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Engine Electrical - No one seems to know

mark97213

By mark97213
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mark97213 Enthusiast

mark97213

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I posted an electrical question on a forum but no one seems to know.  I have an 1981 K5 Blazer that the previous owner put a big block in.  I'm trying to get the factory gauges to work but I am having a tough time with the Oil pressure sending unit wiring.  I found the oil pressure plug on top of the bell housing and pulled it out ready to plug into a new oil pressure switch when I notice the middle wire is cut and missing (see photo) and am wondering, it's just a two prong plug but there are three wires coming from the connection.  Can someone tell me what this middle wire is too.... is it just a ground?  Does it need all three wires connected to make the gauge work?

 

IMG_20230215_164458999.jpg

Jsdirt Senior Enthusiast

Jsdirt

Posted
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Definitely needs all 3, or else GM would've skipped one. 

 

What year is the big block? I can't remember when it's final year of production was, but if it's a computer-controlled OBD-II engine ('96 - '98 or newer) you'll have a 5v reference, ground, and signal return (usually less than 5v).

 

Memory is super fuzzy on the older ones, but I think it's the same concept only using 12v and no computer. Best to find a wiring diagram somewhere. Should be easy to find on something that old (I.E., FREE diagrams).

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mark97213 Enthusiast

mark97213

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Yes, I've looked for a schematic but I've yet to find one for that specific year Blazer.  Yes sir, I agree . . . GM put it there for a reason.  I'm hoping it's simply a ground as there is no part of that wire in the loom anywhere.  It's as though he ripped it all the way out.  The big block is a 74.

Jsdirt Senior Enthusiast

Jsdirt

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Ok, definitely not OBD-II, lol. I'll see if I can find some info somewhere ...

Jsdirt Senior Enthusiast

Jsdirt

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So, according to this late 70's to early 80's diagram, that sending unit  and plug should be a ONE wire deal.

 

Dammit - can't post it here due to size. I'll try linking it ...

 

 

https://freeimage.host/i/HMBK7F1

 

Looks like there's 3 wires to the oil pressure SWITCH. Not 100% sure what the function of that is, but it's got 3 wires. The gauge should only need  ONE wire to run the gauge with a SENDER.

 

Actually, the switch is for a warning light now that I think of it.

 

Edited by Jsdirt
mark97213 Enthusiast

mark97213

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30 minutes ago, Jsdirt said:

So, according to this late 70's to early 80's diagram, that sending unit  and plug should be a ONE wire deal

Looks like there's 3 wires to the oil pressure SWITCH. Not 100% sure what the function of that is, but it's got 3 wires. The gauge should only need  ONE wire to run the gauge with a SENDER.

Actually, the switch is for a warning light now that I think of it.

 

Thank you for that.  Yeah, I've seen that schematic before but it's too generic for the 81 blazer... I think.  On the switch, the one I pictured, may actually not be the one for the oil pressure switch.  I'm operating completely in the dark on this.  I search for a new oil pressure switch and it showed a single blade and a two blade.  I bought the two blade but now that I think about it, it may be wrong after all.  But it is factory so it must go somewhere.  It's frustrating working on a swapped engine.  But like you said, it really should only be a single wire to operate the gauge.  I also need to figure out the Temp, which I know is a single wire, and the voltage.  I'm far from a electrical pro but have some general knowledge about them.

Jsdirt Senior Enthusiast

Jsdirt

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Posted (edited)

Temp is just as easy. 1 wire - done. It grounds through the sender, like the oil pressure sender does. Power goes into the gauge in the cluster, then grounds through either sender.

 

If it were me, I'd just install a rack of Autometer gauges, or install them in the cluster. Super simple hookup on those. Oil pressure can either come in mechanically through a tube, or electrically through a sender (also 1 wire). Same with the temp (no tube, though - electric only).

Edited by Jsdirt
mark97213 Enthusiast

mark97213

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Yeah, there were some aftermarket gauges in it along with a mechanical oil pressure one but I'm restoring it back to factory.  Temp is easy indeed but I'm not positive which color goes to it.  If you saw this mess of wiring, you'd be amazed what he was trying to do and it's beyond me.  Anyway pretty sure the temp gauge is an orange wire and I'll have to trace it from the cluster I think.  As far as the volts, will the cluster just pick that up from the Alt or HEI ?

Jsdirt Senior Enthusiast

Jsdirt

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Volt meter is usually hooked directly to the battery, or wiring headed that way. At least that's the way I prefer it, so I'm sure the battery is actually being charged by the alternator no matter what load is being put on it. I prefer to keep the 12v HEI wiring by itself to avoid any ignition problems.

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