1989 one ton (old body style) VSS code

[ahawes]

I've got an 89 crew cab one ton that I am getting a SES light on and every time I run the code it comes up as a 24 which is the VSS. My speedometer still works, odometer does not. My question is this - with the VSS code is my issue more likely the sender on the transmission or the unit behind the speedometer? Or is there any way to know? I realize the VSS doesn't control a whole lot with the motor or PCM but the perfectionist in me just wants to keep the SES light from coming on. Any help or suggestion would be appreciated!
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[oldebondo]

I have an '88 GMC R2500, 350 TBI and a Muncie 465 3-speed w/ Granny Gear. I am having the same Code 24 - VSS. My spedometer works so I know it's not the cable. I don't believe there is a VSS on a standard transmission. I've heard it could be corrosion on the terminals of the VSS Buffer Module but I checked that and if they were bad, they should be good now. Still throwing the code. I am wondering if you figured it out and maybe you could share.

BTW - sweet truck!

[Desert1957]

I thought the VSS was only on the speedo , behind the cluster.

Desert

[hatzie]

Originally Posted by Desert1957

I thought the VSS was only on the speedo , behind the cluster.

Desert

If I Remember Correctly... EDIT I have a case of CRS here... Look down two posts.

[oldebondo]

I'm sure I could repair the buffer if I only knew what component(s) to replace. Anybody have experience troubleshooting this sensor?

[hatzie]

I just looked at the 1987 - 1989 service manuals.
ECM = Engine Control Module
CCM = Cruise Control Module
VSSB= Vehicle Speed Sensor Buffer.

87-89 have cable speed heads with a VSS Buffer Amplifier with the optical pickup screwed to the speedo housing for cruise control. 1987 & some 88-89 use it for the ECM as well.
The optical VSSB on the instrument cluster is the same type used for Cruise III cruise control. The Optical VSSB was all discrete thru-hole components. It's the same type of unit from 1983-1990. According to the Fiero guys the buffer in the later units is improved over the early units. Rock Auto has replacements for these. It's a 2 pulse optical unit. There's a light chopper (a reflector behind two blades or something similar) inside the speedometer head. The sensor assembly on the end of the ribbon shines an LED beam on the reflector that gets interrupted by the blades as the speedometer cable turns. The light is reflected back to a phototransistor twice per revolution of the speedometer cable. This signal is then sent to an amplifier and buffer circuit which turns the signal into a "on" and "off" signal the ECM can interpret as vehicle speed. For every revolution of the speedometer cable, there are 2 pulses sent to the ECM. GM speedometer cables are calibrated to turn 1000 revolutions (or 1001 depending on who you ask) per mile. So the input the ECM or cruise module sees, is 2000 pulses per mile. (PPM)
The optical VSSB was all discrete thru-hole components (TO92 package transistors, resistors, capacitors). None of them are potted so they should, in theory, be repairable unless the LED & optical pickup sensor on the pigtail is kaput.
I won't go into troubleshooting one of these. They can be purchased new for around $50.
Wiring to the edge connector from the top in the picture... Power, Ground, narrow gap, ECM Signal, wide gap, CCM signal. On your Oscilloscope the output signals should be 0.556 HZ / Mile square wave.


Some but not all 88 & 89 RV series trucks have a VSS sensor screwed into the transmission or transfer case with a DRAC or VSS buffer (not the same as the optical unit) in the cab. You'll have to look to verify what you have.
90?? & 91 RV series has electronic speedometer with VSS in the transmission or transfer case and a DRAC in the cab.

The 88-91 VSS Amplifier Buffer or DRAC module, if you have one of them, should be mounted near the ECM under the passenger side of the dash.

[ElGracho]

My 1989 R3500 has the VSS on the TH400 and the electronic speedometer. It is a TBI 350 also.

I think there were some 89's that were still carbureted. (454 with HD emissions?) I would expect if any still had mechanical speedos it would be the ones that didn't have ECMs...

[oldebondo]

Thank you Hatzie. I see when researching the conditions required to set Code 24 that one is: a low voltage from the TPS. Can I check this signal with a DVM? If so, might you know of a write-up describing this process?

[hatzie]

Originally Posted by ElGracho

My 1989 R3500 has the VSS on the TH400 and the electronic speedometer. It is a TBI 350 also.

I think there were some 89's that were still carbureted. (454 with HD emissions?) I would expect if any still had mechanical speedos it would be the ones that didn't have ECMs...

So there's a mix of cable and electronic speedo in 89 MY RV series?? Nothing like being consistent.
The electric speed heads with tranny mounted VSS will have the DRAC or Buffer box. Do you know if they are 4K PPM or 2K PPM? EDIT They are 2K Found the answer here--->GM V8 VSS sensors.PDF

[hatzie]

Originally Posted by oldebondo

Thank you Hatzie. I see when researching the conditions required to set Code 24 that one is: a low voltage from the TPS. Can I check this signal with a DVM? If so, might you know of a write-up describing this process?

You can. The procedure for testing the TPS should be in the Emissions and Driveablility manuals.
Use a meter with a 10Megaohm impedance. Your meter needs to have higher impedence to prevent electricity back flow which can kill the ECM. Low impedance meters or test lamps can damage the unprotected ECM.

From a textbook on testing Engine Control systems.
-----------------
CAUTION: A number of circuits include solid-state modules, such as the Engine Control Module(ECM), used with computer command
control injection. Voltage in these circuits should be tested only with a 10-megaohm or higher impedance digital multimeter. Never use a test
lamp or low impedance meter on circuits that contain solid state modules. Damage to the modules may result.
------------------

[Desert1957]

Great post Hatzie, very good info.

Desert

[oldebondo]

I checked the output of the TPS just to satisfy myself that I did not have a problem. I found .5V at closed throttle rising to 4.2V at WOT.

I replaced the VSS with a brand new one only to find that Code 24 continues to be set. The speedometer works fine. In fact, I lubricated it with Vaseline and it is very smooth and uniform now.

When I read the codes by jumpering the two pins on the OBD I connector, I see Code 24 first followed by Code 42. When the SES light comes on, the engine stumbles very briefly (milliseconds).

I recently wired-in an MSD 6A multiple spark ignition system using the rated supplemental wiring harness for the application. (I previously had the same MSD ignition system on my '89 S-10 4.3L TBI without any problems.) Could the MSD ignition be causing code 42 to be set and simultaneously cascading into Code 24?

I am stumped. I appreciate you guys helping me out with any hypothesis.

TIA!

[hatzie]

Code 42 may mean you have wiring issues in the ESC distributor harness or someone bypassed the ESC.

Hopefully you didn't get Vaseline into the optics.

Assuming the VSSB is the only VSS unit on the truck. Do you have continuity from the VSSB plug to the ECM plug on the signal wire?

[oldebondo]

I cleared the codes by disconnecting the battery for 30 minutes while I continued to verify continuity from the VSS BRN wire to the ECM (Pin A10 on the 24 Pin Plug). I cleaned to connector with electrical contact cleaner and replaced the Plug back into the Receptacle of the ECM taking care to exercise the motion so that any surface corrosion had a chance to be displaced.

Since I had a simultaneous Code 42 related to the ESC, I also abandoned the MSD 6A Multiple Spark Ignition System and reverted back to stock.

I drove the truck about 10 mile with several starts in between without setting any codes. I'm keeping my fingers crossed that I won't have to continue this thread.

I'd like to know how Ahawes, the OP solved his problem. Keep in mind, my rig is a stick shift and I don't believe I have another VSS on the Muncie 465 transmission.

Thank you gentlemen for all your help!