73-87 Chevy _ GMC Trucks > Electrical

1977 K20 Engine Harness

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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.



Do you have two batteries? Batteries Isolators are often used with two batteries.

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.

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.

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.


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