Cant figure EEC out (Differing results)

[Hoffman]

I posted earlier, but hit a dead end. In my earlier post I had a major misfiring problem. So I checked the engine codes and came up with 22, 35, 51, 53, and 54. Which are all problems with the tps, egr, map, and act, evr sensors. So Jrichker sugessted i check the sig return line on all of these sensors with the battery ground. All checked out with resistance above 1 ohm. So I checked resistance to the 10 pin connector and to the ecu on the sig rtn line, all checked out fine. I then cracked open the computer to see if pin 46 was broken inside the case (it wasn't). Then checked the two ground pins to the battery to see if they had connection, which also checked out fine.
While I was messing around with this i also checked the fuel pressure, which was 40 psi at idle, with a 5 to 10 lb drop when it was revved to about 2500 rpm (don't know if this is normal) I checked the vaccuum at idle which was useless since the idle was nonexistant to begin with. But when i held the throttle at 2000 rpm the gauge held a steady 15 inches which ruled out valve train problems. And I checked the timing which was set at about 12 deg.
So I went back to the sig rtn line problem and made a jumper wire from the rtn line on the 10 pin harness to the battery, then ran the car for several miles, at part and full throttle. I then dumped the codes and came up with 22, 67, 29, and 34. The car seemed to run better but the idle still sucked. So I cleared the memory, took the jumper wire off, and test drove it again to see if I would get the same result i had to begin with, sure enough the same original codes popped up, so the jumper wire did clear up some of the problem. I cannot figure out if i should be looking at one sensor for the idle problem or if the computer needs to be replaced, wiring harness etc. etc.
While by now i'm getting pretty damn familiar with the wiring on this car I still cannot figure out what the main problem is. The car is a 1989 5.0 with a holley systemax intake, mass air, and a b303 cam. Which i find odd because a mass air vehicle shouldn't pull a 22 MAP error or a 29 VSS error.
Any help would be greatly apreciated.

 

[jrichker]

All of the resistance reading between the sensor signal ground (black/white wire) and the negative battery post should be less than 1 ohm.

Measure the resistance between the black/white wire on each sensor and the negative battery post. All of the reading should be less than 1 ohm. Then measure the resistance between each sensor using the black/white wire. All of the reading should be less than 1 ohm. More than that indicates bad wiring or bad connections.


Code 22 MAP (vacuum) or BARO signal out of range. The MAP or BARO sensor is pretty much the same sensor for both Mass Air & Speed Density cars. The main difference is where it is connected. Mass Air cars vent it to the atmosphere, while Speed Density cars connect it to the intake manifold vacuum. Its purpose is to help set a baseline for the air/fuel mixture and changes in barometric pressure. The MAP or BAP sensor puts out a 5 volt square wave that changes frequency with variations in atmospheric pressure. The base is 154 HZ at 29.92" of mercury - dry sunny day at sea level, about 68-72 degrees. You need an oscilloscope or frequency meter to measure it.

Baro or MAP test using frequency meter - run the test key on engine off. The noise from the ignition system will likely upset the frequency meter. I used a 10 x oscilloscope probe connected from the frequency meter to the MAP/BAP to reduce the jitter in the meter's readout.

If it is defective, your air/fuel ratio will be off and the car’s performance & emissions will suffer

Your Baro/Map sensor is still not functioning the way it is supposed to. The Baro/Map sensor is still a valid part of the 89-93 model Mass Air computer setup & is used to set the initial fuel/air & spark values accroding to the barometric pressure when you first turn the key to the Run or Start position.

Code 29 - Vehicle Speed Sensor (VSS) is an electronic sender mounted on the speedo pickup gear on the trans. It works the cruse control for both 5 speed and auto trans cars. The VSS is used to tell the computer to speed up the idle as you slow to a stop. This helps keep the engine from stalling when you slow down for a stop sign or stop light.
Check to see if the electrical connector is plugged into it. Clean the connector & contacts with non flammable brake parts cleaner prior to replacing the sensor, as that may fix the problem. The sensor cost is under $30 and it is easy to replace.

The Vehicle Speed Sensor (VSS) is a valid part of Mass Air cars. It is used to tell the computer that the car is slowing down and to increase the idle speed. The cruse control also uses the VSS signal.

Code 34 Or 334 - EGR voltage above closed limit - Failed sensor, carbon between EGR pintle valve and seat holding the valve off its seat. Remove the EGR valve and clean it with carbon remover. Prior to re-installing see if you can blow air through the flange side of the EGR by mouth. If it leaks, there is carbon stuck on the pintle valve seat, replace the EGR valve ($85-$95).

If the blow by test passes, and you have replaced the sensor, then you have electrical ground problems. Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1 ohm. Next check the resistance between the black/white wire and the negative battery post. It should be less than 1 ohm.

Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter.

Let’s put on our Inspector Gadget propeller head beanies and think about how this works:
The EGR sensor is a variable resistor with ground on one leg and Vref (5 volts) on the other. Its’ resistance ranges from 4000 to 5500 Ohms measured between Vref & ground, depending on the sensor. The center connection of the variable resistor is the slider that moves in response to the amount of vacuum applied. The slider has some minimum value of resistance greater than 100 ohms so that the computer always sees a voltage present at its’ input. If the value was 0 ohms, there would be no voltage output. Then the computer would not be able to distinguish between a properly functioning sensor and one that had a broken wire or bad connection. The EGR I have in hand reads 700 Ohms between the slider (EPV) and ground (SIG RTN) at rest with no vacuum applied. The EGR valve or sensor may cause the voltage to be above closed limits due to the manufacturing tolerances that cause the EGR sensor to rest at a higher position than it should.

This will affect idle quality by diluting the intake air charge

I still believe that you may have wiring problems due to bad signal grounds to the sensors.

 

[Hoffman]

I did what you said in the other threads, and yes i do believe there is a short in the sig rtn line but i cannot figure out where and how. If I check resistance between the sensors and the computer they all check out to less than 1 ohm and in some cases zero on my meter. I also checked the two ground pins on the computer to the ground terminal on the battery and they show less than 1 ohm. So how is it that when i check the resistance (power off) between the sensors and the ground terminal on the battery that i have no continuity? I'm going to clean the egr, and check the vss/map sensor, since they are the only repetitive code flashing on the ecu, so they are obviously bad. I tried grounding the sig rtn line to the battery ground using a jumper, and that cleared the ect, tps and the air temp sensor, but the computer still needs that line to recieve info from these sensors correct? So how could breaking that signal fix those codes? Thanks for all of your help.

 

[jrichker]

Always make resistance reading with the power off. Doing it with the power on will give false readings. In your case, I would even go so far as to recommend that you disconnect the Positve battery cable from the battery prior to making any resistance readings.

Repeat your mesurements with the above changes and see what you get for resistance readings. I do not believe you have short to ground or power, but an open circuit to ground and that is the source of your problems. There is a broken wire or bad connection somewhere in the computer pin's 46, signal ground path.