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  1. 75gmck25

    Defroster without AC

    Also check under the hood and make sure you still have the vacuum lines and vacuum reservoir that connect to the HVAC dash controls. Without vacuum you can’t switch the air flow between dash, heat, defrost, etc. Many guys start pulling out vacuum lines and devices under the hood because they...
  2. 75gmck25

    Fuel Goes to Empty when shifter moved from Park

    Column shifter linkage under the drivers seat area is probably grounding out the sending unit wire as it moves into drive. Check for bare wire areas that touched the linkage.
  3. 75gmck25

    Putting 1984 harness into 1982 truck

    The wiring diagrams for both years are posted on this site. Just pull them up and start comparing. My guess is that most wiring differences from ‘82 to ‘84 were implemented with small supplemental wiring harnesses. GM liked keeping it simple for the assembly line. For example, if it needed a...
  4. 75gmck25

    82 GMC sierra C15 Inline 6 to 454 swap?

    The 454 was a factory option, so no special mods needed to fit it in your truck. You do need a cooling system and transmission that is up to the job, and probably need to upgrade the front springs. In my opinion, building and swapping in a 454 is often not worth the trouble unless you just...
  5. 75gmck25

    Fuel gauge and wiring diagram for 1978 Suburban C20

    On a pickup there is a tan or pink sending unit wire that runs up the passenger side frame rail, then across the back between the engine and the firewall, and connects to the wiring for the instrument panel.
  6. 75gmck25

    Fuel return changes?

    A 3 port sending unit has ports for fuel feed to engine, fuel return from pump, and vapor line to canister under the hood. Fuel return line is a good idea to prevent vapor lock, but if you have dual tanks then you then need a 6 port solenoid. A ‘74 350 may have had a vented gas cap and...
  7. 75gmck25

    Ignition timing

    You need to first decide if you want to keep that K3500 as a heavy load hauler, or make it more of a daily driver truck. HD pickup tires are usually listed as load range E, or in the newer specs a load rating of 120, 125, or higher. To keep the full K3500 load capacity of your truck, I assume...
  8. 75gmck25

    Ignition timing

    Using 235/85R16E tires would be one stock (about 31.5”) tire that has the load rating you need for a K35. It’s a common tire size for SRW high GVW pickups. Other tire sizes may also work, but you have to determine whether they have the load rating you need, and that they will work with a stock...
  9. 75gmck25

    Ignition timing

    The procedure above will work fine, but it doesn’t tell you how quickly your centrifugal timing increases as you go up in rpm. Most stock centrifugal setups add 18-20 degrees of timing as rpm rises. A stock unit may need to get to 3k rpm to hit the max, but by using a kit and swapping...
  10. 75gmck25

    78' K20 Dual vacuum actuator issue maybe....

    There are two actuators mounted on the HVAC plenum Benin’s the glove compartment. IIRC, one actuator is easy to see toward the front, and the other is on the back side between the plenum and the firewall. If I am oriented properly (a picture from farther back would help), I don’t recall any...
  11. 75gmck25

    Electric Choke Conversion

    The original idle screws have a double-d head, and when assembled by GM they also had a tamper-proof cap over them. Your caps have been removed/drilled out. You can get an adjustment tool made specially for that screw head, or buy replacement mixture screws with a standard screwdriver head.
  12. 75gmck25

    1978 k30

    Start out by checking the timing. Stock base timing may be as low as 4 BTDC, but it should handle 12 BTDC without any issue. Then use a dialback timing light to verify you get 18-20 degrees more by about 2500-3000 rpm (total of about 30-32 degrees). That might help a little to wake it up...
  13. 75gmck25

    1 inch bolts on a SB Chevy?

    1” is fairly common for fittings on some plumbing or hydronic systems, but usually you need an open end wrench. I had to use a 1 1/8” open end just last week to hold the fitting while I tightened up a gas line on a duel-fuel range. I don’t remember when I bought that odd size open end...
  14. 75gmck25

    73 K20 Driver Side Fuel Tank Sending Unit - 1 Port or 2 Ports?

    A ‘73 K20 might have a very simple system, but I’ll explain some of the things to look for. My ‘75 K25 is kind of unusual because it has HD emissions and 8400 GVW (no catalytic converter), but I do have a fuel return line and evaporative canister. Your sending units, fuel pump, gas cap, and...
  15. 75gmck25

    how to test if my parasitic draw is my fuel system?

    I’ve been following squarebody forums for at 20+ years, and have never heard of that fuel system timer. It’s really an unusual option, and I wonder why it was needed. I would have to study the wiring diagram for a while, but it seems you could disconnect the relay and just run a standard...
  16. 75gmck25

    What do those of us with '73s do if we want to replace the HVAC controls?

    I’m a little confused about what you have purchased. The back of the controls for my ‘74 look similar to the first picture, and the faceplate looks like the 2nd picture. However, the vacuum connector and electrical assembly on the back of the 2nd picture is totally different than mine. The...
  17. 75gmck25

    ‘86 heater problem

    Is this an A/C truck? You need to troubleshoot the heater blower motor a little differently. On most years of these trucks, the lower blower speeds run through the dash switch and a resistor pack, but max/high speed runs through a separate fuse and the relay on the plenum under the hood. Have...
  18. 75gmck25

    100 amp Powermaster

    I bought the 94 amp 12si to replace my 63 amp 10si because it was a direct bolt-in, and I added a bigger gauge charge wire just in case. It’s a simple upgrade, with no change of brackets, and the voltage regulator works just like the original.
  19. 75gmck25

    100 amp Powermaster

    I replaced the charge wire for my 63 amp alternator when I switched to a 94 amp 12si, and I used the charge wire from a GMT400 from pick and pull. I would have to check the size (I think it may be 8 gauge) and it already had a fusible link. I ran the new wire directly over from battery...
  20. 75gmck25

    Alternator BAT Wire Gauge

    On my ‘75 the original alternator charge wire for a 63 amp alternator is 10 gauge with a 14 gauge fusible link.
  21. 75gmck25

    A/C & HEAT slider is froze up

    There is no cable connected to the upper slider, but there is a heat blend door cable on the bottom slider. When you move the upper slider it is connected to a large plastic assembly on the back of the controls, and all the HVAC vacuum lines connect to the plastic assembly. If that plastic...
  22. 75gmck25

    Electric Choke Conversion

    I’ve seen recommendations to tie it to the ignition-on wire for the wipers, but I’ve never tracked it down. My ‘75 K25 has a brown wire under the hood that is used for the relay that disconnects the aux battery when the ignition is off. I don’t know if the same brown wire is on all trucks, but...
  23. 75gmck25

    Blower Motor Fan Speed does not work on High

    Ricko1966, you are correct. Lower speeds do run through the resistor, but high speed is separately powered directly through the relay (as I explained a couple posts back). If high speed is the only one not working, its the direct power circuit and fuse under the hood and it does not go...
  24. 75gmck25

    Switching AC to R134

    I used a Vintage Air Sanden compressor and Alan Grove brackets for my ‘75 350 engine with v-belts and it all mounted very well. The compressor is high on the driver’s side. It may not work the same for an ‘89.
  25. 75gmck25

    Blower Motor Fan Speed does not work on High

    In a ‘74, that relay is powered by a separate power wire that runs from the power junction next to the brake booster over to the relay. Midway in the wire there is a glass fuse holder and 30 amp fuse. I had to replace my fuse holder with a newer style fuse holder and 30 amp fuse because the old...

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