Truck: 1990 b2600i 4x4 5sp

Situation: When the temperature is less than about 70 degrees Farenheit outside, my truck idles higher and has NO power. This problem will come and go for about the first 45 minutes of operating from a cold start (shorter duration from a warm start) I have noticed that voltage to the IAC jumps from about 3 volts (normal operation of engine) to about 5+ volts (problematic operation) when this occurs. Anyone know what sends this signal to the IAC?? I assume ECM but what sensor or combo of sensors tells the ECM that more IAC is needed?? Maybe MAF??

Already replaced: REbuild ECM by AvPro, coolant temp sensor (for ECM not Guage), fuel pump/filter, fuel pressure regulator, Fuel Relay, O2 sensor.

Any info would be GREATLY appreciated (is is getting colder in Nebraska)!!

Thanks again

 

The IAC is directly controlled by the ECM and uses a great variety of input to determine how much is needed. Am a little confused on the voltage signal reading you are getting. The ISC ( or IAC as you call ) should be getting battery voltage from the main relay at the Black w/Yellow stripe wire. The other wire (Blue w/ green stripe) goes to the PCM and voltage here should vary according to how much idle up is demanded by the ECM. Should be around 11 volts or so with key on engine off. The BAC or bypass air control valve on your truck how two separate, independent sections. The air valve section is a thermo controlled valve that is open when cold and then closes off completely above approx 130'F. It is only there to help provide high idle speed when colde. The ISC section is the electrical part. The voltage readings you are getting do not match with what they should be. Even when at normal operation. 10 volts at idle should be more the normal. Check that signal from the main relay and make sure the correct voltage is getting to the IAC. I was wondering about the IAT or intake air thermoensor causing some of your problems as well. It should have a resistance of around 33,000 ohms at 75'F and drop down to 3500 at 185'F. May give that a check as well. Let know what voltage you find at the wire from the main relay to the IAC with key on and sensor unplugged.

 

Checked the voltage at both of the wires at the ISC (idle SPEED? control). The Black w/ Yellow wire does have battery voltage as you stated (thanks BabyHuey). The Blue w/ Green wire is at about 11 volts when the truck is running fine. I had tested it at a time when the truck was fluctuating between Running well and poorly and when running poorly this wire was giving about 9 volts. Do you think such a drop (2 volts) may indicate another sensor fluctuating in its reading and causing the ecm to fluctuate its Idle Up demand?? I am curious about how to determine if this ISC is bad. Does anyone know the resistance readings for it?? (assuming the Temp related function may be bad and causing this fluctuation in voltage)

I checked the Intake Air Temp sensor too. When it was warm: high 9000's and slowly raising to 10,000's in Ohms. I will do some more monitoring of this sensor. I may have to wait till next week though since I'll be in Cali for a BlueGrass festival this weekend. ( A Nebraskan likes Bluegrass music and drives a pick up. Go figure!!)

 

Another sensor could be causing a fault but it would have to do with the input voltage. The Blue w/ green runs directly to the ECM but the voltage supply Black w/ yellow also supplies power to the injectors, crank angle sensor, purge solenoid, pressure regulator solenoid and mass air flow sensor. If any were causing a drop in input voltage would definitely suspect the mass air flow sensor first. Has been know to do this before. The ISC resistance should be between 7.7 to 9.3 Ohms with cold engine ( 73'F ). Give a check to one other thing as well. Have not seen it cause the problem you are describing but has caused some unusual ones. Make sure the clutch switch is ok. The one to check unfortunately is the one that is hardest to get to. It is the most rearward and is contacted by the pedal when it is at released position. Pedal released should be open. Pedal depressed should be closed.

 

BabyHuey, Thanks for the heads up on that clutch switch, I'll be sure to check it. I was wondering if you (or anyone reading this) may know how to check the MAF. I don't have access to any specs for it. And it is costing some HEFTY loot!! I don't wanna buy one just to find out it my old one was fine. I was also wondering if my truck may have a vacuum advanced timing. I haven't seen anything to support that but it almost seems like the timing is slightly advanced when the truck is running poorly (will rev, but not if push the accelerator pedal down fast. It'll cut out and then back fire like it is getting too much gas if you push the pedal down fast). I had a Honda Accord that did that too. Since it was really cooled down today in Eastern Nebraska I also noticed that while the truck is warming up (same times that it goes from running fine to running poorly) the brakes are REAL touchy (locking up). This makes me think there might be some type of vacuum trouble (reasoning behind suspecting Vacuum Advance timing) cuz the brake master cylinder has a vacuum line running to it. This may be a seperate problem too. Any suggestions would be great.

thanks

 

No your truck does not have vacuum advance on the timing. All electronic control. The brakes locking up is probably due to moisture in the morning on rear shoes. Not too uncommon on these trucks. If you were having a vacuum related issure it would manifest as a hard and very high brake pedal. I will list the voltage specs for the wires at the MAF sensor. They are rather vague but if you could compare readings at idle and maybe 2000 rpm when operating normal and then compare with readings when acting up a fault may show up. The Black wire with yellow stripe should have 12volts at all times key is on or running. This is power from main relay. The Green w/ Yellow stripe and Black with Orange stripe wires are both grounds. 0 volts and continuity to ground all the time. The Green w/ Orange wire is for burn off ( this is designed to clean off the sensing wire ). It should have 0 volts all the time with one exception. It should have 8-12 volts momentarily 2-5 seconds after ignition switch is turned off. The last wire is most important as it is the air flow signal wire to the PCM. It is Green w/ Black stripe. With ignition on should have 1.0-2.0 volts. With engine running should read between 1.9-5.0 volts. The higher the air flow amount the higher the voltage. Hope this will help. Might also check the throttle position sensor signal too. The signal wire is the Green w/ Yellow stripe. Get a few good reference voltages when running ok and then compare with voltage when running poorly.

 

Thanks BabyHuey. I'll take those voltage readings and check 'em out.