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  1. 75gmck25

    Debugging my AC problems

    Fan speed will always be on low. That is the GM HVAC design and you can’t easily change it. Not sure what you mean by the “vacuum module”. A/C mode turns on power to the compressor, and then it is routed through a low pressure cutoff in the hard line near the evaporator. Switch has rubber...
  2. 75gmck25

    Towing with my 86.... hints, tips, essentials?

    1st priority - electric trailer brakes and a good controller. When you hit the brakes you want that trailer to drag you down like an anchor, not push the back of the truck into a jackknife. It weighs about the same as your truck, so it will definitely push the truck around. Also verify the...
  3. 75gmck25

    Removing instrument panel - dealing with the oil pressure line

    I’ve always disconnected the oil line in the engine compartment. It’s not real hard to reach, and the two nuts for the compression fitting are usually accessible with a wrench. I’ve been through just about all of my HVAC, and pulling the cluster is not usually needed. - remove glove...
  4. 75gmck25

    Carb Adapter

    I also recall an Engine Masters episode where they found an open hole spacer on a dual plane was a good combination. It got a little more power in the upper range, without losing any low end. You want a phenolic spacer or a wood spacer, not aluminum, to provide heat separation. Also remember...
  5. 75gmck25

    Quick and easy engine rear support when transmission is removed?

    If you can buy a short piece of angle iron, it seems you could use blocks or spacers to bolt it to the frame and support the engine. Wood may also work, but I would have more trust in angle iron. Then run a ratchet strap up around the engine and the support to keep it from bouncing and pivoting...
  6. 75gmck25

    Exhaust Question

    If you want a nice mellow, straight-through muffler, take a look at Magnaflow. I have them with duals and an H-pipe and it has strong V8 sound without being excessively loud. Flowmasters are also a common choice, but I'm not really crazy about the tone and/or the drone at certain speeds. If...
  7. 75gmck25

    AFR GAUGE addition?

    I bought one of these several years ago, and it's been really useful. https://www.innovatemotorsports.com/db-red-gauge-lc-2-kit.html It's not the cheapest choice, but it's a complete setup for a Wideband gauge. You can also save data to a PC, or add other sensors.
  8. 75gmck25

    Turn signal switch

    When I bought a new turn signal switch the wiring connector on the end had "almost" the right spacing. However, I couldn't get all the wires to line up and make a good connection, so I cut the end off the old switch and spliced it on the new switch.
  9. 75gmck25

    Smoking Ground Wire

    Start with the dash cluster. There should be a black ground wire up high under the dash. On an '87 I think there is a ground bus it connects to on the firewall, and older trucks the ground is a post on the top of the emergency brake. Then make sure the large 2 gauge ground wire on the...
  10. 75gmck25

    exhaust question, heat riser valve connections

    There is a flat gasket with 3 holes on the manifold side, and a donut gasket to mate with the exhaust pipe. My pipes had a slight flare on the end, but then there was an insert dropped int o the end, and it had a larger flared flange that clamped up against the bottom of the donut. It all...
  11. 75gmck25

    Faulty rigged

    My truck had a dealer-installed aux tank and it looked just like the square one in your picture. I got a factory tank, solenoid, hard lines and all the wiring from a truck in the pick and pull, and now mine is a near stock setup. Still kind of a PIA because there are so many different...
  12. 75gmck25

    Mechanical fuel pump return line

    If that 2nd metal line ends very close to the fuel pump, then is probably the fuel return line. The picture I attached shows my three-port fuel pump and all the lines attached. I would have to take a look at the vent line location in my truck, but I think it ends a different location that is...
  13. 75gmck25

    gas gauge

    As AuroraGirl said, make sure the sending units are grounded. Unscrew the bolt, sand down the frame to bright metal, and screw back in. Also follow the wire up and make sure it’s still connected to the sending unit. An 87 uses a solenoid with 6 terminals, so you will have to determine which...
  14. 75gmck25

    TRC Actuator and Air Conditioning Solenoid Question

    My ‘75 has no solenoid to increase the idle with A/C on (factory A/C and no evidence it ever had a solenoid) and it does not cause any issue. I do run the vacuum advance on full manifold vacuum, so I get a little idle speed increase from the vacuum advance.
  15. 75gmck25

    Transmission or rear diff?

    They are probably 4.56 gears. 4.10s were also common in the 3/4 tons with the same axle, so a swap should be easy, and you may be able to also use 3.73 gears without any change to the carrier. 4L80 transmission would give you a good overdrive automatic, but I’m not sure how much you might...
  16. 75gmck25

    Starter and Alternator wires

    Your truck is probably wired exactly like my '75 and Rick1966's '75. One quirk is that you may have an extra wire on the starter that runs up to the wiring loom for the in-dash ammeter (ammeter was used up through about '76). This wire has a small fuse in the harness, usually in the wiring...
  17. 75gmck25

    Carb replacement recommendation?

    The Edelbrock AVS2 gets a lot of good reviews for easy setup and good street manners. It is available in 650 or 800 cfm, and your engine and heads have the flow two use the larger carburetor. However, the 650 would also work fine. I used a 213/217 @ .050 and 112 LSA cam in my 350 and it’s a...
  18. 75gmck25

    Starter Wiring Issues

    The '73 is wired a little differently than later trucks because it has points (not HEI) and it has an ammeter in the dash. 1. large gauge wire/cable running to the battery positive. original factory was 2 gauge black, but many have been changed to red. These first two smaller wires are also...
  19. 75gmck25

    Emissions question

    The GVW vs emissions rules changed over the years. My ‘75 with 8400 GVW had no factory catalytic converters and very minimal emissions gear, but that exemption only lasted a few years. In 1980 they raised the HD standard to 8500 lbs, so Chevy then marketed trucks with the 8600 lb GVW package...
  20. 75gmck25

    Replacing turn signal switch in 75 High Sierra

    I did this job on my '75 GMC several years ago, and I don't remember how far you had to tear into it to get to the turn signal switch. I was fixing a broken ignition switch rack and my emergency flasher switch would not stay on, so I replaced both at the same time. I got the turn switch from...
  21. 75gmck25

    LMC AC controller has too many detents

    I have not found a source for the vacuum connector attached to the back of the dash switch. Several sources sell the switches, but they do not come with a new vacuum connector. The vacuum connectors have a parts number stamped on them, but I have not found a reference to explain them.
  22. 75gmck25

    The squeaking is getting on my last nerves

    I used poly bushings for my cab mounts, and greased them lightly as was suggested. They still squeaked when first installed, but settled in and quieted down after about a month of wear in.
  23. 75gmck25

    Fuel gauge to tachometer, clock to fuel gauge

    I have a '74 cluster with a factory tach in my '75, and the tach does not use any wires from the large multi-pin connector. The tach harness is a separate three wire plug, and only has power, ground and signal from distributor. I'd have to take a close look to see if the illumination bulbs are...
  24. 75gmck25

    Engine mount, old or new?

    I think it always pay off to go with new, but spend some time looking for a higher quality product. Maybe the off-road vendors sell some HD engine mounts? With a C20 you will probably pull the engine and trans as one unit, so reinstallation might be not that difficult. With a K25 like mine...
  25. 75gmck25

    power steering and input shaft

    First - Set the wheels pointing straight ahead and off the ground and the steering wheel at center. From there, turn the steering wheel full lock in each direction, and count the number of turns in each direction. If it is not equal, the box is not centered. I’m not sure off-center would...

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