Same hereOriginally Posted by EagleMark
Same here
1990 GMC C1500, 5spd, 1993 LT1 (ECM 16159278 $DA3)
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sorry for the huge pics. Top is what i believe is the v.s.s. plug, those 2 wires run to the plug in the second pic. From there the diagram on here shows a brown wire to pin f12 which i have. Anybody have any idea what the plug in the last pic is for? Looks like its part of the chasis harness but i have no idea where it goes.
The twisted pair is probably it (128k ppm vss). In 1990 truck it was purple/yellow, in 1993 camaro it is also purple/yellow.
1990 GMC C1500, 5spd, 1993 LT1 (ECM 16159278 $DA3)
Looks like a relay plug, probably for fuel pump. Can you show the face of it?
1990 GMC C1500, 5spd, 1993 LT1 (ECM 16159278 $DA3)
Thanks. I labelled it vss when i first pulled it from the truck looking at a generic diagram for 88-98 c/k trucks. The chilton diagram has different wires for this year and engine/ trans, and the diagram on this site shows the brown wire pin that i have. I really dont like wiring. I have a pink wire hot in run/start going to the trans from the fuse panel. Can i just clip that? Dont know how else id get it out of the plug.
This is the face of said plug
I think this plug is for the fuel pump relay though it has some wires going to another small plug from it
The first plug pictured goes to the same plug as the twisted v.s.s. wires
Last edited by conwaymuddy; 07-26-2013 at 10:52 PM.
They are both for relays it looks like. However, you are probably right that the 2nd one for the fuel pump relay (that's the style my 1990 truck uses). It gets interupted if the oilpressure switch is reading low pressure. In these trucks, the oil pressure sending unit also has a switch, making it a 2 purpose device. If pressure is too low, it opens which shuts off the fuel pump. you can by pass that though pretty easily as the computer doesn't get this on/off information.
What that other relay is for idk what thats for. The camaro used the same plug for HVAC controls of some sort. Not sure what exactly because I didn't use AC in my truck and it has manual cables for heating controls.
So in summary, 2nd pic is probably fuel pump, 1st pic I would guess it is a relay for some sort of HVAC control.
1990 GMC C1500, 5spd, 1993 LT1 (ECM 16159278 $DA3)
I just read this. I'm not exactly sure what pink wire you're talking about. However in general pink wires on these gm vehicles are VERY imporant as they typically go to crucial places to make things work. If you do need to clip it, 2x and 3x check that you're doing the right thing.
Yea I'll agree wiring is a pain in the ass, as it will easily take you as much time setting up your new harness in a spread sheet as it will actually splicing connections. I can give you an example of the spread sheet I used to marry two harnesses if you like to get some ideas.
1990 GMC C1500, 5spd, 1993 LT1 (ECM 16159278 $DA3)
I think the VSS buffer is for the speedo or drac module?It will change the speed pulses from 4k to 2k for cruise or speedo maybe.
I know how it works on the fiero's but not positive on the trucks,the few i've messed with used a DRAC or VSSB module.
I don't think you'll need to mess with it for what you're doing,just the VSS input for the ecm.
These guy's will know better than I.
The pink wire comes from the firewall plug near where the steering column comes through. Chilton shows it coming from a 20A fuse to the transmission. The only wire coming out of the trans on the diagram that doesnt go to the pcm is a purple that splices to to the purple at pin B4 which goes to the map sensor. Could the oil pressure switch be the 3wire plug that comes off the fuel pump relay? Thanks again for all the help.
Could be,alot of GM's use the oil spressure switch as a cutout.If you have a guage and light than one wire will be guage,one for light and one for fuel pump relay iirc.
ECM powers the pump for about 4 seconds to charge the line and then shuts off till the oil pressure switch closes with enough pressure/cranking rpm-which completes pump circuit.
Good thing is if the relay goes out they will generally backfeed voltage thru pressure switch and you'll never know it.
Bad part is it will eventually kill the pressure switch and you'll be stranded with two issues,bad relay and bad pressure switch.
Again,I have more experience with some of the cars on this,not the trucks.
After more tracing the black from the big plug goes to a common ground which goes to these two plugs.
Those to plugs wind up at this plug.
the one on the left looks like an ac compressor plug, the one on the right my be ac high pressure switch.
Probably, it should be oval looking. The fuel pump relay is supposed to sit on the firewall under that black cover real close to the oil pressures ender/switch. 2 of the wires are like 2 wires on a switch, where the switch is actually controlled by oil pressure (high pressure = closed, low pressure = open).
For those other 2 pictures at the bottom, that 3 pin connector looks like it is for AC. So I would assume those other two connectors are also for AC (maybe one for the AC pressure switch and one for AC clutch control???)
Last edited by bentrod; 07-27-2013 at 03:58 AM. Reason: clarity
1990 GMC C1500, 5spd, 1993 LT1 (ECM 16159278 $DA3)
After looking at wiring diagrams and my other trucks i figured that stuff was A/C stuff and pulled it. I still dont know what the plug with the 2 yellows, white, and black wires is for. any thoughts on the pink wire that supplies power to the trans? I dont run an e-trans so im assuming its safe to clip it. There was also a loose pink/white wire coming from the fuse block plug. Wasnt connected anywhere when i undid the tape and wire loom. Again, safe to just clip it?
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