Author Topic: 1977 K20 Engine Harness  (Read 1195 times)

Offline ehjorten

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1977 K20 Engine Harness
« on: April 12, 2022, 10:01:08 AM »
So my starter failed on my '77 K20. The previous starter lasted for decades, but the replacement starter only lasted a few years. Being frustrated with the lack of quality in these OE replacement starters, I went and purchased a RobbMc mini starter!

Before installing it, I took off my engine harness to go through it and clean it up and make sure all connections are clean and in good repair. Tearing off the old convolute and looking at the 45 year old mess that had been hacked-up by the previous owner to add a battery isolator on the firewall, and add in an electric choke wire, spliced in off of the HEI power feed, I kind of want to just buy a new engine harness!

That creates questions. It appears that I have the blue 18 ga wire with a broken connector that was just dangling at the back of the engine. From the wiring diagram, that is for a oil pressure sensor, that I don't appear to have. I have an electric choke, but that is on the aftermarket Holley 4-bbl. This truck is a Silverado with gauges. My Oil pressure gauge works. Does that extra oil pressure sensor run an additional idiot light? I see the Canadian version has a two-blade oil pressure sensor? That runs a jumper to the electric choke?

I am just trying to make sense of it all and add the proper oil pressure sensor that doesn't seem to be there, but I had the pigtail for it.

BTW...this truck has a 350 engine in it, but originally it had a 400, so the previous owner must have swapped out the 400 and maybe didn't replace the oil pressure sensor? I have owned this truck for over 20 years.

Thanks,

-Erik
« Last Edit: April 12, 2022, 04:23:25 PM by ehjorten »
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline Mike81K10

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Re: 1977 K20 Engine Harness
« Reply #1 on: April 12, 2022, 03:55:44 PM »
Do you have two batteries? Batteries Isolators are often used with two batteries.
"Tell me and I forget. Teach me and I remember. Involve me and I learn." -Benjamin Franklin

Offline ehjorten

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Re: 1977 K20 Engine Harness
« Reply #2 on: April 12, 2022, 04:19:34 PM »
No I do not have two batteries. The previous owner had it setup for a cab-over camper. I long ago pulled out that extra wiring and the silly wiring for the aftermarket radio. I can't even begin to describe the craziness that went on there. Lets just say there were 4-6 20-amp circuit breakers in the negative and positive wiring for the radio that went around opposite routes along both LH and RH fenders to the battery and went to the radio in-line 5-amp fuse!

Anyways...really trying to understand the '77 oil pressure sensor and electric choke functionality. I did find this: http://forum.73-87chevytrucks.com/smforum/index.php?topic=36868.msg309686#msg309686

Is that what I would have had from the factory? Mine is not '81-'86 so not sure if it was different.
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline bd

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Re: 1977 K20 Engine Harness
« Reply #3 on: April 12, 2022, 05:22:55 PM »
Studying the 1977 Wiring Manual, that single dark blue wire is for a low oil pressure warning lamp in a base (non-gauge) cluster (Ckt 30), but can double as a choke heater OPS wire for an electric choke (Ckt 931).  If the truck doesn't have oil pressure control of the choke heater, it would be a nice addition since it delays choke opening until the engine is running.  Use the link you posted to the electric choke wiring article by Henry for details of its operation.

Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Henry

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Re: 1977 K20 Engine Harness
« Reply #4 on: April 13, 2022, 10:15:22 AM »
Hi EH:
Well, your 77 K-20 is pretty close to my 76 C-20 as far as build time but the GM guys were making a lot of changes in those days and had a lot of different configurations to deal with on the wiring harness so what is on my truck, although stock, may be different from yours. Anyway, my truck has a green wire and connector dangling loose behind the HEI distributor...this is for an electric oil pressure gauge used on the 454 engine (Mark Engine) and since my truck is a 350, it uses the mechanical oil pressure gauge...so it has been dangling loose there since it came from the factory. For 1976, the 454 engine also had the electric choke Q-jet carb while my 350 had the divorced mechanical choke. I may have that light blue wire (and a couple more related to the 454 oil press sender) wrapped up in the engine harness plastic conduit that I have never seen or have been exposed to the light of day.

My advice to you is to buy the 1977 GM Truck Wiring Diagrams book for models 10-1500 thru 30-3500 to really understand what your harness should look like. It is not a big book so I think it is inexpensive. That blab I did a few years ago about the electric choke really applies to early 1980s and I am not sure if the electric choke on 1970s models was as sophisticated...everything regarding the carburetor in the 1970s and 80s was constantly evolving. Do you have a choke warm up light on your dash? What flavor of carb choke do you have on your truck now? Do you have an electric or mechanical oil pressure gauge? I dont think the 400 engine was considered a "Mark Engine" (big block 454) so I would expect your truck originally had the mechanical oil pressure gauge....but maybe BD can shed more light on which engines got the electric oil pressure sender and which did not.

For 1976, if a truck had the electric choke, the light blue wire ran from the choke to the electric oil pressure sender on the block. The sender on the block then had two wires going back to the firewall: one wire was dark blue and this went to the gauge and the other wire was brown/white which went to the big main electrical connector on the firewall.
Regards,
Henry

Offline Henry

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Re: 1977 K20 Engine Harness
« Reply #5 on: April 13, 2022, 10:48:26 AM »
Hi EH:
In my previous discussion I first mentioned I had a green wire and connector dangling behind the HEI. Well, it is so old that my notes say that it is either dark blue or dark green...it is probably actually dark blue and is related to the unused electric oil pressure sender. It is a real short exposed wire from the conduit and hangs behind the engine between the HEI and the driver side valve cover. My 1976 Supplemental Manual actually shows it hanging loose on the LF4(400)/LG9(van V8 engine?)/LS9(350) engines but does not describe what it is for.
Regards,
Henry

Offline ehjorten

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Re: 1977 K20 Engine Harness
« Reply #6 on: April 13, 2022, 11:07:54 AM »
Henry and BD...Thanks for your replies. I have an original 1977 Truck Wiring Diagrams book! I am a junky on all of that old documentation. I have Parts and Illustration Books, Assembly Manuals, Wiring Diagrams, etc. on all of my old cars!

I was getting confused on what the difference was between the "Canadian" engine harness and the standard V8 engine harness and what I "should" have.

What I see is that if I was to add the two prong OPS I would need to add a jumper wire to the choke (circuit 901) from the circuit 931 Dk Blu wire. I would need to upgrade the oil tell tale circuit 31 from a 20 ga wire to a 12 ga wire. Then I would need to add the Circuit 300 Orange 12 ga. wire to my engine harness to the other side of the OPS. I then would have to investigate if my dash harness has the necessary wiring in it to function properly. I don't believe I have a choke light anywhere on my dash, so I am not certain what circuit 31 would be going to. I see a 20 ga dk blue 931 wire going to the gauges instrument cluster, but it is a 12 ga dk blue 931 for the idiot light cluster.

I need to get this truck back on the road, so it sounds like a longer term project to do this.
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline bd

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Re: 1977 K20 Engine Harness
« Reply #7 on: April 13, 2022, 01:08:59 PM »
Unless you wish to adhere to strict "period correct" construction you can dispense with the "Choke" light in the dash.  And, you certainly don't need 12-gauge wiring!  A pink 18-gauge I+ wire protected by an inline 4- to 5-amp fuse to feed power to the non-grounding 2-prong OPS and an 18-gauge wire in your choice of color (OEM is light blue) from the OPS to the choke heater element will suffice.  If the factory dark blue (931) wire dangling from the harness is not connected to an I+ powered bulb in the cluster, don't use it.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Henry

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Re: 1977 K20 Engine Harness
« Reply #8 on: April 20, 2022, 12:09:18 PM »
Hi Eric:
If I am not mistaken, the choke indicator light on the dash cluster would be in either of the two big round gauge faces of your cluster...either the speedo face or the other face which is either a fuel gauge face or tach face depending on your options. It will be at the bottom of the face and be normally blacked out lens so you dont see the writing on it which should illuminate the word "CHOKE" when it gets it is on. It should be a small black rectangular lens that is curved to match the bottom of the gauge face. So if you dont have this someone has swapped out the original dash cluster.

I also think that all C/K-10s through C-30s that had a V8 engine with a Q-jet carb in 1977 had the electric choke and hence should have had the instrument cluster with the choke light. I think 1976 was the last year C/K-20s and 30s with the small-block V8 had the mechanical (divorced) choke and had an instrument cluster without the choke light...all V8 C/K-10s in 1976 had the choke. It was very confusing but it all had to do with the staggered introduction of emissions equipment of the heavier duty trucks.
Regards,
Henry

Offline ehjorten

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Re: 1977 K20 Engine Harness
« Reply #9 on: May 04, 2022, 09:03:21 AM »
Ordered the engine harness from NPD. It was a pretty good representation of the original. The water temp sender pigtail was a little short, but did reach and connected to the sender, just the pigtail looks a little stretched. The black dielectric that they provide to seal the bulkhead connector was a little skimpy and they could have provided another ounce. My stock one was filled-up more and the amount they provided was barely enough. The alternator leads were sufficient in length, but could have been shortened a little. That could be easily fixed. The biggest issue I had, and it is just a bit of a nit, was with the starter leads. The starter leads are long enough that I could clock my RobbMc mini starter so that the solenoid could be pointed down and tucked up near the oil pan (I ended up not doing that), but the starter wire ring terminal was too small to fit over the stud. I ended up cutting off that small ring terminal and putting on a standard small, yellow solderless ring terminal and some good shrink wrap. The new RobbMc mini starter is SO MUCH better than the stock starter! I ditched the metal tube that mounted to the stock starter solenoid for the wire routing. The new starter is so much smaller and the solenoid has much more clearance to my headers in the stock solenoid location, that the wires naturally route far away from the headers. There is no simple way to have that tube connect to the solenoid of the mini starter anyways.

I haven't done anything yet with the choke circuit to try and integrate it with an oil pressure switch, but it is on my to do list. Right now the choke is just getting power off of the coil feed circuit. Glad to have my '77 back on the road!
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6