I’m new here, but I have a 2002 Chevy blazer with the 4.3L vortec. I recently put this car back together after wrecking it, and I’ve got one last problem to work out before it’s fixed. The car runs and drives, and the dash works perfect when you have it at idle and not moving. Once the car starts to move the spedo, and fuel gauge drop to 0, and every light imaginable comes on the dash. We have checked for our wiring for shorts, but can’t find any. We have checked our grounds, and they are all connected. We do get a specific speed sensor code, and the fuel pump stays on with the key on, it does not shut off after priming. If anyone has had the same problem and could lend some advice it would be very helpful. Thank you in advance.

 

Welcome to the forum, lets get you fixed up.

What areas of the truck did you work on to repair it?

Does the truck run OK during this dash event?

Does it act up when you put it in gear at rest or does the truck need to be moving?

What is the exact code?

Do you have and know how to use a digital multimeter?


George

 

We had to replace the oil pan and front clip of the car. The oil pan required us to pull the motor, but the original wiring harness and motor were still used. We just stole parts from another wrecked one. The car runs perfect even when the lights come on, and the car must be moving for the event to occur. Putting the car in drive, but not moving, does not trigger the event. I don’t have the computer handy so I couldn’t tell ya the code, but I do know it’s a vehicle speed sensor code, and I think a fuel pump relay code. I do have a multimeter, and scan computer to hook it to. We have identified that there are two positive 2.5v readings when one of them should be negative. That points to a short in the wire, but we tested at the computer, and sensors both and they don’t show a short.

 

We are going to need the exact speed sensor code, there are more than one. There are speed sensors on the front wheels if you have ABS and a speed sensor on the transmission. Was work done on any of these or near them? While we are waiting for the codes you can disconnect the three sensors ona at a time to see if the problem goes away.

Have you tried a new fuel pump relay? What happens if you pull the fuel pump relay?

What wires have improper voltage on them?


George

 

I’ll get the codes tomorrow when I go out, but the fuel pump relay is good. When we pull the relay, and turn the key almost all of the lights go away. The speed sensor code is the one on the transmission, not the wheel speed sensors. We have checked the modules one at a time, but we still can’t track down where the “short” is. I’m not sure it’s a short but everything is pointing to it being one.

 

"When we pull the relay, and turn the key almost all of the lights go away". What lights go away?

Checked the modules how and which ones? Checked for power and ground, pulled the connector to them? ...

Why do you think there is a short? To power or ground?


George

 

One possibility for some of this may be data corruption on the class 2 data buss. When the scanner is hooked up tomorrow go to live data and observe some live data elements with engine on, in park - rpm, vehicle speed, ect, etc. Then start moving and see if any of the live data gets weird. Since I am sure that you don't have a scope this is a first order look with the tool at hand.


George

 

Back again. Went to use the computer this morning and it cannot connect to the computer. This has happened before, but I forget how to exactly hook it up to get around it.

When I checked the modules I used a multimeter that hooked to the computer. I then went to the plug near the obd2. It’s a thin white plug that connects to each module. We checked for both power and grounds to each, but couldn’t find what module was messing up. Independently each module works fine, but when hooked together there is not a typical box wave form for voltage. The wave will box at first and then start to curve and then return to a box and repeat.

When we pull the relay most of the lights went away, until the engine was running again. These lights were the ABS, security, low fuel, and service lights.

If the class 2 data buss is corrupted how would it be resolved? Thank you for all the help so far and hopefully this clears up a lot of your questions.

 

The modules, sensors and computers all need a few common things. Proper clean power and ground connections, good clean wiring and connectors and of course proper functioning of each of those elements. Some of them can fail in isolation like say the MAF sensor. If it dies completely the truck might run perfectly ok as the engine computer transitions to the back fuel calculation method which is speed density which does not need the MAF. Other failures have a much wider impact. A failing engine computer is of course one. Another is the class 2 data bus.

This is a collection of modules who's data lines are all tied in parallel at the comb connector near the ODB port. It is a serial communications protocol called j1850 where serial data in the form of digital square wave pulses are the language. Each module transmits on the serial bus and then listens for messages directed towards that particular module. It saves a bunch of wires and is configurable in software so more flexible but if one module corrupts the data then all modules on the bus may struggle to work correctly. A scanner that will not connect is one possible symptom. The first fail safe way to diagnose is to monitor the square wave pulses and look for irregularities. The absolute best way need a gm tech 2 scanner and class 2 commands and evaluation along with gm proprietary codes. For us mere mortals you pull out the comb at the connector and re-enter each module in turn until the poop hits the fan and that’s your problem. Then you have to determine if it’s the module power/ground, it’s wiring/connectors or the module itself. You can either make some diy testing leads or buy pre made stuff:











You may have both class 2 bus problems and separate wiring problems like with the fuel pump but let’s get to a clean bill of health for the class 2 bus first since you already know that the data is corrupted..


George

 

Your class 2 bus and comb connector layout:



George