eallanboggs

iTrader: (1)

You need a meter and a schematic. You need to know the pinout on the PCM. You also need to know if there are any intermediate connectors between the O2 and the PCM. Do the obvious first like checking fuses and looking for pinched, frayed or damaged wires. If you see soot on a metal part that's a sign a HOT wire has found ground(HOT wires are NOT supposed to find ground, at least not before they go thru a load). When you see soot or signs of arcing do some closer examination it that area. Then start probing with the meter. You can either start at the PCM and work towards the O2 or do it the other way around. You need to know what voltage your supposed to see at any given point to know whether the results you get are correct or if you have found the problem. Without knowing this how are you supposed to be able to interpret you readings?

budhayes3

iTrader: (19)

Correct, but a wire shorted to another circuit may just power up the load on that circuit (assuming that the load is already grounded) and remain undetected unless the PCM picks up a fault. After checking the fuses, if they're all OK, power can be removed from the suspect circuit and a short to ground can be checked by using an ohm-meter and seeing if there is continuity with ground. If the meter shows .5 ohms or less, then you have a direct short to ground.

I've seen critters chew through harnesses and create direct loops in harnesses, with no loads, that did not blow any fuses and surprisingly did not smoke the PCM. In most of these cases though, there was no check engine light with the key on, and no way to communicate with the PCM as it was being bypassed by the looped circuit.

eallanboggs

iTrader: (1)

Many times these kind of problems show themselves AFTER a mod. had been completed such as installing headers. That's why extra care is needed when installing aftermarket parts or even when replacing OEM parts. You need to be sure wires are not pinched or near HOT or moving parts. Wire ties, black plastic tape and standoffs help here. Insulation gets damaged for all sorts of reasons whether it's caused by a careless installation or a car wreck. Imagine trying to trace out electical problems after a car has been in a wreck with the potential for all kinds of pinched, cut and shorted wires. What a nightmare. Regardless of how it happens Ohms law still applies. If a wire is touching ground, but doesn't blow a fuse it's probably because of dirt, rust or corrosion on the wire itself or on the metal surface it's touching(chassis). Even shorts need good conductivity to qualify as shorts. If a wire is touching ground that usually qualifies as a short, but if there is enough rust, dirt or corrosion between the wire and ground the "short" might not conduct enough current to excede the rating of the fuse and it won't blow as expected. It might intermittently arc leaving behind soot or other signs of arcing so you have to get under the vehicle and look closely at your wire harnesses. The likelyhood of a wire having its' insulation stripped back and touching another stripped wire thereby piggybacking itself onto that circuit is slim to none. If you can get that lucky and everythings still works properly you better go out and buy a Powerball ticket. When troubleshooting these sorts of problems you have to know what you're doing. Can you read a schematic? Can you use a DVOM? Do you know how to proerly backprobe a circuit? Do you know what voltages you're looking for on each wire so you properly interpret your readings? If you don't know what voltages your looking for you have no idea if you have found your problem or not when you take a reading. Good luck. If you don't have experience with this stuff you're going to need it. Go out and buy a Powerball ticket. That way if you do win you can afford to pay someone who does.

budhayes3

iTrader: (19)

Well, it appears that kirtondog23 has his problem narrowed down to a 4 wire circuit (assuming that the O2 sensor circuit is the problem), there is a schematic posted in this thread, and he has described some parameters that would lead one to believe that he has some info on how the circuit works. He posted here for help, so that's what we're trying to accomplish with the limited resources that we have...before he goes somewhere and gets charged $xxx/hr labor for someone else who doesn't have a clue to try and diagnose an electrical problem.

This statement is open for debate....if you've worked in the field than you've seen wires pinched together, especially on cars that have been wrecked and had bodywork...or been modded or had aftermarket wiring installed. Even more common are pins in the connectors shorting together after they've been unplugged and re-plugged...this can create major problems and still not blow a fuse. Yes, maybe it's not a true "short", but it does happen. I had 2 cars last month alone that came back from body work with the male pins of connectors "shorted" together because some hack bodyguys couldn't figure out how to plug the connectors back in straight.

Just curious, are you directing these questions at me? If so, the answers to all of them are yes

You bring up a good point here...maybe someone could post up the proper voltages on the circuit in question, if the thread starter hasn't already found that info yet.

eallanboggs

iTrader: (1)

Bud, my statements were not directed at you. You sound like you know what you're talking about. Troubleshooting electrical circuits isn't something you can wake up one morning and decide you want to do just because you have a problem. At least with automotive stuff the voltage is low and you can't electrocute yourself. About the worst you can do it grab a spark plug wire. With the O2s he can use the swap part and comparison method to troubleshoot, but once he narrows it down to one or the other he is still going to have to do some point to point voltage checks. He needs to know you to backprobe, read a schematic, use a pinout sheet to locate a wire,etc. These kinds of things come from experience not a textbook.

budhayes3

iTrader: (19)

Cool deal man...

You bring up alot of good points also...electrical diagnosis gone wrong can equal more damaged wires, distorted and elongated terminals, smoked modules, and alot more cash out of pocket than necessary in the long run. Then when someone who is qualified has to perform the diagnosis, they not only have to deal with the original problem, but all of the other problems that were created afterwards

eallanboggs

iTrader: (1)

One more thing he really needs for this job is a good scan tool. They are very helpful in viewing sensors that switch voltages very quickly. The O2 sensors are at the top of this list because the PCM constantly adds and subtracts fuel trying to maintain the EPA mandated 14.7 AFR. The sensor reading switches above and below 450mVdc during normal operation and this happens too quickly to see on a DVOM or a Simpson. You need a scan tool, labscope or oscilloscope for that. I use AutoTap and it kicks butt. I can view over 20 pids at the same time. Try doing that with a TechII. You can display the pids in gauge, table or even graph format. That's like having a 20 channel Oscope if you can imagine that. With a good scan tool he could diagnose his problem in minutes.