiny screw driver tip into the switch side of the connector body on the flat side of the terminal.. [_v_] see the small V.. it will be a flat tab sticking out below the underlines. if you were looking at the terminal..

then the fun comes.. what does the tan pair power... where does it go..

find out which bulbs don't work with the tan disconnected..
 

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#27 · (Edited)
when you plug it back in with the tan disconnected.. see if it still blows the fuse.. kinda a dangerous situation..

you can also ohm the other wires except for the reds.. the reds are power..

edit
well scratch that idea.... there are no more wires.. the grey wire is the dome light... the switch grounds that to turn the dome lights on..

if you have the switch out.. you may want to ohm between the tail light power where one of the red wires hooks up on the switch to the white wire for the instrument cluster ... that should vary.. but i don't know what the readings will be.. you may have to measure between the white terminal and the metal switch bracket also.. it it a POT... so it will have 12 volts at one end of the wire wound resistor.. ground at the other thru the mounting bracket.. and the white wire is hooked to the wiping contact ..
 
#28 ·
If it were mine...... I'd start at the light switch plug and use a test light or multi-meter and make sure the two red wires are hot.

I'd make fused jumper wire (two short pieces of wire with a male and a female terminals, plug in a 10 amp fuse into the female ends) plug one of the male ends into the red terminal and the other into each of the other wires and see what they do and which one blows the fuse..... at least then you can narrow down what to chase.
 
#29 ·
I understand your method, sorta the reverse of what was previously suggested, either get a fused jumper and see which wire blows the fuse, or unplug individual wires and see which ones don't blow the main fuse.

As an update, after spending about a hour upside down trying to get the gauge cluster out of the dash, wiggling and poking every wire, all the sudden everything is working. Took a 20 mi drive and all worked fine. Backed it into the gargage and "bink" out went the fuse. So....at least its a 90% chance that its something behind the gauges.

Wayne, thanks for the PM and phone call. I'm taking an Advil for the sore muscles from Saturday and letting her sit today then start anew. I probably will just disconnect the dash light lead in the short term so at least I have tail lights and can be legal at night. I really need to get the cluster out of the dash and that seems to require a special allen wrench just to get to the back brackets off. The panel must have been installed by someone with midget hands.
 
#31 ·
I would bet that the dash and tail lights are on the same circuit and with out physically separating them adding more fuses will not solve the problem. It's apparent, at least to my way of thinking and troubleshooting, that the wire feeding the dashlights at the dash panel is intermittently shorting to ground and blowing the fuse.
 
#33 ·
well, today I took the headlight switch out and removed the white spade connector out of the HL plug, put back together and the tail lights now work, no fuse blow, and the dash lights are deactivated. So at least that settles the where question. I did a continuity check with my meter, one probe on ground and any gauge spade connector I touched on the back of the dash cluster gave me an audible "beep" so the whole thing is grounded at the moment.

Next thing is to buy some tools to get the cluster out.
 
#34 ·
do you happen to have one of the probe video cameras... so you can fish the end up behind the cluster to see if you can spot any pinched wires..

you might be able to pop a gauge out and access it instead of having to pull the whole cluster..

edit... and what was the OHM reading on the white wire.. almost any resistance will cause the beeper to read...

last cluster i had out that kept blowing fuses.. a several bulb sockets had been knocked out.. one bulb broke and the filament support wires were touching ground sometimes.. another that fell out had the bulb come out also.. the center wire was extended thru the socket and was also shorting out. talk about a nightmare.. who ever installed that cluster had really wrapped the various bulb wires around the main harness so there was NO SLACK... that is as far as i could tell why the bulbs were yanked out of their positions..
 
#35 ·
The ohm reading on the white wire was about 1.6. The beeper on my meter was just for continuity, like looking for which wire is which on a bundle of wires. so I was grounded on any spade I touched on the back of the gauges. Yesterday when it worked for a while I did have ALL the dash lights, so at least the bulbs were all good. And as I told you yesterday, just for kicks I did put in a 30A fuse (rather than the 20A) and tried everything for about 5 seconds and all worked fine. In fact I was questioning why the 20 keeps blowing and the 30A does not if it was a really bad grounding problem, you would think it would blow a 50A fuse
 
#36 ·
It didn't blow the 30A fuse because it's not a hard ground. You could have a filament in one or more of the dash light bulbs that is partially shorted to the bulb's case, and yes it can and does happen, that will blow a 20A fuse but not a 30A. The 30A will carry more load and it will also make the associated wiring hot which is a fire hazard.
 
#38 ·
At the moment with the white wire off the head light switch, I'm going to track it back to the fuse box and from the fuse box back out to the gauge cluster and verify that its OK and not grounded and just not worry about the cluster just yet. Actually the idea of removing a couple of the easier lower gauges individually just might get me access to the bastart of a bracket screw to take the whole cluster out.

thanks guys for all the responses. I have mostly hung out in the "lounge" last couple of years and this is the first time I posted a question in the tech sections and figured it would take days for a couple of responses. I'll try to surf the tech sections more and see if I can help someone (on something other than electrical). In meantime its a work week so next weekend will try to get back to the problem. Will post whenever I get a resolution.
 
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