Instrument lights fuse blowing

MFPvette80 · 06-30-07, 09:54 PM

I've got a 1980, that has developed a nasty habit of blowing the instrument (gauge) lamp fuse (7.5A). It first showed up when I turned the headlight rheostat to activate the dome light. The dome light worked, but the instrument light fuse fried. The problem worsened quickly to a point now that whenever the headlight switch is pulled out, the same fuse blows.

I pulled the fuse and put an ohmeter on the circuit that didn't show a problem. There must be a short somewhere though. Has anyone experienced this before, and where did it lead you? Also, can the headlight switch cause this problem? It doesn't appear so from the wiring diagram, but the wisdom of this group has proved to be greater than the printed word before. Thanks.

Fast Freddie · 06-30-07, 10:20 PM

i would start by changing the headlight switch.....

AKRAY4PLAY · 06-30-07, 11:13 PM

i'm not near my car or service manuals for another 10 days, so i can't check this myself just yet. i would look at the tail light wiring for your problem. most Chevy products have the tail lights wired to the dash lights (or vise versa) so you know when the dash goes black, your rear running lights are also out. if you have a short in the tail light line, it will pop the fuse when the switch is pulled to turn on the parking lights. did you ohm the fuse block (ohm leads connected to the two places the fuse would contact) and turn on the lights?? if you have a wiring diagram, you could pull the light switch and make a jumper (with a fuse in line) to test the different electrical circuits that the switch controls. this would remove the switch from the diagnostic woes. good luck, Brian.

MFPvette80 · 07-01-07, 11:13 AM

Funny you should mention the tail light circuit Brian. When I by-pass the 7.5A instrument light fuse (with the ammeter) the 20A tail light fuse pops. It was not obvious from the wiring diagram that the circuits communicated with each other, but your comment has me re-thinking that possibility.

I'll take a look at the tail light wiring to see if anything looks bad. There's been no work done on that area of the car, so I can't re-trace steps back to something obvious. However, with the instrument light fuse out (or blown) there are no problems seen with the tail, turn signal, or brake lights (I tested this).

I will also check the resistance across the instrument light fuse connections. If it looks high (and I don't know what "high" is), I'll remove the tail light fuse to see if it changes. That should tell me something.

Thanks for your insights.

MFPvette80 · 07-01-07, 08:30 PM

I did a circuit resistance test on the instrument light loop to see if a short was present. First, the battery was disconnected and the instrument light fuse removed. I drilled out a blown fuse and attached a few inches of wire to the contacts so I could hook up my meter. With an ohm meter in-line, I turned on the headlight switch and watched the resistance. With the rheostat turned down all the way (lights out), there was an open circuit, as there should be. When I turned up the rheostat, the resistance varied from 1 to 4 ohms. Just before the point where the switch would turn on the dome light, the resistance dropped to 0.8 ohm. When the 20A tail light fuse was removed, the results were the same.

Now, Ohm's law (I=E/R) tells me that, assuming a 12 volt source, any resistance lower than 1.6 ohm will exceed the 7.5 amp fuse rating. It is clear now that the current was high enough to blow the fuse. I still wonder if the problem is the light switch or in the circuit?

I pulled the dash panel and unplugged the connector to the speedo/tach assembly. No changes in circuit resistance were noted, thus eliminating that as a source of the problem.

I suppose the smart thing is to take the advice and get a new light switch. Is there any help available on light switch replacement? The manuals I have show a great view of how to replace the switch with the dashboard floating in space. That's not my situation however. Any practical tips on how to pull the bugger out would be appreciated.

AKRAY4PLAY · 07-02-07, 12:49 AM

i have not removed my light switch, so unfortunately i can't give you any tips. i know that my knob and the attached post pulls out of my switch, i "assume" this makes it easier to pull the switch body from the back of the panel. if a new switch does not cure your ills, try pulling the running light bulbs and ohm each circuit. i have seen corroded bulb sockets short out systems and plague people to death. other than that, tracing the wires from source to termination is the only thing left. hope you get it figured out.

MFPvette80 · 07-02-07, 11:07 AM

When you say that the knob "pulls out" do you mean that literally? Is there a clip or something that locks it in place, or will a good stiff tug dislodge it?

AKRAY4PLAY · 07-02-07, 11:49 AM

yep, the knob pulls free from the switch if i pull it to far. it does not take a hard pull, the same amount of effort to turn on lights, just a longer pull and the actuator rod comes out. my switch was most likely damaged from a previous owner, but it still works. i think there is a retaining clip that is supposed to be internal to prevent the actuator rod from coming out, but i can't verify it.

bmotojoe · 07-02-07, 12:39 PM

If you remove the dash pad screws and the speedometer/tachometer cluster screws and wedge up the dash pad with something you should be able to reach between the pad and cluster. Reaching in with my left hand I removed the vacuum hoses and electrical connection plug and if I remember correctly the side of the switch towards the center of the car there is a little spring loaded button about the size of a #2 pencil eraser. When the switch knob is pushed in to the (Off Position) push in the little button all the way and pull out headlight switch. The shaft should release from the lock and pull out. Unscrew the locking nut at the front of the switch pull old switch out. Just don’t drop the nut inside dash. If you have very large hands it may be impossible to do it this way but it worked for me. It would be a good idea to park the car in the sun to heat up the dash pad so it is flexible. I let mine sit in the sun over an hour or so before starting the job.
Brian

MFPvette80 · 07-04-07, 08:50 PM

Excellent advice. Thanks for the critical information on how to remove the knob and shaft. I gave it a little tug yesterday, and it didn't budge. That's when I decided to wait for additional comments.

I already had the speedo/tach cluster out, and the dash pad pulled back. It seems possible to get enough fingers on it to do the job. The switch is on order, so next week I'll give it a shot. With the new switch in hand, I'll be able to see how the shaft lock mechanism works. I just hope this is the cause of the fuse problem.

MFPvette80 · 07-20-07, 09:28 AM

I just finished installing the new headlight switch, and then promptly blew the instrument light fuse on the first test. I guess it wasn't the switch. So just to recap, my instrument light fuse is poping when the lights are on (obviously). When I by pass the fuse with an ammeter, I pop the tail light fuse. I have had no problems with the taillight circuit unless I performed this test. With the 7.5A instrument fuse removed, all the other lights work fine. Since the taillight fuse is not popping under normal circumstances, I have to think that the problem still remains with the instrument light circuit.

I'm going to try to make some additional current measurements with a resettable fuse in-line with the ammeter this weekend.

Any ideas folks?

bmotojoe · 07-20-07, 10:14 AM

I would then try this to help isolate where it could be. 1977 through 1982 used a printed circuit board for the console gauge cluster lighting. 1978 through 1982 there is also a separate printed circuit board for the dash cluster. Not sure about 1978 and newer but my dash lighting and console gauge lighting is from the same fuse INST LPS fuse. Also my car is an automatic and there is a bulb that lights the gear selector panel area, also on the same circuit the bulb that shines on the heater control area. Disconnect one ribbon cable at a time that is connected to the printed circuit boards and see if the fuse still blows. If both ribbon cables are removed and fuse still blows then I would look into interior/exterior lighting sockets etc because something is back feeding.
Brian

MFPvette80 · 07-20-07, 10:35 AM

I started down this path already Brian. While I had the dash cluster (speedo & tach) out, I checked the resistance of the circuit and found no change. I did re-install the shift position lamp and a exterior corner lamp recently. I need to start pulling components out of the circuit to isolate what appears to be a short.

Does anyone have advice on pulling the console gauge cluster, or know if the light circuit for the cluster can be disabled in some way?

MFPvette80 · 07-20-07, 10:39 AM

One more quick question: I never did get the entire dash cluster out; just far enough to remove the connector. With the steering wheel removed, the cluster still didn't clear the steering column. How do you get it out?

bmotojoe · 07-20-07, 10:59 AM

On my 77 I had to remove the front and middle screws from the center consol side boards, pulled the radio knobs off and removed nuts on the stem of tuning and volume. There are screws into the dash pad, a screw on each side of the gauge cluster bezel towards the top going into the speedometer/tachometer dash and passengers side map pocked, also a screw on each side to the center consol at the bottom of gauge cluster bezel. As you tip the gauge cluster forward from the top the printed circuit board ribbon cable will be in view and also the heater control bulb socket and wiring pigtail. Just remove the ribbon cable from the printed circuit board to isolate the gauge lighting from the rest of the circuit.