I am a newbie with a 1978 F150 XLT 4X4 that I just upgraded the radio system in. Now I have no instrument cluster lights or A/C control lights. All other lights controlled by the hheadlight switch are working. Whenever i put a fuse in there is power to the top of the fuse and not the bottom. The fuse then blows as soon as it is touched with the test light. On the fuse panel without a fuse there is power to the top of the fuse slot but not the bottom. I have a very limited electrical background and am kind of at a loss of what could be causing it. Any help would be appreciated.

 

Sounds like you have a short somewhere. Are you using the right size fuse? Having power at the top and not the bottom is normal. The fuse is what sends the power from the top to the bottom. Either the fuse is too small to handle the current or something is grounding out.

 

First & Foremost "Welcome Aboard".
I also would check the prongs "clip" that holds the fuse into the fuse panel; over time some of these clips lose their contact strenght / means due to becoming worn etc. I've had to pull my panel off the fire wall (daughter's 78 F100) and replace a clip or two by either riveting a new clip or using a sheet metal screw to tighten the contact point to hold.
Just another consideration!

 

Thanks for the welcome. I am using a 3A AGA glass fuse. I haven't pulled off the fuse panel to check the backing as of yet. That was going to be my next step. Shouldn't I get power to the bottom with the Fuse in place? I have none with or without the fuse.

 

Make sure you didn't hook up the Light Blue/Red stripe wire to the aftermarket radio ground wire.

The Light Blue/Red Stripe wire in the radio power plug is NOT a ground wire but it is infact for the factory Radios dial illumination. The Light Blue/Red stripe wire also provides power to the instrument cluster and A/C heater control illumination lights.

This is a very comon mistake that is made when installing aftermarket radios into these trucks. There have been four such cases in the last month alone.

The confusion happens because the 12V power wire for the radio is in the same plug, as the radio dial illumination.

A: You test the Yellow/Black wire, it's hot, so you correctly think it's the power wire.
B: You test the Light Blue/Red wire, it's not hot, (because your headlights or parking lights are not on) then erroniously think it must be the ground, then cant understand why the instrument cluster lights no longer work.

The factory radio did NOT have a ground wire. It grounded directly through the radios case, to the radio support bracket, to the dash metal, as well as through the radio antenna wire.

So If the aftermarket radio has a seperate ground wire, you will have to add a seperate ground wire to dash metal.

And if you have the Light Blue/Red stripe wire hooked to the aftermarket radios ground wire, remove it and cap the end of the Light Blue/Red stripe wire.

Hope this helps...

 

Yes, as long as the fuse is good, and the fact that the fuse blows as soon as you put your test light on the bottom side, suggests you have a bad connection at the fuse panel connector, (bottom side).

The reason why the fuse blows is most likely covered in my previous post.

Two seperate problems on the same circut.

 

The light blue/red wire is not connected to anything as of now. Figuring the wiring of all that was fun, but I did figure it out. The accessory is hooked into the Yellow/black wire as it was only hot with ignition and the constant 12v is connected to the cigar lighter wiring. There is a spaid connector on the end of the light blue/red wire. I was going to wrap it with electrical tape to make sure the metal of the connector is not contacting the radio case and shorting out. The radio ground is connected to the ground point on the case itself. Thanks for trying to help me figure this out.

 

Good to know.

Make sure you cap or wrap the light blue/red wire with electrical tape to make sure it's not the problem as you described earlier.

Let us know what happens. Then if none of that described earlier fixes it, we will go from there.

 

Helpful post. I have an aftermarket deck in my truck and I have no dash lights at all. Ill have to take a look at it when I get home. Do most "professional" installers tend to make this mistake too? Like someone from best buy, car toys, etc?

 

stop buying cheap trucks tj and you wont have that problem lol

 

Are you blowing fuses, or do the interior lights just not work? I've had to clean the contacts that hold the fuses in before, they get rusted/corroded and wont transmit power to the fuse or from the fuse to the wiring.

Wiring is a simple system, if broken down by circut and traced out on a schematic. Schematics are like a road map, showing you what the wires do and telling you the colors of them (sometimes the wire colors in the book are wrong).
Most frequently I've found loose/corroded/dirty connections to be the culprit of most electrical problems, when no-one has tampered with a known good system. But loose/corrode/dirty connections don't blow fuses, whatever it is just doesn't work.


Since when are the installers at best buy or car toys, "professionals"?

 

Well, it has been known yes. Unfortunately.

You would hope the pro wouldn't though..

 

Thank you all for all the help on this one. I finally got some time to tak a look at the issue. I covered up the blue/red wires that were contacting the radio body with electrical tape. Additionally I took a look at the fuse that was in it. There was a 3AMP SFE fuse so I replaced with a 3A AGA and then there was light. Problem solved but Belle just likes to keep me busy so she has started leaking from the front axle gear now...Yay more time with my girl.