86 c3500 check engine light no ecu?

[fishingdave]

Thanks for all the great info, the schematic is a great help, i have been looking everywhere for it! The truck has electric choke is there a timer for it or does it only run off the oil pressure switch? It could be 80 degrees out side and fast idle only until it gets to operating temp and i still have to press the gas pedal all the way down to disengage. I cleaned all the carb linkage still the same. Shouldnt throttle down on its own?

 

[CSFJ]

No, the choke will release its tension on the linkage, but you still have to manually move the linkages to come down off choke.

 

[chengny]

Thanks for all the great info, the schematic is a great help, i have been looking everywhere for it! The truck has electric choke is there a timer for it or does it only run off the oil pressure switch? It could be 80 degrees out side and fast idle only until it gets to operating temp and i still have to press the gas pedal all the way down to disengage. I cleaned all the carb linkage still the same. Shouldnt throttle down on its own?

The choke is set closed by pressing the accelerator to the floor before cranking. This allows the plates to snap shut (and also strokes the accelerator pump to provide a shot of fuel for initial start). Opening of the choke plates is a combined effort of the heater and the vacuum break(s). If those components are operating properly, the choke will open without any assistance from the driver.

All carburetors use either electrically or hot air heated choke coils. The heated coils provide choke valve closing force for cold starts, as well as correct opening timing during warm-up. Vacuum break assemblies control initial choke valve opening during starting and warm-up periods.

You can release the closing tension (supplied by the coil) somewhat. Loosen the 3 locking screws and rotate the black coil cover as indicated (I think that's the correct direction, if not, go the other way). Do it just a bit at a time and use the marks to keep track of how far you have moved it:

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On most vehicles, an electric assist choke is incorporated onto the carburetor to aid in reducing the emissions of hydrocarbons (HC) and carbon monoxide (CO) during starting and warm-up (choke-on) period. The electric assist choke is designed to give a more rapid choke opening at temperatures of approximately 60-65°F., or greater and a slower choke opening at temperatures of approximately 60-65°F., or below. The electric assist choke system does not change any carburetor service procedures and cannot be adjusted. If the system is out of calibration, the heater control switch and/or choke unit must be replaced.
The main components of the electric assist choke system, Fig. 39, consist of a thermostatic coil, ceramic resistor, cover, bi-metal snap disc and contact spring. The electric actuated ceramic resistor heats the thermostatic coil, gradually relaxing coil tension and allowing the choke valve to open.
At air temperatures below 50°F., electric current applied to the small section of the ceramic resistor, allows slow opening of the choke valve for good engine warm-up. As the small section of the ceramic resistor continues to heat, a bi-metal disc causes the spring loaded contact to close and apply electric current to the large section of the ceramic resistor which increases the heat flow to the thermostatic coil for more rapid opening of the choke valve.
At air temperatures between 50-70°F., electric current applied to the small section, or both the small and large sections of the ceramic resistor, will produce the amount of heat required to control the choke valve position for good engine operation in these temperature ranges.
At air temperatures above 70°F., electric current applied to the small section of the ceramic resistor and through the spring contact to the large section of the ceramic resistor, provides rapid heating of the thermostatic coil for quicker choke valve opening when leaner air-fuel mixtures are required at warmer temperatures.

Diagnosis & Testing
Possible conditions for the electric choke not operating properly are listed as follows (use a voltmeter to check oil pressure and choke coil circuits):

1. Low or no engine oil pressure.
2. Faulty oil pressure switch.
3. No current to oil pressure switch due to:

• 1. Burned out fuse.
  2. Broken wire to oil pressure switch.

1. No current between choke and oil pressure switch due to:

• 1. Broken lead wire.
  2. Wire terminal not properly secured on coil terminal.
  3. Faulty ground circuit between choke assembly and housing grounding plate.

1. Faulty choke coil assembly.

If it is determined that the choke coil assembly is inoperative, the following test must be made:

1. Remove choke coil from carburetor. After removal of choke coil, allow coil to cool to room temperature.
2. Connect a ``jumper'' wire between positive battery terminal and choke coil terminal assembly. Connect a second ``jumper'' wire between negative battery terminal and choke coil assembly grounding plate.
3. The tang on the choke coil should rotate 45°in 54-90 seconds.
4. If choke coil does not rotate or exceeds the 54-90 second time limit, replace choke coil.
5. If coil is within specification, check steps 1 thru 5 for possible cause.

 

[fishingdave]

Thanks for the great info i am going to tinker with it during the week will let you know what i find. Also do you have or know where i can get the heavy duty service manual for the truck ? I am only able to find light duty ones online .


Thanks
Dave

 

[chengny]

Thanks for the great info i am going to tinker with it during the week will let you know what i find. Also do you have or know where i can get the heavy duty service manual for the truck ? I am only able to find light duty ones online .


Thanks
Dave

Unless you mean some other truck than the one we have been discussing - you don't need a Heavy Duty Service Manual, that's because your C3500 is only considered Light Duty. It is the heaviest of the Lights but Light Duty nonetheless.

The term Heavy Duty - when used in reference to the Lt. Duty truck line - only refers to the emissions package and was part of a kind of cat/mouse game that GM/Ford were playing with the EPA and the State of California. It really had very little to do with the GVWR of a truck.

GM's Heavy Duty series was way up there - they had a GVWR of like 49,500 to 69,800+. For example, even when (GM) classifies a truck as Medium Duty it only means the 4500 -7000 series. When they say Heavy Duty, they were talking about their two really BIG truck lines (i.e the 8000 & 9500 models). You know, the ones with air brakes and that require a ladder to get in the cab.

You might want to read this - it's a good story about why our little Tonka Trucks are sometimes referred to as Hvy Dty. It is specifically about a ploy to avoid installing catalytic converters in some 1/2 ton trucks (AKA the Big 10's). And it worked - up until around 1978. After that, unless you had a K20/30, a diesel or a Big Block, dream on - like it or not, you were getting a Cat.

Note the last change California made for a Cat to be required and you'll understand why your truck has a GVWR of 8600:

Even in the case of California emissions packages, the F44 RPO designation allowed a 1/2 ton truck to avoid the requirement of catalytic converter(s). This was done by tweaking a regular 1/2 ton up to a point where the GVWR was above the limit that would require the use of a cat (6000 lbs) Minor upgrades were made to the springs, brakes and tires to accomplish this increased load capacity.

When the F44 package was added, the 1/2 ton truck was then essentially a 3/4 ton truck. These upgraded 1/2 tonners were commonly called Big 10/Heavy Half.

This practice began with advent of cat con requirements (for all vehicles with a GVWR below 6000) in 1975. It didn't fool anyone for very long. California raised the minimum GVWR for no cats from 6K to 8.5K in 1978 - the rest of the country lasted until 1979.

So if I am reading everything correctly, your truck (F44 RPO and 1977 MY) was never equipped with any catalytic converter(s) as supplied from the factory. Consequently it is now - and always has been - in compliance with CA emissions requirements.

A dead giveaway is the design of the transmission crossmember support. The supports on 1/2 ton trucks from that era will be different depending on whether cats were installed or not (don't ask me what the difference was - I just learned all this tonight).

What probably cannot be used - to determine if a truck came with an original exhaust that featured cats - is the other emissions components. There were too many variations and work-arounds across all the engine/trans/altitude packages to use them as an indication.

Even up till 1986 (like my truck for example) on HD emissions packages, it was common to see an A.I.R. injection system complete with 2 air pumps and exhaust manifold injection - but the truck never needed cats.

Some Big 10 history:

The Big 10 was promoted as a heavy duty two wheel drive half ton for the 1975 through 1980 model years. Although it offered truck buyers somewhat more load carrying ability than the standard C10 half tons, its real advantage- -and the motive behind its conception- -was that it provided buyers with the opportunity to purchase a half ton truck that was unencumbered by the dreaded catalytic converter, which was first introduced to a skeptical American car buying public just in time for the 1975 model year.

For several years prior to that time, all trucks with gross vehicle weight ratings of 6,000 lbs and below were forced to comply with the strict light duty emission standards that also affected passenger cars. The EPA's selection of the 6,000 lb. threshold likely reflected its awareness that that number had become the traditional de facto dividing line between half ton and three quarter ton trucks. And since the vast majority of trucks purchased by consumers were half tons, limiting the emission constraints to them probably seemed to be a reasonable compromise to the government, while avoiding potential protests about cost and implementation by the truck manufacturers- -and their commercial customers- -that might have occurred had the laws been expanded to the heavier vehicles.

Before the advent of the catalytic converter, the distinction between light￾ and heavy duty emission controlled trucks was largely ignored by consumers. But the converter introduction and it's nearly universal application in 1975 light duty￾ emission vehicles changed things dramatically. Unlike the EGR and evaporative canister devices that preceded it, the converter had an exclusive appetite for expensive unleaded fuel which outraged truck buyers- -and especially fleet purchasers- -in a nation that still had access to relatively cheaper leaded fuel.

Recognizing a new marketing opportunity, or necessity, light truck makers made relatively minor spring, tire, and brake modifications to their existing half ton models to push the GVWR just over the 6,000 lb. threshold. Ford lead the way by introducing the heavy half ton F150 as an alternative to their traditional half ton F100, while Chevrolet introduced the F44 Heavy Duty Chassis package as an option for their C10 pickup.

These new-for-1975 models moved into the heavy duty￾ emission classification enjoyed by three quarter ton and heavier rated models. This allowed the F44 equipped C10, later dubbed the Big 10￾ for market visibility, to comply with government emissions regulations using only a PCV valve, heat stove, and relatively loose controlled combustion system (CCS) tuning. In contrast, the 1975 light duty ￾emission certified standard capacity C10 required a PCV valve, heat stove, stricter controlled combustion system￾ (CCS) tuning, EGR, evaporative canisters, a vacuum activated early fuel evaporation (EFE heat riser) valve, special outside air ducting to the air cleaner, and a catalytic converter. But by 1979 the EPA- -following legislation enacted in California a year earlier- -raised the light duty/heavy duty emission dividing line from 6,000 to 8,500 lbs. GVWR which brought all heavy duty half ton and almost all three quarter ton pickups into the light duty emission fold. Following this legislation, consumer appreciation for the heavy duty half ton concept- -so gelded- -largely faded away.


All the above information comes from some guy who appears to know what he is talking about. It is just a tiny part of his answer to the questions in the post shown below - which is almost identical to your situation :

What is a Big 10 or Heavy half?
i just purchased a 77 half ton pickup and the 4th digit in the vin is a 4. the gross weight for this truck is 6050 lbs. this makes it a Big 10. this is a calif. truck but no cat converter. also the intake manifold(v-8) dosent have an egr valve. is this correct? or do all trucks regardless of weight require an egr valve? what are emissions for the heavier trucks? thanks

 

[fishingdave]

thanks great info!!

 

[Craig 85]

One other note for Chengny's post. K5's had heavy duty emissions too. I had a California '77 K5 with a factory SB400, dual exhaust and ran on regular fuel. I don't recall if it had an EGR, but I don't think so.

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