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Electrical issues on 1978 U-Model Mack -- Need help!

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Hello everyone,

I need some help in figuring out electrical issues on a 1978 U-model that I purchased.  I have the service manual and a (generic) wiring diagram for mid-70's R/U model trucks but I am still having difficulty.  The truck is a positive ground system.  I have three questions:


Question #1

  • The previous owner had disconnected the wire at the fuse panel that supports low air buzzer, low air warning light, and oil pressure gauge --- not sure why he did that.  That loose wire (white) is shown in picture below near accessory relay.
  • I reconnected the wire to a hot portion of the fuse bar (3 position bar shown in image) and everything seems to now work (buzzer, light, etc).....However -- I can't figure out where that wire was supposed to be connected.  There is no label for buzzer on fuse bar, however wire length seems to indicate it goes somewhere on the right side (with 3 fuse positions), near the accessory relay

Question #2

  • I assume that the way this is supposed to work is that the left fuse bar stays hot all of the time (with key off) and the right side of the fuse bar is only hot when the key is on.  When I turn the key to on position, I hear the click in the accessory relay to suggest that it is working.  However, the right side of the fuse bar does not activate and the right terminal of the accessory relay does not go hot.  Does this mean accessory relay is bad?

Question #3

  • The headlights and outer marker lights of this truck stay on even when light switch is in off position (and key is off).  Movement of light switch to on position does also turn on the dash lights and tail lights...headlights continue to stay on regardless
  • The only way to get headlights to turn off is to disconnect the battery.

Any ideas on questions above?  Much appreciated!   We saved this truck from the weeds (had been abandoned) so previous info is limited.  It runs great aside from these electrical issues.  Picture of fuse bar is attached below



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I'm not an electrical knowledgeable person but I would use a test light on the relay. I think with  key off the large right lug should have power then key on the left lug should then have power and energize the right bank of circuits.

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I don't know why previous owner added that red wire.  If you notice it is connected on the left side to a circuit breaker at the far end that is not connected to the others.  It looks like that circuit on the far end  is for Stop Lamp Switch (does that mean tail-lights?).  Perhaps the red wire was added to bypass the accessory relay and provide power to the right bank?  If that is true then it is not working now because I used a test light and both the red wire and entire right bank is not getting power with key on.  

It sounds like my accessory relay is bad and once I replace that I may find other problems.

Edited by Mdmccurr
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2 hours ago, JoeH said:

Never worked on a positive ground vehicle, does that affect what you hook up the test light to?

Test light hooks up is the same on Pos or Neg system.


 “Life’s journey is not to arrive at the grave safely, in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy shit, what a ride!’


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