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1981 gmc 6-cyl

Discussion in 'Chevy Trucks' started by 81c1500, Nov 4, 2003.

  1. 81c1500

    81c1500 Junior Member
    Messages: 29

    HELLO ALL,

    I CAME ACROSS YOUR SITE SEARCHING FOR KNOWLEDGE ON MY NEWLY ACQUIRED 81 GMC LONG BED WITH THE STRAIGHT 6. FIRST QUESTION I HAD WAS THE ORIGINAL OWNER DID NOT LIKE THE MANUAL TRANNY SO HE PUT IN A 350TH FROM A PONTIAC TO OBTAIN AUTO TRANSMISSION. MY QUESTION IS THE SPEEDOMETER IS ACURATE TILL ABOUT 45 MILES PER HOUR BUT AFTER THAT IT VARIES ON ACCURACY, ANY INPUT ON THIS. ALSO MY GAS GAUGE IS WAY OUT TO LUNCH. I VERIFIED THAT I HAD A FULL TANK AND IT INDICATES 3/8 OF A TANK AND WHEN I WENT TO FILL UP THIS MORNING IT INDICATED 1/16 OF A TANK AND IT COULD ONLY HOLD 14.65 GALLONS. ANY INPUT WOULD BE HELPFUL. ON THE SPEEDO ISSUE I NOTICED THE STOCK TIRE IS A 205/75 R15 AND I HAVE 235/75 R15, AND WITH THE SPEEDO CABLE COMING FORM THE TRANSMISSION IT APPEARS TO BE A LITTLE SHORT(I MAY HAVE TO GET A LONGER SPEEDO CABLE). fOR $1,000.00 THE TRUCK SEEMS TO BE PRETTY SOUND. DOES ANYBODY HAVE ANY WORDS OF WISDOM ON THE STRAIGHT 6 I BELIEVE 250 CUBIC INCH. THANKS, CHUCK HAVRILA
     
  2. wyldman

    wyldman Member
    Messages: 3,265

    Welcome to Plowsite :waving:

    Try not to type all in caps,it's considered shouting.

    The inline six was a good strong motor.The carb and cold driveability was the big problem.

    You can change the speedo gears to recalibrate the speedometer.I'd replace the speedo cable first,as it sounds like you need one anyways,and if it's binding it can throw the speedo off,especially at higher speeds.

    The fuel gauge or the sending unit could be at fault.If you disconnect the wire to the sending unit,you can ground the wire to verify the gauge has full travel and is working properly.If it does,then the sending unit is bad.If not,then it's the fuel gauge or a wiring problem.
     
  3. 81c1500

    81c1500 Junior Member
    Messages: 29

    Hey Chris,

    Sorry about the caps, I have never posted anything before. Thanks for the input, I will try the cable when I can get one. Chuck
     
  4. 81c1500

    81c1500 Junior Member
    Messages: 29

    Well I troubleshooting this gas guage problem I pulled the gauge housing out of the dash and found a flexible printed circuitattached to the back. The contacts for the gas Gauge were extremely corroded and one contact not even making. So I am going to hard wire the gauge to the main connector. Again Thanks for the input.
     
  5. wyldman

    wyldman Member
    Messages: 3,265

    Great.Hopefully that is your problem.Let us know how you make out with it.
     
  6. 81c1500

    81c1500 Junior Member
    Messages: 29

    Well I hard wired the gas gauge and the needle moved towards the F position but it kept going past the full indicator. So my next job will be to change the sending unit in the tank.
     
  7. wyldman

    wyldman Member
    Messages: 3,265

    are you sure you didn't ground it ?
     
  8. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    Just a little friendly advice - The sending unit is probably the last resort to try, since you have to pull the tank or the bed to reach the top of the tank. Also, new sending units go for about $75-80, so they are relatively expensive.

    Check all the wiring carefully and you'll probably find a grounded wire, or a wire that's disconnected.

    The wire to the tank is vulnerable where it runs across the back of the firewall behind the distributor and down to the right side frame rail. You should also be able to see the ground wire where it connects to the frame rail near the tank. You can probably see where the two wires (power and ground) connect to the sending unit if you look down at the top of the tank between the bed and the cab.

    Bruce
     
  9. 81c1500

    81c1500 Junior Member
    Messages: 29

    Well I worked on the sender and found the resistance feedback was shot and the gauge was too. I bought the generic gauge and sender form JC Whitney and cut the center of the stock fuel gauge out and place the new gauge in. Also ran new sending wire and a good ground. The gauge actually works now. I have recently finished working on the center support bearing. I figured might as well change the u-joints since I'm under the truck anyway. While under the truck I saw the oil pump on the 350TH had half of the mounting bolts backed out. So I pulled the tranny back and all but 1 bolt was loose. I also installed Sun-pro gauges in the stock gauge holes( I only had idiot lights). The truck is really coming around. I do appreciate the feedback on some issues that I have had. Hope you all have a great new year. Take care
     
  10. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    I was wondering how the Sun Pro gauges fit in the stock gauge holes in the instrument panel? What size of gauge did you use? Also, how does the gauge lighting work? I assume the factory panel lights don't illuminate a replacement gauge, and that you have to use a light for each gauge.

    I've already replaced my instrument panel with one that had a factory tach, but I'd like to have some gauges might be more accurate than the '75 vintage factory gauges. My factory tach is also flakey, and sometimes does not read at all. Its probably a loose connection, but that will take time to troubleshoot.

    Thanks,

    Bruce
     
  11. tuna

    tuna Senior Member
    Messages: 488

    That 6 is either a 250 or a 292 depending on if its a 1/2 or 3/4 ton.Just a little FYI that trans cant a Pontiac the Pontiac,Olds, Buick and Caddy use a different bolt pattern.
     
  12. gpin

    gpin Senior Member
    Messages: 390

    For an '81, your problems sound minimal. I have late 90s trucks with more problems.