1990 GMC Sierra, no voltage to VSS

TylerW

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#1
Hey guys..got another electrical gremlin:

This is on a 1990 GMC Sierra 2wd w/700R4 trans. 350 V8.

When I got this truck the tranmission was on its way out, so I picked up a used 700R4 trans with about 47,000(out of a 92 Roadmaster wreck) and swapped it in.

The trans works fine, but the electric speedometer and lockup converter are non-op.

I checked for voltage at the VSS with key on and got none, both in Drive and in park.

I'm pretty sure that the speedo did work before I swapped transmissions, but converter operation unknown since it wouldn't shift above second.

I checked the harness all the way back up to the main harness at the engine and cannot see anything damaged.

No SES while driving, lamp does work.

So, can someone point me in the right direction to see where the power for the VSS is getting dropped?

Thanks!
 
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#2
Is the speed sensor putting out any voltage when drive shaft turning?
Is transmission out put shaft and speed sensor gear the same as you old transmission?
Which speed sensor are you using off of old one or buick transmission?
 

EricC

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#3
VSS generates an ac signal. Disconnect the sensor and hook up DVOM to the terminals at the sensor and rotate the rear propshaft and look for voltage.
 

TylerW

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#4
Well once again I've learned something in short order from asking on this site ;)

Now that I understand that there IS no voltage going to the VSS, I ran some tests and also got some pertinent info:

The VSS on the Roadmaster trans is part # 10456088, which references to 91-93 all, with D8N option.It appears to run from a gear, with a rod.

The VSS that was on the old trans is part # 8654750. It operates from a reluctor ring.

I never got any voltage output from the Buick VSS by turning the driveshaft, but then again its not easy to get much wheelspeed that way, lol.

So, are these VSS' deemed to be compatible as far as output, or am I dealing with 2 different types of sensors?

I definately let the fact that these two transmissions came from behind TBI 350s fake me out. I've already had to swap TV cables, the selector arm on the trans, and now this issue.Live and learn.

Thanks again for the help...
 

EricC

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#5
Description and operation of the GMC VSS:

The Vehicle Speed Sensor generates a signal that indicates the speed of the vehicle. The signal is then processed in the Instrument Cluster, which provides inputs to the Rear Wheel Antilock Brake Module, the Electronic Control Module, the Cruise Control Module, the Speedometer, and the Odometer.
The Vehicle Speed Sensor is mounted in the transmission. A magnet rotating within a coil creates 40 voltage pulses for every revolution of the Transmission Output Shaft (Propeller Shaft). The frequency of this AC signal is dependent on the vehicle's speed. As the speed increases, so does the frequency.

The AC signal goes to the Instrument Cluster where the Input Buffer and Ratio Adapter convert the signal to 128,000 pulses per mile. This is done by dividing the AC signal by the divide ratio. The divide ratio is determined by the pin configuration of the Programming Clip. The Programming Clip configuration is determined by the tire radius and the axle drive ratio, and manually set.

The signal output from the Ratio Adapter is 128,000 pulses per mile. This signal is used to close a Solid State Switch every time a pulse occurs. The switch grounds terminal D of the Antilock Brake Module 128,000 times a mile, giving an accurate speed signal.

The signal from the Ratio Adapter is also used to close switches to the Cruise Control Module and the ECM. Before the signal is sent to the Cruise Control Switch, it is divided by 32, creating 4000 pulses per mile. From here it also goes to the Speedometer and Odometer. After being further divided by 2, creating 2000 pulses - per mile, it is used to close a switch grounding terminal A1O of the ECM.
The Output Switches in the Instrument Cluster are Solid State Switches, not mechanical ones. Self-powered test lights or ohmmeters should not be used to test them. Do not measure the resistance at the Outputs of the Instrument Cluster.

Description and operation of the Roadmaster VSS:

The Vehicle Speed Sensor, mounted to the Transmission, produces an AC signal in the form of a sine wave. The frequency of this sine wave is proportional to the speed at which the Transmission rotates. The speed at which the Transmission rotates is proportional to the speed of the vehicle.
The AC signal produced by the Vehicle Speed Sensor is amplified and converted to a square wave by the Vehicle Speed Sensor Module. The Vehicle Speed Sensor Module produces the square wave by opening and closing internal solid state switches to ground. The square wave signal is supplied to the Engine Control Module (ECM), Cruise Control Module, Audio Alarm Module, Power Steering Control Module (Sedan), and Speedometer by the Vehicle Speed Sensor Module. The square wave to the ECM is at a rate of 2000 pulses per mile, while the Cruise Control Module, Audio Alarm Module, Power Steering Control Module (Sedan), and Speedometer see a square wave at 4000 pulses per mile. The ECM, Cruise Control Module, Power Steering Control Module (Sedan), and Speedometer internally convert the number of pulses per mile per second to determine the vehicle speed.


Looking at the way the various processors interpret the signal, I'd say the sensors are not cross compatible.
 

TylerW

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#6
Thank you for a very good description of the 2 different VSS' I'm dealing with.

I arrived at the same conclusion after spending much time on the internet but nothing quite as detailed as that.

Obviously my next task will be to swap the reluctor over to the new trans in place of the gear drive it now has in order to use the magnetic VSS. That's not going to be easy..would anyone be willing to offer tips on that? ;)

Continued thanks....
 

Transman

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#7
Your problem is the Relucter is "Press fit" on the output shaft. This means a puller will be needed to remove it from the "old" output shaft and a installer (pipe) to install it onto the "new" shaft. Both ways being careful not to crack it as it will become useless at that point. Transman