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73-87 Chevy _ GMC Trucks => 73-87 Chevy & GMC Trucks => Topic started by: bryanedp9 on July 31, 2016, 09:06:11 am
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1989 Crew Cab TBI 7.4 TH400 2WD Haven't owned it long.
Okay, so my speedo consistently reads about 47mph when I'm doing 55. The gap seems to increase proportionally. It'll bob a bit then quit working at times, working again later. The cruise and variable wiper speeds don't work, but hi and low wiper speeds work. So it may be the switch, but I got a code 24 which is supposed to be low voltage or inactivity on the VSS. I also got a egr and lean exhaust code, but that seems totally unrelated.
Anyhow, could the VSS and cruise be inoperable due to the speedo cable? It was a bit loose at the transmission so I hand tightened it a bit, but it seems like it's been fooled with. I don't fully understand where the VSS comes into play with the cable and all. Any help is appreciated.
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VSS with cable is probably the optical pickup and buffer "VSSB" on the back of the instrument cluster.
http://67-72chevytrucks.com/vboard/showpost.php?p=6162822&postcount=5 (http://67-72chevytrucks.com/vboard/showpost.php?p=6162822&postcount=5)
Delay not working is probably the 15598496 wiper pulse module. They're around $50.
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So would you swap the cable first or fool with the VSSB?
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You have three completely separate problems. Troubleshoot. Hail Mary repairs are usually very costly.
- The speedometer is reading is 8 MPH low. You'll need to change the speedometer driven gear or install a ratio adapter to correct this. I posted a Spreadsheet that includes speedometer gear and ratio adapter calculations based on axle ratio and tire size. Tire size can make a big difference in the speed shown on the speedometer.
http://forum.73-87chevytrucks.com/smforum/index.php?topic=30546.msg274738#msg274738 (http://forum.73-87chevytrucks.com/smforum/index.php?topic=30546.msg274738#msg274738)
Mike Ervin Tech article on speedo gears... http://www.73-87.com/7387garage/drivetrain/speedo.htm (http://www.73-87.com/7387garage/drivetrain/speedo.htm) - The Delay module is likely your problem with the wipers.
- You have a cable operated speedometer. Cable speed heads have an Optical VSSB module with two speed pulse outputs for cruise control and EFI ECM starting in 1981. GM used the same Optical VSSB for automobiles with early EFI systems prior to their introduction on the Light Truck platforms in 1987. This is why there's a VSS output signal for an engine ECM on carburetor and Diesel trucks.
A good Optical VSSB with good power will work as long as the speedometer is working. Since the speedometer is working it's not a cable problem so we look elsewhere.
The optical pickup of the VSS is located in the speedometer frame. A reflective blade and optical pickup mount is integrated into all GM Light Truck speedometer heads starting in 1981. The reflective blade rotates, with its blades passing through a light beam from a L.E.D. in the optic head. When the rotating reflective blade enters the LED light beam light is reflected back to a photocell in the optic head. The photocell emits a raw low power speed signal to be sent to the buffer for amplification and signal conditioning. This amplified and conditioned signal is then sent to the buffer output terminals for the ECM and Cruise Module.
The speed head reading 8MPH low will affect the point where the Cruise III controller allows you to engage speed control but it will not set a VSS code 24 error. Code 24 means the ECM is not getting a speed signal from the VSSB. Cruise III does not communicate with the ECM.
I would park on a hard level surface (not gravel or dirt), set the rear axle on jackstands, run the truck in gear, and probe the VSSB output at the ECM and Cruise control module with one of my Oscilloscopes.
If you don't have a scope you can ohm out the signal wire from the VSSB to the Cruise controller and the same to the ECM. If the signal wires Ohm out OK then check that the VSSB is getting power and ground with the ignition switched on. If those all check out OK replace the VSSB.