Help with my 87 suburban

[shoeless3231]

I have a 1987 gmc suburban 4x4 with the T.B.I. 5.7l V8 and 700R4 tranny with the 208 transfercase that I just got a couple weeks ago. It had some issues and I fixed some of them but I cant figure out these problems. My emergency brake light is staying on all the time, The speedometer quit working right ( it either dosent work at all or is jumping all over the place), When the speedometer is jumping around it runs ruff but dosent die. After talking to a few people the suggestions are; fuel pump, fuel pressure regulator (all ready replaced), VSS (vehicle speed sensor), Bad speedometer gear(replaced), or Bad speedometer cable( messed around with it and it seems to be ok). I took apart my whole dash and the VSS is located on the back of the speedometer and it seems like it is all hooked up right. Any help you can give me would help cause this is my daily driver and I am trying to keep it running good.

[Lt.Del]

First, welcome aboard!

I don't beleive you have a VSS behind the instrument cluster.  Since you don't have anti lock brakes on a '87, your speedo is mechanical, and your trans is not computerized, you may not have a VSS (I could be wrong).  On my '91 sub, I have two VSS's and my speedo is not mechanical but electrical, my trans (4L80E) is computerized, and I have RWAL anti locks, therefore the need for VSS's.    

Try a test:  turn key half on.  Notice the light, wait a few seconds for the computer checks.  Then note if it blinks at all.  Pull your brake release and push up on the pedal to ensure it is not engaged.   Note the brightness of light.  PUsh down on parking brake pedal and see if brightness changes....it should.   If it does, your brake light comes on because of something else besides your parking brake.

If your light goes out when you pull up on parking brake, you need to adjust parking brake, spring, etc...

[shoeless3231]

Ok well a little bit of a update. I tried to mess with the emergency brake adjustments but the light still stays on. It dosent get brighter though. The speed-o dosent work most of the time now and the service engine soon light comes on after driving for a little while. Code 24 something with the VSS sensor. Still runs ruff.

[VileZambonie]

CODE 24 - VSS CIRCUIT FAULT







 Circuit Description :

The ECM applies and monitors 12 volts on CKT 437. CKT 437 connects to the Vehicle Speed Sensor (VSS) which alternately grounds CKT 437 when drive wheels are turning. This pulsing action takes place about 2000 times per mile and the ECM will calculate vehicle speed based on the time between "pulses".

A "Scan" tool reading should closely match with speedometer reading with drive wheels turning.



 Test Description : Numbers below refer to circled numbers on the diagnostic chart.

Code 24 will set if:



CKT 437 voltage is constant.
Engine speed is more than 200 RPM .
Vehicle speed signal (voltage on terminal "A9" is less than 10 mph (16 km/h)
All conditions must be met for 10 seconds. These conditions are met during a road load deceleration.
This test monitors the ECM voltage on CKT 437. With the wheels turning, the pulsing action will result in a varying voltage. The variation will be greater at low wheel speeds to an average of 4-6 volts at about 20 mph (32 km/h).
A voltage of less than 1 volt at the ECM connector indicates that the CKT 437 wire is shorted to ground. Disconnect CKT 437 at the vehicle speed sensor. If voltage now reads above 10 volts, the vehicle speed sensor is faulty. If voltage remains less than 10 volt, then CKT 437 wire is grounded. If 437 is not grounded, check for a faulty ECM connector or ECM.
A steady 8-12 volts at the ECM connector indicates CKT 437 is open or a faulty vehicle speed sensor.
This is normal voltage which indicates a possible intermittent condition.
 Diagnostic Aids :

"Scan" reading should closely match with speedometer reading, with drive wheels turning.
Check park/neutral switch diagnosis chart if vehicle equipped with automatic transmission.
If park/neutral switch is OK, see Diagnosis By Symptoms - No Trouble Codes Stored.