Author Topic: Odd wiring harness attached to engine block/transmission casing see pictures  (Read 3878 times)

Offline usnrcwo

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OK,

So, finally got the engine running very well on the 81 GMC and the transmission went out.  So, I was working on it this morning and saw this set of wires attached on the drivers side at the corner bolt where the transmission casing attaches to the engine block.  I detached it an pulled it through the top to get a better look at it.  It was broken below and couldn't see it well enough to figure out how to fix it.  I don't know what this thing is.  It looks like it might be a fusable link but whether it is or not, I don't know what this runs or does at all.  And since the engine is running well, I expect it relates to a sensor or a gauge but I really don't know.

Any ideas?

Offline bd

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Wash the wires so you can determine original wire colors.  If they are pink, light blue and dark blue, it connected to a two-prong oil pressure switch that controlled power to an electric choke thermostat.
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 usnrcwo

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Thanks.  Seemed sort of janky that they would just bolt this to the engine block and leave these wires essentially hanging in the wind.  Some of that marvelous GM engineering.

Offline VileZambonie

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Looks like someone made a jumper for the oil pressure switch and it broke at the crimp. It isn't GM engineering, it's backyard engineering. The right way to fix it is to connect it to the oil pressure switch.
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline usnrcwo

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OK, I've looked all over for a two prong oil pressure switch but can't find one.  The only oil pressure sender I know of is a single prong cylinder on top of the manifold next to the distributer.  However, I did notice this bit that looks like it's supposed to be some kind of sender but it has not ports and just a hole in the end.  I've had some folks work on the truck over the years that I eventually discovered had been either shady or incompetent.  Anyway, could this be some sort of dummy that should have a two prong sender into which this plug is supposed to go?

Offline VileZambonie

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No that's the block drain. The only thing that was screwed in there was a knock sensor if equipped.
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline usnrcwo

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Ok, not too proud to admit that I've never heard of a block drain and don't know anything about it.  Still don't know where this oil pressure switch is supposed to be.

Offline VileZambonie

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There would be a tee where your sending unit is. Like this one
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline bd

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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 usnrcwo

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OK,

So here's what I figured out.  It was likely the guys who installed my header that put the shorting wire on this oil pressure sender plug.  They clearly had an issue with clearance with the sender which was likely mounted just above the oil filter.  So, rather than figure out how to fix this problem, they just hid the problem. 

My solution was to put a Y on the input for the oil pressure gauge sensor with a 45 degree adapter.  I've got a picture below.  Hopefully this set up will stand the pressure that is put on it.  This is just a cruising around town truck and so I don't put it through rough conditions.  Hopefully it won't have any issues.

Once I get the truck back up and running (still need to fix the transmission and am having the headers powder coated) I'll update on how my solution worked.

Offline 75gmck25

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GM used that oil pressure switch for different purposes over the years.  In some trucks it was used to control the electric choke, so that the choke only had power when the engine was running. 

I've also seen some guys use it to control the relay for an electric fuel pump, so the pump stops if the engine stops running.

Bruce


Offline usnrcwo

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Mine is definitely the choke.  When I pulled that short wire off, the choke light came on.  I've got the engine back together and it's running nicely.  So far, I'm not detecting any leaks where I rigged that sensor onto the block.  Hopefully it will hold.  It's at the transmission shop now awaiting the rebuild of the turbo 350.