700r4 speed sensor cable

DaveKlein

Newbie
Joined
Jan 30, 2021
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3
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Make
Chevy
Model
Bel Air
Year
55
Miles
?
Engine
'88 5.7L
So on an '88 chevy 5.7L, what is the speed sensor cable for from the 700r4 to the ECM OBD1? This setup is currently in a '55 Chevy Bel Air and running fine without it connected. I'm just not sure if it has anything to do with lockup?

Thanks for any help!!

55bel.jpeg55bel.jpeg
 

billr

wrench
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I'm not sure what the question here really is. When an engine/trans swap of that scope is done, there are many mechanical, wiring, and plumbing issues that must be resolved with custom/modified components. The VSS wiring should be trivial compared to many of the other problems. Are you looking for an "off the shelf" cable for this special application, rather than fab one more custom part?
 

DaveKlein

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Jan 30, 2021
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So here's the deal.... This car was built 30 years ago by my girlfriend, her dad and ex husband with a 5.7L TBI OBD1 engine...... It would had been nice if they blew off the TBI and just went with a carb. Anyways, they had to hand wire everything to the ECM and its just been prone to many problems. So we bought a new dedicated engine harness from Hotwire Auto to replace this hand wiring. It's very self contained and nicely done. But I have yet to connect the speed sensor cable that came with the harness. It needs a mechanical to electrical sending unit attached to where the mechanical speedometer comes out of trans. I'm having a difficult time understanding what it's for or finding the correct sending unit!! Even the harness builder can't give me a real answer as to what this feedback loop does for the ECM. So for now, we are driving the car without it. But the real deal is want to get a Dakota Digital instrument cluster and that comes with it's own 3 wire sending unit for transmission. So I'm trying to understand if I can blow off this wire for the harness or manually wire it in somehow???? Sorry if that's a long story!!

Speed Sensor.jpeg

newharness.jpeg
 

billr

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I think there are two questions here:

1) How can an electric VSS be fitted on this 700R4 that has a mechanical speedo drive?

2) What does the VSS do besides just drive a speedo; can the ECM/engine function OK without the OEM VSS signal?

The 700R4 evolved into the 4L60E, so I suspect there was a VSS that would go in and engage the tail-shaft gear at some point. However, if you want to drive the stock '55 speedo, then it could get trickier.

I don't know about that older ECM, but in later years the VSS is used to "fine tune" for emissions and fuel economy. As a simple example: if the TP is "closed" and the VSS is "0", then fueling must be for a good idle. However, if the TP is "closed" and the VSS indicates speed above a certain threshold, then the engine is in over-run (braking on decel or downhill) and the fueling can be cut off and/or the spark retarded.

I forgot to say explicitly: if the engine operation (drivability) is acceptable to you without the VSS signal to the ECM, then there is no real need to add it. And, a brief web search indicates there are many ways to fit a VSS to the 700R4/4L60 trans.
 
Last edited:

DaveKlein

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Jan 30, 2021
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Great! Thanks so much for the help! Engine runs fine along with drivability so we will go with new instrument cluster without the ecm getting the vss signal..
 

bp042665

hunting nut
Joined
Nov 26, 2012
Messages
707
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Location
Iva South Carolina
FYI transgo makes a kit to make the lock up come on with out any electrical hook up and as for speedo you can get a speedo gear conversion kit
 
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