What's the ECM Voltage suppose to read?

[jorgensensc]

Ok guys, dumb question of the day. What is the ECM voltage suppose to read on the scantool? I'm trying to narrow down why my l92/6l80e went into limp mode last week and won't get out of 3rd gear. I've got no codes in the scantool, I've checked all fuses and checked voltage to the tranny with both key on and key off. All appears fine. After research I see that people are saying bad alternator or battery can cause this same problem. The battery seems to be staying charged at about 12.5 volts with the truck off. With the truck running at idle the alternator shows 13.5 volts at the junction block and the battery. However, the ecu varies drastically from 12.02, down to 11.5, all the way up to 13.5. It will literally bounce around while my scantool is hooked up and driving. Tends to hover around 12.02-11.7 while driving. As the rpms go up, or when first started in the morning tends to be around 13 or so.

When this happened I was literally driving just fine, stopped and got gas, then when I left the gas station it was in limp mode.

So where should the voltage be at? I'm thinking bad voltage regulator? Ideas, opinions, or suggestions are always welcomed!
Thanks all!
Shawn

[BR3W CITY]

Voltage reg or alternator if the voltage reg is internal. Bad grounding or small wire can cause the "scaling" in voltage with RPM.

My truck alt puts out over 14v, battery sits at 13.1 most of the time when off. Cold starts see 15v. I vaguely recall something about 11.1v being the function point on the ecm in a thread about case-grounding. This isn't scientific, but when I had an alternator go bad WHILE on the freeway, the engine kept obdII connectivity until the voltmeter just about hit 11....so take that for what its worth.

[jorgensensc]

Thanks Br3w. I am going to change out the alternator. I had the auto parts store test the system, but I didn't trust their results. First it said battery good, in last quarter of its life. Then it said voltage regulator inside alternator was bad, then it said the battery was bad, then it said the battery was good. I know when I crank the engine on the voltage drops to about 9.5 or 10, so I think that's tripping the trans into limp mode, either the TCM isn't seeing correct voltage, or the ECM isn't seeing it and putting the trans in limp mode.
I know there have been discussions regarding power up order for ECM and TCM, but I haven't had any issues with that previously. What does your ECM voltage while idling?
Thanks again

[BR3W CITY]

Not really sure, its too cold for me to start it and put the laptop on.

I'm on a genIII so I don't have any TCM or BCM to deal with.

If you are using gel-cel batteries, never test the stores result unless they did the long term discharge test. Gel-cels can force-fail if put on the flash-charge tester.


all those hyphens in my post.

[truckdude239]

My gel cell batteries always fail a battery test i slow charge them when they get weak 2amp for about a day and they always come back i got one over 10 years old still 4est good

[jorgensensc]

Yeah, I'm running an optima battery. He ran the tester on the truck. First he ran it (although he did choose optima on the drop down) but had the wrong cranking amps, he did 800. Mine is 720. That's when it came up as good, but in the last quarter of its life. I had him do it again, but told him to put the correct ca of 720. Of course he didn't pay attention and did it at 800 ca again. This time it said bad. Then he ran it again at the right ca and got that it was good. Hopefully the alternator will fix it. I might need to get a new battery though, the one I have is 800cca and 720ca. It was the same battery I used in the suburban before. Apparently the escalade wants a 1000cca and 800ca. We will see.
Shawn

[truckdude239]

Cca really mean nothi g a good starter will normally onky draw like 200-300 amps

[Hart_Rod]

Interested...

[solidaxel]

Will this answer anything?

[solidaxel]

Will this answer anything?

[BR3W CITY]

HP Tuners implicitly says not to program a vehicles with less than 11.5v, so I'm guessing thats the point at which things become unstable. Doesn't help you, but for reference.

[Hart_Rod]

My gauge usually reads 13.5-13.8v. For whatever reason, the ECM/TCM always shows exactly one volt less...

[jorgensensc]

Thanks guys! All that really helps. I ended up changing the alternator and still had the problem. I had tried a bunch of different things before changing the alternator. I ended up changing a ground. Come to find out that was a really crappy ground. The screw was showing grounded, but the built in washer was not letting it get a good ground to the wire. I figured it out while checking the power to the trans. I was showing 10.5 volts when using the TCM ground. On a whim I checked the same voltage using the exhaust as a ground on my multimeter. The same wire then read 12.5. Ahaa! Crappy ground to trans TCM! I ran a new screw and wire and got a good ground. Suddenly she was working great again. I still read about 12.5-12.8 volts at the ECM. It initially was showing 13.4, but I think I agree that it reads 1 volt less, and I think it was trying to charge the battery. So still believe it may have been an alternator issue, maybe...

Was hoping it would fix the p0606 code, and I thought it did, but alas no. On the freeway doing 75, went wot, then it dropped to safe mode, no pedal response.

Tuner is reflashing my extra PCM and reloading it to see if that will fix the issue.

Now, for another dumb question. Sometimes my ECM shows fuel pressure of 57.9 psi , sometimes 0psi (even though I'm driving just fine). Goofy thing is that I don't have a fuel pressure sensor hooked up. How does the ECM determine fuel pressure without a pressure sensor? With it being 0 sometimes and 57.9 sometimes, there might be an issue somewhere I can hunt down that may explain the p0606 code.

Thanks again guys!!
Shawn

[Hart_Rod]

Originally Posted by jorgensensc

Thanks guys! All that really helps. I ended up changing the alternator and still had the problem. I had tried a bunch of different things before changing the alternator. I ended up changing a ground. Come to find out that was a really crappy ground. The screw was showing grounded, but the built in washer was not letting it get a good ground to the wire. I figured it out while checking the power to the trans. I was showing 10.5 volts when using the TCM ground. On a whim I checked the same voltage using the exhaust as a ground on my multimeter. The same wire then read 12.5. Ahaa! Crappy ground to trans TCM! I ran a new screw and wire and got a good ground. Suddenly she was working great again. I still read about 12.5-12.8 volts at the ECM. It initially was showing 13.4, but I think I agree that it reads 1 volt less, and I think it was trying to charge the battery. So still believe it may have been an alternator issue, maybe...

Was hoping it would fix the p0606 code, and I thought it did, but alas no. On the freeway doing 75, went wot, then it dropped to safe mode, no pedal response.

Tuner is reflashing my extra PCM and reloading it to see if that will fix the issue.

Now, for another dumb question. Sometimes my ECM shows fuel pressure of 57.9 psi , sometimes 0psi (even though I'm driving just fine). Goofy thing is that I don't have a fuel pressure sensor hooked up. How does the ECM determine fuel pressure without a pressure sensor? With it being 0 sometimes and 57.9 sometimes, there might be an issue somewhere I can hunt down that may explain the p0606 code.

Thanks again guys!!
Shawn

Where does your ECM show the fuel pressure at? Scan gauge? CanBus? I know within the tune you can turn off the fuel pump settings that would normally be captured with a factory setup and displayed on the Bus. Send me your tune and I can check.

[dayj1]

Originally Posted by jorgensensc

Now, for another dumb question. Sometimes my ECM shows fuel pressure of 57.9 psi , sometimes 0psi (even though I'm driving just fine). Goofy thing is that I don't have a fuel pressure sensor hooked up. How does the ECM determine fuel pressure without a pressure sensor? With it being 0 sometimes and 57.9 sometimes, there might be an issue somewhere I can hunt down that may explain the p0606 code.

Thanks again guys!!
Shawn

The gen IV setups came from the factory with a PWM (pulse width modulation) fuel pump assembly. In other words, the ECM sends DC pulses to the fuel pump module instead of a steady 12v, so the speed (and pressure) of the pump can be varied based on the frequency and duration of the pulses. I'm fairly certain (but not positive) that the fuel pump assembly no longer contains a regulator like the gen III factory pump modules. So, the computer can calculate what the pressure should be based off what it is commanding the fuel pump to do. It's just an educated guess, but I'm betting that's how they come up with the fuel pressure reading. In other words, I don't believe that there is an actual sensor even in the factory system.

With all that said, are you using the fuel pump module from a gen IV? If not, you may want your tuner to take a look at your fuel pump settings. In HP tuners, at least, there is a drop down for fuel pump type. You definitely don't want it set to PWM if that's not what you're using.

[jorgensensc]

Thanks for the reply.
Rob, unfortunately I don't have a copy of the tune. I can try and see if he can give me one so I can get it to you. I'm just looking at the live data feed on my scantool. Odd thing is that it is always either 57.9 or 0, nothing in between.
Dayj1, according to my alldata schematics there is a fuel pressure sensor that feeds to a BCM, which then feeds via CANBUS to the ECM. I understand what your saying about the PWM, which may be why it's always 0 or 57.9. The ECM is basically saying what it is requesting. I'm definitely not running a PWM fuel pump. It is a simple on/off setup at 80 psi using a c5 filter/regulator to bring the pressure down, on a returnless system. I'll ask the tuner about the fuel pump setting in the computer. I was hoping there might be a way to wire in a fuel pressure sensor and have it feed to the ECM so I can keep tabs on fuel pressure.
Shawn

[dayj1]

Originally Posted by jorgensensc

Dayj1, according to my alldata schematics there is a fuel pressure sensor that feeds to a BCM, which then feeds via CANBUS to the ECM. Shawn

Unfortunately, I lost my access to AllData when a friend of mine closed his repair shop Are you sure the pressure sensor that you see in the schematics is actually for fuel pressure and not the tank pressure for the EVAP system? I doubt this really has anything to do with your problem, but I do like to understand how the factory systems work.

[jorgensensc]

Here is the diagram pic. I thought it was the fuel pressure sensor. I know there is a fuel pressure sensor on the stock fuel line I got from the escalade.

[dayj1]

Thanks for posting that. It definitely does have a sender fort he fuel pressure. I stole the following from GM's description of the system:

"With the increase in electronic returnless fuel systems (ERFS), it is important to know that there is a new module associated with ERFS. The new module is called the Fuel Pump Control Module (FPCM) and it is a serviceable GMLAN device. It is a microprocessor that controls the voltage supplied to the fuel pump (located within the fuel tank) to achieve the desired fuel pressure requested by the Engine Control Module (ECM). There is also a new fuel line pressure sensor, which sends a feedback signal to the FPCM, so the FPCM can determine whether the desired pressure is being achieved"

Sorry for the thread hijack, but maybe we learned something out of it