With the Map sensor plugged in I get 5.8 volts. With the Map sensor unplugged, I now have 7.5 volts at the SIG RTN, not 12 volts, to the to battery ground, with all the sensors plugged in. Reading other post at FTE, I think its ok to have 2.5 volts at all times at the SIG RTN at the MAP. SIG RTN to battery ground. Can someone please check how many volts are at your STO and SIG RTN pins on the DLC self test connector? Also your STI plug too?


With all the sensors plugged in:

I get 5.8 volts at the SIG RTN at the DLC test plug. O volts at the STO pin. and 3.5 volts at the STI plug. When I try to do codes today I get no movement of the volt meter needle. I just remembered, maybe important, when I first tried to get trouble codes a few weeks ago the needle stayed all the way to the right, until I heard a click, then the needle dropped all the way to the left and stayed there.Could not get codes, never did get codes ever. Do I need to put the truck in neutral and press in the clutch to read codes?



With all the sensors unplugged:

At the DLC plug I get 12 volts at the fuel pump pin and nothing at the STO and I get 1 volt at the SIG RTN. At the STI ( self test input) plug i get 3.5 volts. All other SIG RTN's have 1 volt. When I tried to get codes today with my analog volt meter and I get a constant 1 volt with key on, clutch press in and battery connected. When I cut the KNOCK sensor out of the loop yesterday I get no voltage at any SIG RTN's, but when I plug in all the sensors in I get 5.8 volts. Same reading if knock sensor is plugged or unplugged.

 

STI should be between 3 and 6 volts, unless you are trying to run a self-test, in which case you should have jumpered it to ground. Normally we'd use SIGRTN as that ground, but SIGRTN has issues for you, so use something else. But to run a self-test, it has to be less than 1 volt.

STO should be at 0 volts whenever the "check engine" light should be on. It needs a load of some sort to +5 or +12 (like a meter on ohms or maybe volts) to go to a higher voltage. (The PCM grounds STO to indicate a "blink" or turn on the light, and ungrounds it to turn the light off). If you ground STI and then measure the resistance of STO, you should see a high resistance "most" of the time, except when the PCM is trying to output a code. If you try to measure the "voltage" on STO, between STO and ground, you will always get 0 volts on a vehicle without a "check engine" light. This is normal.



Sigrtn should be between 0 volts and maybe 0.7 volts all of the time, assuming everything is connected.

The fuel pump pin on the self test connector should be 12 volts, unless the PCM is trying to run the fuel pump at the time. You ground this pin to force the fuel pump on.

7.5 volts on SIGRTN is bad. SIGRTN cannot be higher than VREF, with the computer disconnected, with any of the sensors that you have on this engine. Find the source of 7.5 volts.

5.8 volts might be OK if your VREF is a little high. The fact that plugging in the MAP sensor pulls the voltage down suggests something less than a hard short circuit.

 

The 2.5V is on the MAP signal wire - there should not be any voltage on the SIG RTN. I have about 20 mV between my SIG RTN and ground.

I checked resistance between SIG RTN and the negative battery terminal, and got about 170 ohms, which seems way too high. I also got about 130 ohms between the fender ground and negative battery terminal, which also seems too high. I got very little resistance between SIG RTN and the fender ground. Don't use mine as a reference, as I don't think these resistance figures are where they should be. I also think that the voltage between the SIG RTN and the negative battery terminal is due to this resistance. SIG RTN shouldn't be anything more than a ground, so it won't have much more than 0 volts at all times.

The clutch has to be held to the floor to enter the diagnostic tests. The computer looks for a return signal from the NDS pin to the SIG RTN pin. I think that the voltage on the SIG RTN is not allowing the computer to see this, so you must track down the short to power and fix it before trying to do anything else.

 

Measuring SIGRTN resistance, or ground resistance, with the key on and current flowing is a dicey proposition. An ohmmeter measures resistance by suppling a "test current" to the probes and then measuring the resulting voltage. E = I * R and all that.

If there is already a voltage differential in the circuit (due to current flow from the battery) your ohmmeter will get confused and give you a false and sometimes wildly high reading.

If you suspect something like this, try reversing the leads on the ohmmeter and see if the resistance changes. If it does, you are either dealing with a diode or with voltage in the circuit.

Another option is to disconnect the battery positive post before trying to measure ground resistance.

 

I think the batteries in my multimeter are getting weak as well. I'll see what happens with the key off next time, as it makes sense what you are saying.

 

Hi Guys got my rebuilt EEC-IV in and it looks ok now. I don't get the 5.8 volts at the sig rtn on the self test connector. I get no voltage at all sig rtns. Got 12 volts at the FP pin on the self test connector. Quick check on Map got 4.5 volts at orange wire and at the blue wire to battery ground. My old spark plugs have soot on them can this cause it not to start? Should I try a set of new plugs i havn't found what is causing the ( carbon) soot yet. Can't check o2 sensor,fuel pressure i dont have the tools, also could not get codes from old or new EEC. Which sensors causes carbon so that it wont start? Last time I checked some of the plugs were wet with fuel.

 

You are making progress. Sigrtn is making sense now. FP will have 12 volts on it when the key is on but the PCM is not commanding the fuel pump to run. If you have a helper turn the key from off to on, you should see that FP stays at 0 volts for 2 seconds (while the fuel pump runs), then goes to 12 volts (when the fuel pump stops).

The O2 sensor is not needed for starting the engine. It only comes into play once the engine starts and fully warms up.

You really should invest $20 in a proper fuel pressure guage -- and another $20 in a timing light. A no-start problem like this is a lot easier to figure out with these tools.
But if you are real cheap, you can kinda get by using an old spark plug as a poor-man's "timing light" and a tire pressure guage on the fuel rail for fuel pressure.

Sooted spark plugs mean the mixture is too rich. Wet spark plugs mean the mixture is too rich or the plugs aren't firing. The good news is that fuel is at least reaching the cylinders. The spark plugs have to be really nasty to cause a no-start -- with the soot it might run a little rough, but it should cough and fart and at least try to run.

Some of these computers have a "clear flood" mode. If you floor the throttle and then crank the engine the PCM (on such installations) does not fire the injectors if it sees full throttle during engine crank. The air flowing through the cylinders will dry out the spark plugs.

On a similar note, you might try opening the throttle partway during crank (push down the gas peddle). If the PCM is overfueling the engine due to a sensor problem, the extra air from the open throttle may balance things out and let it start.

Before you get into sensors, a couple of things can cause a too rich mixture that have nothing to do with the PCM. Fuel pressure that is too high will do this. A leaking fuel injector will do this. You need a fuel pressure guage to check these two possibilities. A bad fuel pressure regulator can let uncommanded fuel flow through the vacuum line. That one you can check by pulling the hose off.

 

"If you have a helper turn the key from off to on, you should see that FP stays at 0 volts for 2 seconds (while the fuel pump runs), then goes to 12 volts (when the fuel pump stops)." With the key in in the off it has 1 volts for two secs then when turn to run it gets 12 volts. Is that ok?

 

I did the codes for the fun of it. It only ran total about 20 sconds on starter fluid. Codes i got were (sweeps from my volt meter)
11 then 1 sec pause
1 then 2 sec pause
11 then 1 sec pause
11 then 2 sec pause
11 then 1 sec pause
1111 then 4 sec pause
11 then 1 sec pause
1 then 2 sec pause
11 then 1 sec pause
11 then 2 sec pause
11 then 1 sec pause
1111 then 6 sec pause
1 then 2 sec pause
11 then 1 sec pause
1 then 2 sec pause
11 then 2 sec pause
11 stop for good. I removed the key then waiting 20 sec then re tried for trouble codes. Did 10 times always the same. I had the clutch in, everything off, park brake on and in nutrel.

 

The PCM has a pull down transistor on one side of the fuel pump relay coil. The other side of the fuel pump relay coil is connected to 12 volts through the EEC power relay contacts. When the pull-down transistor is activated, the current flow through the non-zero resistance of the pull down causes the voltage to be higher than the theoretical zero volts. One volt (where I may have inplied zero volts) is fine.

With the key "off" you should see zero volts on FP (because there is no source of voltage to either side of the coil. When the key first goes to on, FP should go to about one volt for about two seconds as the PCM energizes the fuel pump relay coil. Then FP will go to battery voltage (about 12 volts) as the PCM de-energizes the FP relay coil.