OBD1 help

[85c10owner]

Um...... Not sure. I assumed fuel injected when I saw OBD1. That would require more research.

1985 GMC C10 carbed 305/700r4. It's my understanding that obd1 is from 82-95. I do plan to convert to either TBI or Vortec engine.

 

[chengny]

CODE 23 - M/C SOLENOID VOLTAGE LOW TO ECM

Circuit Description :
Code 23 says the ECM has seen the voltage at ECM 18" stay low instead of rising and falling as the M/C solenoid is turned "ON" and "OFF". This could be caused by an open in the M/C solenoid circuit or a ground on the ECM side of the M/C solenoid.

An open would cause a full rich condition and cause poor economy, odor, smoky exhaust or poor driveability. A ground would cause a full lean condition and cause poor driveability.

Test Description :
Checks for a complete circuit from the battery to the M/C solenoid dwell lead. It should be battery voltage. Battery voltage means there might be an open circuit between the dwell connector and ground. No voltage could be either an open between the connector and battery or a ground on the ECM side of the M/C solenoid.
Checks for B+ on the pink ignition source wire. Test light should light between the ignition source and ground.
2A. Checks for an open in the solenoid to ECM circuit. A normal circuit would read about battery voltage at the ECM Terminal "18".
This step determines whether the fault is in the M/C solenoid, a ground in the circuit to the ECM, or the ECM. A light would indicate a ground in circuit to Terminal "18" or a faulty ECM. A voltmeter can't be used because it is normal to have enough current flow through the ECM even with the circuit open to make a voltmeter read, but not enough to light a test light.
This checks for ground in the wire to ECM Term. "18". If it is grounded, the light will stay "ON".


CODE 34 - DIFFERENTIAL PRESSURE (VAC) SENSOR

Circuit Description :
Code 34 says that the ECM has seen the following:

Pressure outside a specified voltage range (seen by ECM as voltage at Term. 20).
Engine RPM less than a given value (normally curb idle).
Engine at operating temperature.
All the above for a time greater than specified.
The VAC sensor is a differential pressure sensor that measures the difference in pressure between atmosphere and manifold. The VAC sensor supplies high voltage at high vacuum. High voltage increases spark advance while low voltage reduces advance.

Test Description :
Checks output of sensor at idle to determine if sensor is within specification. Normal sensor will read less than 1 volt with key "ON", engine "OFF" and over 3 volts with engine idling with a minimum of 15" vacuum.
A normal sensor should drop below 1 volt when vacuum is removed. This step tests for that drop at the sensor.
Check for a ground in wire from Term. "B" of VAC sensor to ECM. This would be the case if the voltage went up over 2 volts when the line was opened.
Checks to see if the fault is in the sensor, or in the ECM wiring, or in the ECM. If the voltage goes over 2 volts with the sensor disconnected, the sensor or sensor connections are faulty.


CODE 54 - M/C SOLENOID VOLTAGE HIGH TO ECM

Circuit Description :
Code 54 will be set if there is constant high voltage at ECM Term. "18".

A shorted solenoid or a wiring short to 12V on circuit to ECM would cause the solenoid to remain in the full rich position, resulting in potential ECM damage, excessive fuel consumption, and excessive exhaust odor.

*** THIS PROCEDURE HAS BEEN UPDATED BY TSB #86-B-102, DATED 7/86

Some 1985-86 vehicles may experience a shorted out electric purge solenoid valveto the canister purge system. This condition may be caused by moisture enteringthe electrical portion of the canister purge solenoid valve and setting a false Code 54. If this condition is encountered and the inspection of the solenoid indicates that the replacement of the solenoid is required, replace the purge solenoid valve and install a new moisture shield P/N 14066503, which has been released to correct this condition.

The ECM may also have a failed quad driver which controls the M/C solenoid and the ECM should be checked to make sure it was not damaged if the purge solenoid was shorted.

Before the new moisture shield can be installed modifications to the purge solenoid valve bracket and moisture shield must be made.

MODIFICATION: SOLENOID VALVE
The solenoid valve bracket must be reshaped to allow the valve to fit inside the moisture shield. Hold the solenoid valve and bracket firmly and bend the bracket into an "L" shape.

MODIFICATION: MOISTURE SHIELD
The moisture shield must be modified to allow the installation of a tie strap to hold the shield to the solenoid valve. A "V" shaped slot must be cut in both sides of the shields opposite the holed tab side of shield. After the modification to the shield and solenoid bracket have been completed, install the shield on the solenoid. Install the tie stap, P/N 15501907 tightly around the solenoid shield and cut off the excess length. Make sure that the tie strap is positionedbetween the center hose port and the electrical connection to the solenoid valve.

Test Description :
Checks the M/C solenoid resistance to determine if the fault is in the solenoid or ECM harness/ECM. A normal solenoid has about 20 to 32 ohms of resistance.
Checks to see if reason for high voltage to Term. '18" is a faulty ECM or a short to 12V on that wire. If the test light illuminates with both ends of harness disconnected, there is a short to 12V in the wire.



I have the associated diagnostic charts and wiring diagrams for the above if you need them let me know.

 

[chengny]

Since the issue seems to be related to the operation of the MC (mixture control) solenoid. You might want to try disconnecting it. With the MC solenoid de-energized the valve will default to the fully open position. This will result in a somewhat increased rate of fuel consumption but the carb will still function well.



ECM wiring diagram (it's actually for a 1986 V8 but should apply to an 85 as well):

You must be registered for see images attach

 

[85c10owner]

CODE 23 - M/C SOLENOID VOLTAGE LOW TO ECM

Circuit Description :
Code 23 says the ECM has seen the voltage at ECM 18" stay low instead of rising and falling as the M/C solenoid is turned "ON" and "OFF". This could be caused by an open in the M/C solenoid circuit or a ground on the ECM side of the M/C solenoid.

An open would cause a full rich condition and cause poor economy, odor, smoky exhaust or poor driveability. A ground would cause a full lean condition and cause poor driveability.

Test Description :
Checks for a complete circuit from the battery to the M/C solenoid dwell lead. It should be battery voltage. Battery voltage means there might be an open circuit between the dwell connector and ground. No voltage could be either an open between the connector and battery or a ground on the ECM side of the M/C solenoid.
Checks for B+ on the pink ignition source wire. Test light should light between the ignition source and ground.
2A. Checks for an open in the solenoid to ECM circuit. A normal circuit would read about battery voltage at the ECM Terminal "18".
This step determines whether the fault is in the M/C solenoid, a ground in the circuit to the ECM, or the ECM. A light would indicate a ground in circuit to Terminal "18" or a faulty ECM. A voltmeter can't be used because it is normal to have enough current flow through the ECM even with the circuit open to make a voltmeter read, but not enough to light a test light.
This checks for ground in the wire to ECM Term. "18". If it is grounded, the light will stay "ON".


CODE 34 - DIFFERENTIAL PRESSURE (VAC) SENSOR

Circuit Description :
Code 34 says that the ECM has seen the following:

Pressure outside a specified voltage range (seen by ECM as voltage at Term. 20).
Engine RPM less than a given value (normally curb idle).
Engine at operating temperature.
All the above for a time greater than specified.
The VAC sensor is a differential pressure sensor that measures the difference in pressure between atmosphere and manifold. The VAC sensor supplies high voltage at high vacuum. High voltage increases spark advance while low voltage reduces advance.

Test Description :
Checks output of sensor at idle to determine if sensor is within specification. Normal sensor will read less than 1 volt with key "ON", engine "OFF" and over 3 volts with engine idling with a minimum of 15" vacuum.
A normal sensor should drop below 1 volt when vacuum is removed. This step tests for that drop at the sensor.
Check for a ground in wire from Term. "B" of VAC sensor to ECM. This would be the case if the voltage went up over 2 volts when the line was opened.
Checks to see if the fault is in the sensor, or in the ECM wiring, or in the ECM. If the voltage goes over 2 volts with the sensor disconnected, the sensor or sensor connections are faulty.


CODE 54 - M/C SOLENOID VOLTAGE HIGH TO ECM

Circuit Description :
Code 54 will be set if there is constant high voltage at ECM Term. "18".

A shorted solenoid or a wiring short to 12V on circuit to ECM would cause the solenoid to remain in the full rich position, resulting in potential ECM damage, excessive fuel consumption, and excessive exhaust odor.

*** THIS PROCEDURE HAS BEEN UPDATED BY TSB #86-B-102, DATED 7/86

Some 1985-86 vehicles may experience a shorted out electric purge solenoid valveto the canister purge system. This condition may be caused by moisture enteringthe electrical portion of the canister purge solenoid valve and setting a false Code 54. If this condition is encountered and the inspection of the solenoid indicates that the replacement of the solenoid is required, replace the purge solenoid valve and install a new moisture shield P/N 14066503, which has been released to correct this condition.

The ECM may also have a failed quad driver which controls the M/C solenoid and the ECM should be checked to make sure it was not damaged if the purge solenoid was shorted.

Before the new moisture shield can be installed modifications to the purge solenoid valve bracket and moisture shield must be made.

MODIFICATION: SOLENOID VALVE
The solenoid valve bracket must be reshaped to allow the valve to fit inside the moisture shield. Hold the solenoid valve and bracket firmly and bend the bracket into an "L" shape.

MODIFICATION: MOISTURE SHIELD
The moisture shield must be modified to allow the installation of a tie strap to hold the shield to the solenoid valve. A "V" shaped slot must be cut in both sides of the shields opposite the holed tab side of shield. After the modification to the shield and solenoid bracket have been completed, install the shield on the solenoid. Install the tie stap, P/N 15501907 tightly around the solenoid shield and cut off the excess length. Make sure that the tie strap is positionedbetween the center hose port and the electrical connection to the solenoid valve.

Test Description :
Checks the M/C solenoid resistance to determine if the fault is in the solenoid or ECM harness/ECM. A normal solenoid has about 20 to 32 ohms of resistance.
Checks to see if reason for high voltage to Term. '18" is a faulty ECM or a short to 12V on that wire. If the test light illuminates with both ends of harness disconnected, there is a short to 12V in the wire.



I have the associated diagnostic charts and wiring diagrams for the above if you need them let me know.

Wow, thank you. Would you like to just come fix it for me? LOL. I`ll have to print this out tomorrow and get back on it. It does smell rich at idle and has a hesitation just off idle and now giving it gas in the 20-35mph range it has a noticable miss. Hoping I either got the firing order wrong when I did the wires or just knocked something loose or possibly have cross fire between the wires, 8mm AC Delco.

Another thing I forgot to mention is I did spray some carb cleaner around the carb because it was pretty grungy and I also hosed off the engine compartment with the engine running as it was very dusty. Im thinking one of these could have caused or helped cause my issues.

 

[chengny]

Locations of the vacuum differential sensor and the MC solenoid:

You must be registered for see images attach

 

[chengny]

The vacuum differential switch (similar to a MAP sensor) is probably the primary input that the ECM uses to control the operation of the MC solenoid. Unfortunately, I don't know that much about these engine management systems - so I can't speak with any authority.

But, given the information above, I would look closely at the vacuum differential switch. Check the associated wiring and vacuum hose for breaks and poor connections.

Note test # 3. If the ECM throws a 34 it indicates that the signal from the vacuum switch is out of range.



SYSTEM PERFORMANCE CHECK

Test Description :

Checks for ability of carburetor main metering system to change the Air/Fuel mixture. Disconnecting M/C solenoid makes carburetor operate full-rich and reconnecting it with the dwell lead grounded makes it operate full-lean. Normal response - RPM drops as M/C solenoid is reconnected. Usually 400-1000 RPM, but should be at least 300 RPM.
1A. Some M/C solenoids are polarity sensitive and will not pull solenoid plunger down if polarity is reversed.

If plugging the PCV, Purge, or Bowl Vent vacuum hose causes RPM to drop over 300 RPM, that hose leads to the source of the problem. If RPM increases as M/C solenoid is connected, it indicates the system is running extremely rich. This can sometimes be caused by incorrect valve timing.

Checks for proper control of idle circuit.

2A. This is a full-rich command to the carburetor and can be caused by:

Lean engine condition.
Grounded oxygen sensor wire or bad sensor.
Open in wire from ECM term. "14" to ground.
Open wire to ECM term. "22".
Open in coolant sensor circuit.


2B. There is an open loop condition. It can be caused by:
An open oxygen sensor circuit or bad sensor.
An open in coolant sensor circuit.
An open in wire from ECM term. "14" to ground.

2C. This is a full lean command. It can be caused by:
Rich engine condition caused by:
M/C Solenoid wire connections reversed.
Leaking Bowl Vent valve, excessive fuel in vapor canister, fuel in crankcase, faulty carburetor calibration or carburetor.

2D. Normal reading - operates in closed loop - dwell is between 10~-50~, but varying. Running for 1 minute at fast idle is to make sure the oxygen sensor is warm.

3. Checks for proper control of main metering system. RPM must be at least 3000 to get into the main metering system operation. Removing and plugging the hose may set a Code 34 so memory will have to be cleared.

3A. A missing O-ring between the switching valve solenoid and the valve, or a faulty valve, may cause air to leak to the exhaust ports at higher RPM only.

 

[85c10owner]

I have also been experiencing a very low idle at times and it will just die on its own. I realize this is hard to answer question but what are the chances I just knocked some of this stuff loose when I removed the air cleaner assembly to get to the cap and rotor? It really ran no different after the tune up initially and has gotten worse. I dont recall any check engine lights on when I drove it home the night I bought it. I`m going to go check all this stuff tomorrow. Thanks again.

 

[85c10owner]

Guys I need some serious help. If I cant get this truck running right by the weekend I`m selling it. I bought this as my daily driver and I have nothing else. My 97 GMC was totaled so I bought this POS. I dont have a test light or a voltage meter to do all the checks listed above correctly so we have to try this the backyard way.

This is whats going on, idles smooth enough but choke/high idle no longer comes on. When I try to drive it I have a serious misfire. Checked the firing order and its correct. New plugs, wires cap and rotor. I did find a connector coming on the t/stat housing that goes into the carb is missing one side of the connector and someone just used small butt connectors to plug the wires into the other side of the plug. I dont know what any of these parts are because I usually stay far away from smog crap but thought I was getting a smokin deal so I bought it.

I disconnected the battery but that did not clear the codes but when I go to jumper the a/b terminals there is a strange noise from under the hood and the check engine light does not flash like it should in diagnostic mode. I`m at a loss. I`ve been wrenching on cars and trucks for 25 yrs and was a Harley Davidson dealership mechanic for 7 yrs so my mechanical ability is strong. I just dont get this smog crap and dont understand why it has a misfire unless the capand/or rotor is bad.

 

[85c10owner]

Little update--

Changed the plugs and it seems to run better. I used what came with the truck and they were a different length plug than what came out. I also let it warm up a bit since the choke doesnt seem to work. Wanted to stumple and die at first but I kept the idle high and after a couple minutes it idles fine on its own. Still has the check engine light and runs pig rich at idle but actually runs pretty strong on the freeway for a little 305. Sounds like glasspacks right off idle but its a flowmaster so I know the tune isnt perfect. But it seems to be driveable at least. now I just need to get a test light and voltage meter so I can do the checks listed above.

 

[85c10owner]

Can anyone tell me what this is? Today I was messing around under the hood and sprayed some carb cleaner on the carb body and when I fired it up about 10 minutes later the misfire was back. The check engine light was actually flashing in sync with the misfire and I heard something clicking in sync under the hood as well. Turned out to be this. I could feel it clicking and when I unplugged it, it stopped. Took it for a spin and it ran really bad, could barely drive it. Started heading back and then all of a sudden it cleaned up and ran fine. I just dont know what these components are called and havent had the money to go order a repair manual yet. I`m half tempted to just replace the carb with a rebuilt unit from rockauto.com. That might be the next step and probably wouldnt be a bad idea on a 30yr old truck.

 

[85c10owner]

Another small update- Took it to a friends shop today to get a better look at stuff since I dont really have the tools to mess with it at home. The timing was at zero so we bumped it up to 8*. We also did some checks on the EGR Solenoid and that seems to be sticking sometimes so we are thinking that might be the problem. The bad thing is nobody seems to have one available. THe good news is the truck runs 100x`s better. No more popping and slight hesitation right off idle. Once I get the light out I can smog it and drive the crap out of it. Then I can tackle the other problems it has.

 

[Georgeb]

Another small update- Took it to a friends shop today to get a better look at stuff since I dont really have the tools to mess with it at home. The timing was at zero so we bumped it up to 8*. We also did some checks on the EGR Solenoid and that seems to be sticking sometimes so we are thinking that might be the problem. The bad thing is nobody seems to have one available. THe good news is the truck runs 100x`s better. No more popping and slight hesitation right off idle. Once I get the light out I can smog it and drive the crap out of it. Then I can tackle the other problems it has.

Good news!

 

[85c10owner]

Ok, bringing this back up. I need to go back and read some of the comments to refresh myself but here is the current situation,

Finally located the correct 3 port EGR solenoid and replaced that along with a new EGR valve. I disconnected the battery for a few minutes and then started it up and I still have the CEL. I checked and its only Code 54.

I realize the code is mentioning the the M/C solenoid but I dont have the symptoms. The fuel smell I had was a bad fuel pump and my fuel mileage is mid teens so its far from poor. runs super smooth. Sometimes you cant tell its running.

So lets assume my M/C solenoid is good for argument sake. What else would cause it to trigger code 54?

 

[85c10owner]

Ok, another update. Met a guy on another forum and with his help and after he went through my carb and correctly set everything we discovered that my ECM would not send a signal to the mixture control solenoid. He gave me another ECM to try and it worked. Truck ran strong, CEL disappeared. Then that night my alternator went out and the next day after I replaced it the truck would no longer run right. Put my old ECM back in and ran perfect except the CEL is back on. So now I`m on the hunt for a EMC and then try to figure out why I have shorted out the other ECM.

 

[87 Silverado]

engine code 54

I've been having the same issue with mine,I've checked everything listed on here but nothing is coming up,did you find the problem with yours?