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68 wiring help


Dec 27, 2001
virginia beach,va
68' 327/350hp 4sp, 08 Z06
HELP! The dash lights on my 68 are not working. I have followed the wiring diagram I have and everything seems in order. I have good continuity through out the wires (fuse box to gauge console) I have tried putting power to them from the connection on the fuse box marked "bat" to make them work and they will..so I'm guessing that it is something in the fuse connection for the inst panel???? Does anyone know the correct fuse for that? the fuse in there is small (i think 3 amp). Maybe a ground???? any help would be appreciated

electrical engineer in training!
Little fuses

At the bottom of you fuse block there is a SHORT fuse marked Inst LPS is it working? If so then we need to look further ( like tear out the dash)

If not replace the fuse ( and its power not fused ground)

The fuse is 2 amp ( I think )

Cig foil has a 6 amp rating ( 1 wrap )

LPS cont.

There is a small male spade lug in the fuse box to the left of the fuse. to test the panel.

The start of the circut is a batt hot line Crkt 2c
(If the tail lamp running lights work it's a good bet the the wire going up to the headlight switch is OK)

+12 goes to the headlight switch wiper (red) of the dimmer ( big nichrome wire wound resistor) the output of the resistor is a Dark Green. give this POT a good scrub and see if the lights flicker.

This goes down to the fuse panel ( 4 amp fuse )

So you should see the dimmer action on the fuse panel. There is a small male spade lug in the fuse box to the left of the fuse to test it.

The fuse output line is grey 20 gauge and ALL the illumination crkt power feed is grey. The lamps are all locally grounded.

Ta Da


Note All the colors I'm listing are out of a 73 book I'll bet the farm that the colors are the same as well as function.
Thanks Mike,

I'm going out to the garage now and give it a try. Sounds like your 73 info is close to the 68 info I have.
again thanks
hoo yah
Let us know

hoo yah

Master Blaster SIR!

No SIR's I work for a living. HAHA
OK here we go.... Fuse is good...tail lamp running lights work..checked headlight switch everything seems fine gave a tug and shake no flicker on the lights. About the only thing I can find is that I am getting no power to the inst LPS fuse, I know " NO Duh!" but I can't seem to figure out why. every other fuse "port" works. My plan is to go out and see if I can clean up the fuse holder, double check the grey wire running to it.....and @#@@@ a few times. Any help would be appreciated
Hoo yah
The source of power...

On the headlight switch the red wire IS the power feed ( next to the dark green wire )

"+12 goes to the headlight switch wiper (red) of the dimmer ( big nichrome wire wound resistor) the output of the resistor is a Dark Green. give this POT a good scrub and see if the lights flicker."

The fuse is on the OUTPUT side of the pot

So in logical order
Batt to red wire on dimmer switch assy one side of winding.

Wiper of pot to dark green wire down to fuse box 1 side of fuse

Other side of fuse to grey wire to feed ALL instrument lights

New test
Is the red wire on the headlight switch HOT?
Jump red to dark green do lights light?
measure resistance of dimmer.


The Red wire to the headlight switch is HOT
I tried the jumper wire" red to dark green" nothing happened
I am not sure how to measure the resistance of dimmer. I did do a continuity check of the green wire from switch to fuse panel and that was good. Next??

Hoo yah

The Red wire to the headlight switch is HOT

That's good

I tried the jumper wire" red to dark green" nothing happened.

That's bad!


The Dark green HAS to go down to one side of the fuse panel. Try this, take a jumper wire and go from +12 ( Batt ) to one side of the INST LPS fuse
( Pull the fuse ).. then try the other.. The one that lights the lights is the gray wire..

If the lights DO NOT light you have to measure from a gray wire ( like speedo Illumination) down to the
fuse panel, if that circut is open you COULD make a connection between any grey wire and run it dowm to the Male lug on the fuse box marked (LPS)

The dark green. Measure that for continuity,I know you checked it but do it again with the fuse out.

I am not sure how to measure the resistance of dimmer.

On the dimmer there are 3 connection points IN A ROW

.........Red.X.. X( not this outside one)
DarkGreen.X < Shaft of switch

The two outside ones (Red & N/C) are the winding of the Nichrome wire.. this should measure as a fixed resistance, I'm guessing about 50 ohms.

The center connection point is the wiper(Dark Green). So with your Ohm meter across the middle & one of the ends as you turn the knob the resistance should change

Repeat for the opposite side.

Now try the following
Reconnect the headlight switch
I'm assuming you are using an automotive style test light ( Cliplead on one end..sharp pokey thing on probe)

The cliplead goes to ground
The pokey tip goes to the Dark Green
Pull the lights to the first stop ( on the switch )
the testlight SHOULD LIGHT! and as you twist the knob it should dim.

Try it...

just got in from the garage. here's what i got:

- The fuse is good...& cleaned up the fuse block
- found out which side the green and grey wires are in the fuse block
- the dark green checks out good for continuity
- The dimmer:
- Red & N/C no resistence ( unless it is very slight)
- Dark Green/NC Yes..and when I turn the knob resistence changes.
- Red/Dark Green - nothing
- Cliplead to ground/" Pokey tip to DarkGreen pull light to first stop and testlight lights and twist knob and it dims.

Oh last thing is I tried a connection from a grey wire(good continuity) to the male lug marked LPS. NO JOY!

will try to get at it again tomorrow. Once the honey do's are done!
thanks again mike. I'm sure we'll get it.
Hoo yah

By the way ..my other half is from sunny Simi-Valley. i guess thats close to you huh.
Weird Wires, More research!

Well you've proved we have power up there & the dimmer works.

We have to chase the dark green wire to see if it makes it to the fuse box... which you said measures out for continuity.....

And if you poke into the dark green
( at the switch ) and dim.. the test light follows.

So the Inst LPS fuse.
The side of the fuse that is closest to center of the fuse panel is the Dark Green.

The other side with the lug goes to the gray.

The lights in the speedo, center console & tach get their ground from the sheet metal. Make sure that that metal IS grounded, use an ohm meter to test.

There are 2 black wires with single connectors that ground each instrument.

Look on the back, they are facing each other about where the steering shaft would be coming through.

Remove the connectors & clean em & the metal point on the instruments.

The frame ground for these items is on the drivers side of the frame above where the hood release is
and above the hole where the door open switch is, You should be able to see it if you have the instruments out.

clean & tighten this too!

Clue here
The ONLY light to have it's own ground is the one in the heater/AC control panel... Is that one dead too ?


The side of the fuse closest to the center of the Inst LPS is GREY
The other side is Dark Green

Will check the ground as you indicated
also will check the heater/AC control
Hoo yah

The schematic is inverted...oh well
the important part it the BOTH het there

Let me know


Never give up!! Just had to do a little military stuff....I should be able to work on my vette tomorrow...latest Sat...so I'll let you know aas soon as I do the things you suggested.. Thanks for hanging in there with me.
Hoo yah'

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