LT1 Based Engine Tech 1993-1997 LT1/LT4 Engine Related

Codes 26, 27 and 77 and Fuse #6 Blown on 1995 Z28. HELP! <long>

Old 09-05-2004, 10:32 PM
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Unhappy Codes 26, 27 and 77 and Fuse #6 Blown on 1995 Z28. HELP! <long>

THE PROBLEM:
My SES engine light went on last night and my scanmaster pulled three codes. Prior to this event, no codes were recorded by the computer.

Here are the codes and what they mean:
#26: EVAPORATIVE EMISSIONS CANISTER PURGE SOLENOID CIRCUIT
#27: EGR SOLENOID CIRCUIT
#77: COOLING FAN RELAY CONTROL CIRCUIT

I also noticed that Fuse #6 (Underhood Fuse and Relay Box) was blown. The Fuse is for the "Power feed from fans/actuators". The fans would not activate and the car would overheat. I tried using the AC, my manual fan switch button and getting it how, and the system remains dead until I replace the fuse.

SOME DIAGNOSIC WORK:
I reset the computer and I replaced the Fuse with another 10 amp and drove for a short period of time. After driving for several minutes, the Fuse blew again and the codes popped up again.

I then placed a 25amp fuse (I needed to drive with at least the fans on) and while the SES and codes pop up again, the fans continue to work and the fuse does not blow. However, a huge amount of current is drawn as my voltage cruising on the highway remains at 11.8 to 12.0 volts (it is usually close to 13 at least) with no accessories/AC on. The car sets the code after idling for a while or when driving for a short period of time (may be timed with Closed look but will check for sure tomorrow)

FIGURING OUT THE WIRING:
I looked at Shoeboxes wiring schematics and found out through this schematic http://shbox.com/1/1995_pcm2.jpg that both the EVAP and EGR are on the same circuit, a circuit that passes though Fuse #6. Therefore I have found a relationship between the codes shown. I have an A4, so the 2 Manual Tranny circuits are not relevant, however, the circuit does also relate to the heating of the 02 Sensors.

DETERMINING A SOLUTION:
Of course if anyone has a solution to the problem, I'd love to hear about it as soon as possible... since I need to get this fixed. Here are some questions I needed answered though that would help me figure it out if you don't have a definitive solution.

WHAT I HAVE FIGURE OUT SO FAR.
1. It has nothing to do with the Fan electrical system, as the failure does not occur when only the fans are running but only when the car is driving or idling and another system is begins working.

2. It is not a relay as these have been checked and Relays often fail or do not fail however this problem does not create an absolute failure of any system with a 25amp fuse. (Ie. EGR seems to be working and driveability remains great)

Questions?
1. Would a problem with EITHER the EGR or EVAP system cause a failure in the other system.... give a code?
2. What could possibly drain enough power to drain the battery, but not blow the 25amp fuse?
3. Would an O2 sensor HEATING SYSTEM failure cause a code as it is in the same system as EVAP and EGR but O2 sensors seem to be working fine and no codes are thrown for them?
4. What does "Other Circuits" mean in the Diagram http://shbox.com/1/1995_pcm2.jpg . Are they important or may be causing the problem?
5. Does anyone know the normal resistance of EVAP, EGR and O2 Heat circuits? How would you test them?

I know this has been a long message, but I am hoping to be as thorough as possible so i can fix the problem. I will be looking more closely tomorrow at each system, but if you guys can help, that would save me a lot of time.

Thanks in advance.
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Old 09-05-2004, 10:59 PM
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Re: Codes 26, 27 and 77 and Fuse #6 Blown on 1995 Z28. HELP! <long>

Risky to put a larger fues in there. The fuse is there to protect your wiring. Burn your wiring up and you will really have an expensive mess. I see you have headers. the most common thing to happen is the O2 harness to get burned on them. Check that first.
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Old 09-05-2004, 11:06 PM
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Re: Codes 26, 27 and 77 and Fuse #6 Blown on 1995 Z28. HELP! <long>

Thanx for the quick reply Shoebox.

I put in a larger fuse only because I was in need of getting the car home without overheating.

As for the headers, they have been on the car for the last 4 years with no issues..however I will check to see if the O2 harness have been burnt. Where is it actually located?

Tomorrow I will also be seeing when the SES actually turns on. From my understanding, the EGR, EVAP and O2 sensors would be activated at different times. If I can pinpoint when the SES light goes on, I may be able to determine what part of the circuit is causing the problems.

O2: Circuit is ENERGIZED when the car is warming up as that is when Heating the O2 sensors is important and will deactivate after the car is in closed look. THEREFORE THE SES LIGHT SHOULD START RIGHT AWAY OR WILL ACTIVATE WHEN GOING INTO CLOSED LOOP.
EVAP: This solenoid is energized during a cold engine condition and is de-energized after warm-up when the engine is above a specified RPM. I will look at the CCP values as well. THEREFORE THE SES LIGHT SHOULD GO ON IF THE CAR IS IDLING AFTER WARM UP AND I REV THE MOTOR ABOVE A CERTAIN RPM (CCP VALUES WILL BE OFF AS WELL I THINK).
EGR: When the recirculation of exhaust gases is not desirable, the EGR solenoid is energized, blocking engine vacuum pressure to the valve. This condition usually occurs during periods of wide open throttle, engine cranking, low coolant temperature, or engine idling. THE SES LIGHT SHOULD NOT COME ON DURING IDLING IF IT IS THE EGR SINCE IT REMAINS ENERGIZED.

What do you think Shoebox?

Last edited by C Man; 09-05-2004 at 11:18 PM.
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Old 09-06-2004, 07:42 AM
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Re: Codes 26, 27 and 77 and Fuse #6 Blown on 1995 Z28. HELP! <long>

Note, as you had asked for the resistance of the devices but I could not find them. The 2 solenoids will have very low resistances since they are coil windings. I can tell you this, all of the codes you got were due to the PCM detecting the wrong voltage on the control lines for each device. Meaning that more than likely the fuse blows then the SES light comes and therefore the fuse blowing is your whole problem.

So you need to figure out why the fuse keeps blowing. You can't simply read resistance at the fuse plug since 2 of the devices I mentioned are coils. After you check the devices for wiring problems like Shoe mentioned, unplug all 4 of the devices. Now you can measure the resistance from the fuse contact to ground. You need to find the correct fuse contact. First measure both sides of the fuse contact to ground with the meter set to DC Voltage. One side is 12 volts and the other is 0Volts. Now set the meter to ohms and measure the side that was 0Volts to ground. I high lighted this passage as it is the most critical and may be hard to follow at first. If it is measuring low ohms, you have a wire shorted to ground somewhere. The only thing you can do at this point is start wiggling wires to see when the meter jumps higher. (See the note at the bottom) If the fuse contact is not measuring low resistance (wire not shorted) then your problem is one of the 4 devices. To find it try the following: Check the fuse in the multimeter to be sure it is not blown cause if it is, the current setting of the meter will not work. (this fuse has nothing to do with the other functions of the meter). Set the meter and move the wires accordingly to measure 10 AMPS. Put it across the fuse holder contacts. One lead on one side and the other lead on the other side. Now one at a time, plug in one of the 4 devices and test each to see how much current you are drawing. The one that pegs the meter is the one that is bad. Now that you found it, try running with all but that device plugged in with the 10AMP fuse in. NOT the 25AMP fuse.

2 things that you can do to help you out are get a multmeter with a buzzer for low ohms so you don't have to be looking at the meter all the time. The other is, unplug the PCM while your doing your final search with the ohm meter as they do put out voltage and you don't want to cause anymore damage than is already done. Also, if you are using the older type analog meter with a needle then the leads of the meter must be correct. To measure voltage or current the red lead must be on the supply side.

Good luck
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Old 09-07-2004, 07:03 AM
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Re: Codes 26, 27 and 77 and Fuse #6 Blown on 1995 Z28. HELP! <long>

Originally Posted by shoebox
Risky to put a larger fues in there. The fuse is there to protect your wiring. Burn your wiring up and you will really have an expensive mess. I see you have headers. the most common thing to happen is the O2 harness to get burned on them. Check that first.
I should have listened to you Shoebox on both points. Regarding the burning of wires, this morning I started the car and saw some smoke coming out of the wiring harness. Turned the car off and checked the harness. I found a brown wire, over the intake manifold had been a little burnt. No big damage though, as I was able to replace the wire easily and no other damage was found.

The burnt wire did help me find out the point in your diagram on http://shbox.com/1/1995_pcm2.jpg where all the 541 Brown Wires joined. I found 5 wires crimped and soddered together located on top of the intake manifold in the numerous wires wrapped together. 3 wires went to the Passenger side of the car while 2 went to the drivers side.

The 3 to the passenger side went to:
1. passenger O2 sensor heating element (it was a thicker wire too)
2. evap circuit
3. to a connector (C100) near the computer

The 2 on the Driver side went to:
1. Driver O2 Sensor heating element (it was the thicker of the 2 wires)
2. Egr solenoid

The burnt wire was the one going to the C100 connector. After replacing the wire, I made connectors for each wire and one by one started attaching them to the circuit until my fuse blew (now 10 amps again). I determined that I am getting a short on the passenger side O2 sensor heating element and have left that part of the circuit disconnected until I can solve that problem.... it took me more than half a day to get to this point and I didn't have any more time.

I am now getting no codes and the car is driving fine with the O2 Heating Element disconnected.

Thanks for your replys guys.... and especially to Shoebox, without your diagram available on your website, it would have been a significantly harder problem to solve.

I have started another post regarding the possibility of Driving without the O2 Heating Element at this Link for those interested. http://web.camaross.com/forums/showthread.php?t=298468

Last edited by C Man; 09-07-2004 at 07:15 AM.
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