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Lights & transmission 4L80

Discussion in 'Truck & Equipment Repair' started by scubapi1522, May 18, 2008.

  1. scubapi1522

    scubapi1522 Junior Member
    from US
    Messages: 2

    I just swapped vehicles with another guy to double my gas mileage. Upon getting what was supposed to be a "great truck with no problems" I found several. The headlights on this 97 Chevy would only go on when the switch was turned off. When turned on, running lights and dash lights work, but no headlights. When lights are supposedly turned off, the headlight dash indicator stays lit. I replaced the headlight switch with no change. Another forum suggested it was the ignition switch so I replaced that. Now the lights stay on whether the light switch is on or off. At least now I can drive at night, but I would like to fix. The truck has a BOSS plow which I haven't picked up yet, but I am wondering if they possibly wired the electrical for the plow up wrong which affected the lights....
    The bigger problem is the tranny. It has a 4L80 which works fine in P, R, N, 3,2,1. Overdrive does not work when going above 33 mph-it starts revving up the RPMs, but will not upshift-it will not downshift until truck speed slows to below 30. I thought possibly an electrical overdrive solenoid might be the problem, but wouldn't that show up as a trouble code? Any ideas?
    Last edited: May 18, 2008
  2. scottL

    scottL PlowSite.com Addict
    Messages: 1,600

    Plow lite wiring - sure. Have you contacted the original party, anything from them?

    Trans - it's hard to kill a 4l80. No suggestions - sorry. Codes may not actually show up. How's the fluid look? leaks? Wiringing into trans? Linkeage?
  3. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    The dash indicator should be on when the light switch is in the "off" position since then the day time running lights should be on. That's what the indicator is for, to show you the DRL's are illuminated.

    When you turn the switch on to illuminate the headlights, the headlights should illuminate to full intensity and the indicator in the dash should go out.

    By your description it sounds like the headlights are working just fine. And since the DRL's and normal head lights both use the same bulbs, if you have head lights at all then your plow wiring is correct.

    As to the trans...there's virtually a dozen things that can cause an OD problem with a 4L80E. The fact that it's not throwing any codes isn't uncommon either.

    Could be electrical in nature...or could be a hard part on it's way out. Need a good data logging scanner with bi-directional controls to narrow it down, like an OTC Genesis or Snap-on pro scanner. A good trans shop should be able to pin point it.
  4. scubapi1522

    scubapi1522 Junior Member
    from US
    Messages: 2

    4L80 & Lights

    The problem with the lights was that with daytime running lights on (switch off), the taillights running lights did not work. Assuming they were working, I was pulled over at night for having no taillights. If I turned the headlights on at night, I actually had taillights, but no headlights. I could go with headlights only, or taillights only, but not both. I replaced the headlight switch with no change at all. I then replaced the ignition switch wiring. I now have headlights that are always on when the key is turned, and also have taillights at night when the headlight switch is switched on as it should be. The only problem now is that there is no way to turn the headlights off. Not usually a problem, but I am also a p.i., and sometimes you need to switch them off...

    Still haven't fixed the tranny, but noticed leaking from pan?? and will investigate further.
  5. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    If you push the E-brake down a couple clicks before you turn the key forward, the DRL's should stay out (until you release it again of course). If they do stay off with the E-brake on, then it's now working as GM designed...which could still be a problem for someone in your profession. :D

    You could always pull the DRL fuse to eliminate them altogether... ;)