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Username Post: O2 sensor harness burnt.        (Topic#345423)
ArdyC 
Forum Newbie
Posts: 8
ArdyC
Reg: 05-22-17
06-14-17 06:32 AM - Post#2695892    

So I bought this truck not too long ago, all has been great with it but recently a CEL came on for an o2 sensor. Not thinking much of it I bought a new one (bank 2 sen 1) and was going to replace it. Come to find out after a quick inspection the harness is burnt almost all the way up to the block (including the connector, etc).

Now, my immediate thought is a short somewhere
and there was a ground strap that was ripped/broken - so I hooked it to he starter.

With all of this in mind, would it be a terrible idea to go to above where these wires are fried and rewire from there down, or should I look into other options?

Would love some advice for some of you that know the 5.7 Vortec better than I do.

What do you guys think the best option for me is?

1998 K1500 5.7 vortec auto/4x4

Edit - or is it maybe possible that the o2 sensor failed, burnt wires, and then they shorted on themselves constantly burning further and further up?

Regardless of why, is there an easy solution?



Edited by ArdyC on 06-14-17 06:40 AM. Reason for edit: No reason given.

 
Low priced Genuine GM Auto Parts
CowboyTrukr 
"6th Year" Silver Supporting Member
Posts: 3732
CowboyTrukr
Loc: Salt Lake City
Reg: 06-20-09
06-15-17 05:36 AM - Post#2695993    
    In response to ArdyC

Hey, Ardy.

So, the ground strap was probably originally connected to the frame rail by the starter and up to the firewall, then to the back of the head. Check that one out.

As for the O2 sensor lead, it's very possible that it happened just as you said. I'd replace/repair the wiring first before replacing the sensor.

The sensor should have three wires intact. One is ground, one is key switched battery voltage, and the third is the signal output to the computer. After you've separated the leads, check to make sure you have switched 12V. The output lead has likely shorted to the ground.

Greg

'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500
'00 Explorer XLT 4.0 V6 Auto
'94 K2500 5.7 NV4500 ECLB - SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke


 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18770
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
06-15-17 05:24 PM - Post#2696072    
    In response to CowboyTrukr

The heater for the O2 sensor draws a lot of current, about 20 amps. That is easily enough to burn up wiring that is 14 gauge or smaller.

If you plan to rewire it, I would use 10 or 12 gauge wiring, and I would use a relay to take the majority of the load off the circuit. A relay is an electrically activated high current switch. It's a lot like a large knife switch in old movies, it handles large amounts of current.

The three wires on your heated O2 sensor have three different functions. One is the signal wire that sends a signal to the ECM about the fuel mixture, you should just wire in a new piece of wire to replace this. The second is a ground, you should wire in a thick piece of wire (to carry the high amount of return current out of the heater) to replace this and send it to a known good ground.

The third wire is the power source for the heated circuit on the O2 sensor, and this is the one that should be wired to the relay. You wire up a Bosch relay like this:
You send the old power input wire for the heater to the 85 terminal, this turns on the heater.
You ground the 86 terminal to a good ground.
Wire the 30 terminal to a battery source, 12 volt positive all the time. Find a good power source like the battery or a large battery power source on the fuse block.
Wire the 87 terminal (NOT the 87a terminal) to the power input of the heated O2 sensor using the larger gauge wire previously talked about.

This should eliminate problems with the wiring going to the O2 sensor, make sure it can't contact the exhaust piping or another source of high heat along it's run.



 
2plus2 
Member
Posts: 1057

Loc: Langley,BC
Reg: 02-03-05
06-15-17 07:22 PM - Post#2696097    
    In response to Vaughn

They draw about 4amps ea and are 4 wire for 98

Heater + & - usually both white
Ground and signal

69 Canadian Pontiac 2+2
factory L48 350 Chev,TH350,PW,PT
99 Silverado RCLB 4.8 NV4500 14BSF 4.10's locker 6" Procomp 35 bfg m/t's 9k winch cutout flares
07 Town Car Loaded,Tinted,Getting Bagged


 
65_Impala 
Very Senior Member
Posts: 3847

Reg: 12-29-02
06-15-17 08:28 PM - Post#2696100    
    In response to 2plus2

Yes, the heaters are typically 5 ohms or more so no more than 3A of current draw. They don't draw anything close to 20A.



 
ArdyC 
Forum Newbie
Posts: 8
ArdyC
Reg: 05-22-17
06-16-17 04:55 PM - Post#2696174    
    In response to ArdyC

Thank you all for your reply. I did a ton of reading on getting the wires to match up and so far looks good.

What a pain it was to rewire everything behind the motor. Anyway, I rewired it all and ended up getting it wired properly...I think.

All seems good since I've restarted the truck and ran it a couple times. No CEL so far.

With regards to the ground strap it was going nowhere other than to the top of the frame rail. So for now I hooked it to a starter bolt, however one of you mentioned it should go to the firewall. So I'll have a look and see if I can find it the original location and replace it. I assume you can buy a new ground strap at an auto parts outlet.



Edited by ArdyC on 06-16-17 04:55 PM. Reason for edit: No reason given.

 
CowboyTrukr 
"6th Year" Silver Supporting Member
Posts: 3732
CowboyTrukr
Loc: Salt Lake City
Reg: 06-20-09
06-19-17 05:53 AM - Post#2696504    
    In response to ArdyC

I would be surprised if you find one at the parts stores.

The factory routing went from behind the head to the firewall and down to the frame rail. There would be two straps landing on one bolt on the firewall.

Happy to hear that is running welll again.

Greg

'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500
'00 Explorer XLT 4.0 V6 Auto
'94 K2500 5.7 NV4500 ECLB - SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke


 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18770
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
06-27-17 09:58 PM - Post#2697814    
    In response to CowboyTrukr

“Sensor heaters can draw a lot of current, so battery voltage is usually supplied directly to the heaters through a relay and a fuse."

http://www.autoserviceprofessional.com/article/958...

There are three common methods of controlling the heating element in oxygen sensors. The first method provides a power source to the heater from the ignition switch or a relay anytime the ignition is turned to the run position. This method was used on many pre-OBD-II vehicles without heater diagnostics. A second method supplies power to the heater through a PCM controlled relay. By controlling the heater power relay with the PCM, the circuit can be checked during key-off/engine-off periods. The third method is limited to newer vehicles equipped with Fast Light Off (FLO) oxygen sensors. These sensors have a larger heater for quick sensor warm-up and are current flow limited through the PCM.

http://www.crypton.co.za/Tto%20know/Sensors/oxy gen...

Mid 80s to 95 o2 sensors use a sensor heater that runs around 5 ohms at room temperature. OBDII sensors, like those in a 1998 truck are even less, for faster light off of the sensor (35 sec on some). You have to remember that the sensor gets to 660 F or about 350 C (pre-OBDII) and 1300 to 1472 F (post OBD II), and that will cause the resistance to drop.

If you get near 1 ohm, which isn't all that uncommon,

P=VI, V=IR, V/R = I, so P= Vx(V/R)

an operating voltage around 14.1 (makes the math easier) will give you P=14.1x14.1/1 = 200 watts, sometimes more if the voltage is higher.
If you divide 200 watts by 1 ohms again, that will give you the current squared. P=IxIxR/R or P/R = IxI = 200, which gives you a current of 14.1 amps through the heater. The greater the supply voltage, the more current will end up going through the heater.

By the way, replacing the wires is usually a no-no on the newer wide band AFR (has 6 wires), you can read why in the first link.

If you want general information:
https://en.wikipedia.org/wiki/Oxygen_sensor



 
65_Impala 
Very Senior Member
Posts: 3847

Reg: 12-29-02
06-28-17 01:53 PM - Post#2697894    
    In response to Vaughn

You can write all you want about it, but it doesn't change the fact that the sensors used in these 88-98 trucks are about 5 ohms and draw about 3A. Heater element resistance goes up with temperature, not down.

Show me a factory narrowband O2 wiring harness from the 88-98 years of truck that use 10 gauge or 12 gauge wiring and a relay to power the sensor..... You can't, they all use about 16 gauge wire (could even be 18 gauge on some) and power the sensors from the ignition switch.



 
2plus2 
Member
Posts: 1057

Loc: Langley,BC
Reg: 02-03-05
06-28-17 04:49 PM - Post#2697913    
    In response to 65_Impala


The 99+ trucks use 1 15amp fuse for 2 heater circuits,so thats 2 15 amp fuses for 4 o2 sensors

69 Canadian Pontiac 2+2
factory L48 350 Chev,TH350,PW,PT
99 Silverado RCLB 4.8 NV4500 14BSF 4.10's locker 6" Procomp 35 bfg m/t's 9k winch cutout flares
07 Town Car Loaded,Tinted,Getting Bagged


 
65_Impala 
Very Senior Member
Posts: 3847

Reg: 12-29-02
06-28-17 06:36 PM - Post#2697939    
    In response to 2plus2

99+ and 2 O2's run on a 15A fuse proves what about the 88-98 trucks?

Many GM vehicles in this age range run 4 O2 sensors on a single 15A fuse.



 
ArdyC 
Forum Newbie
Posts: 8
ArdyC
Reg: 05-22-17
07-03-17 03:08 PM - Post#2698574    
    In response to CowboyTrukr

Quick update:

So for the last couple weeks I've driven the truck mildly around town here and there. We then decided to take the truck for a camping trip (about 600KM total trip) and everything was fine. Parked it with no CEL, no problems. A couple days later started it and the CEL came on for a P0430 code. (Catalyst Inefficiency bank 2 I believe).

So now I'm interested if my converter on that side is actually faulty or if the O2 sensor I wired in is getting strange readings from the resistance being outside of the allowed amount from the ECU.

Any basic tests I should do to confirm what actually is causing the problem?

I've been talking about removing the downstream O2 sensors with a tune or a "tricked" system when I do my new exhaust so just FYI for those of you who have the same situation pop up this is where I've gotten myself into.




 
Shepherd 
Contributor
Posts: 884

Loc: Lake George, NY
Reg: 11-11-15
07-03-17 04:38 PM - Post#2698590    
    In response to ArdyC

Using an infrared thermometer, fully warmed up,check the the inlet and outlet temps of the cat, outlet should at least 100 degrees higher than the inlet, if not the cat is bad from an emissions standpoint.



 
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