ECU Voltage

Hello all,
I'm in the process of diagnosing a few electrical issues on my new to me Tacoma and I happened to notice the ECU voltage is between 1v and 1/2v lower than the alternator/battery all the time. My fuel gauge has been the main complaint in its erroneous indications until Sunday when my truck developed a hesitation off idle and after shifting gears. The hesitation is hard to describe but after I shift the truck seems to have a flat spot for 1/2 a second before it will pull on through. I don't dog my vehicles so sometimes I shift around 2500-3000RPM and the engine will be below 2000RPM when I get back on the throttle in the next gear. No CEL and the hesitation seems to come and go.

The ECU voltage being low makes me wonder if the reference signal for the fuel gauge sender isn't 5-10% lower than it should be causing my gauge to read 1/4 a tank higher than it is all the time (that's what my truck does). Also, could the hesitation come from the fly by wire throttle position sensor voltage being low too? I know these trucks have frame corrosion problems so I plan to start by checking the engine/chassis grounds.

Curious to see if anyone else with a scan tool capable of measuring ECU voltage notes 1v drops from the battery to the ECU.

For reference, if you list your ECU voltage please specify,

Year-
Mileage-
Corrosion-
Region you live in-
ECU Voltage engine running-
Battery voltage engine running-

My info
Year- 2005
Mileage- 205,000
Corrosion- Some light frame corrosion, original frame
Region you live in- SouthEast USA (HUMID!!)
ECU Voltage engine running- 13.6
Battery voltage engine running- 14.5

 

I want to add in reference to the hesitation, my plugs are new with 1,000 miles on them. NGK Iridium. I cleaned the TB and MAF sensor at the same time as the plugs. Brand new Denso fuel pump 1,000 miles on the pump too, no CEL's, fuel grade doesn't seem to make a difference but colder weather seems to make it more noticeable. At times on level ground, in 6th gear at 60mph I can floor the pedal and the truck will barely accelerate if it accelerates at all. Above 2,000RPM in 1st-5th the truck is very strong.

Do these trucks have an external fuel pressure regulator that can be tested or serviced?

 

I'm willing to check it out on my truck for you but I don't have a scan tool. I could check it with my Fluke VOM though if I knew what pin in the OBD2 plug that you want checked. Mine is a early 2006 Tacoma, 129K, clean AF undercarriage, original everything that's pertinent.

 

I’m not sure of the pinout either. It shouldn’t hurt anything but I’m very hesitant to direct probe the OBD2 port, too much at risk if it were to cause an issue!

 

Its 4:49 AM, I can't sleep as usual. I found this

I'll check it out for you later today.

 

Thank you sir, I’m hoping to have the day off tomorrow and I plan to dig into every ground that I can find. I’m going to record the ECU voltage and maybe the voltage at the cigarette lighter before and after cleaning each ground to see if I can increase the ECU voltage any. I’ll also see if the fuel gauge needle moves any after cleaning the grounds.

 

Early 2006
129K miles
Zero corrosion
So Cal
Volts at battery, engine & key off: 12.40
Volts at battery, engine running: 14.20
Volts at OBD2, engine running: 14.20

Measured with a Fluke 175. Have a great day.

 

Very interesting results! Looks like I know what I’m doing tomorrow! I’ve already found two ground straps just by glancing under the hood, while they aren’t what I’d call crusty they also don’t look as clean as they could be.

I’m going to focus my efforts on the chassis grounds first, then the engine grounds. I’m hoping this clears up some electrical gremlins that I’ve seen others report!

 

So, last night in preparation of the battle I anticipated today I scoured the FSM and found a test procedure for the fuel gauge. Very simple, unplug the fuel pump connector and turn the key to the run position; the gauge should indicate empty and the low level light should come on. Next, jumper the 2 and 3 wires in the fuel pump connector and turn the key to the run position; this time the gauge should read full. I decided to conduct this test before cleaning grounds to isolate any variables diagnosing the gauge.

When tested my gauge responded exactly as the FSM specified.

When I plugged in my new Toyota fuel level sending unit to the chassis fuel pump harness, the fuel gauge first indicated exactly as it should no matter how I moved the float arm. After testing for a few minutes in different configurations the gauge began indicating off target again, at one point showing a full tank when the arm was bottomed on the sending unit assembly. I wonder if the gauge has a delay in the circuitry to average readings over time to prevent erroneous indications? For testing purposes, and curiosity, I grabbed the aftermarket Delphi sending unit and plugged it back in, and now it seems to work??? The Delhi would not indicated below 1/8th of a tank three weeks ago when I tested the exact same sending unit in the exact same way; now the Delphi unit will drop the gauge needle to E and activate the low fuel level light. Puzzling...

I'm on a lunch break now, I'm going to measure the signal voltage on terminal 2 as the FSM specifies. Signal voltage should be 4v-7v I'm leaning towards an intermittent connection or varying resistance somewhere in the circuit. I'm also going to measure each of the three sending units for resistance both isolated and connected to the pump assembly.

On a brighter note my URD kit is all installed and ready to drop in once the sending unit is sorted!

 

If you are reading a 1 volt drop between the PCM and the vehicle battery, I would suspect connection issues at the PCM WAY before I would start chasing down fuel pump issues.

Remember, the PCM feeds the gauge cluster it's signals, that includes fuel level, so if you have issues at the PCM you will have issues at the PCM and general odd behaviors across the board, including running condition and potential flat spots under load.

You may want to check for dirty grounds, if and when you do that and it proves indeed dirty, but you can't clean them for whatever reason but you want to supplement the PCM with an additional ground and you have the skill to do so, I would splice in a brand new clean ground to the body. Somewhere close to the PCM, it should restore your 1 volt drop issue if that is indeed your issue.

 

I agree,
I wanted to test the gauge without adding any variables like disturbing factory wiring, and in doing so I was able to get the gauge to indicate correctly per the FSM without touching any wiring. That tells me the gauge is OK, I'm still going to investigate the voltage drop but by testing the gauge first I was able to eliminate the possibility that a ground was causing the gauge to not sweep through its full range. Hopefully cleaning the grounds will improve the accuracy and reliability of the gauge even further!

Also, I tested the ECM voltage again in several different configurations (lights on, fan on, driving, idling and etc.) and the average voltage drop is about .5v. So, I'll pursue cleaning the grounds when I get the tank back in.

Thanks for the reply, I appreciate your insight

 

Half a volt, you wouldn't think that's a lot, but in the grand scheme of things it is. I've had a few oddball cars which have acted funny because of voltage drops of around that much. I hope you get it resolved, let us know what you find. I'll be intently watching your thread. Just quote me or tag me and I'll be back in here to give my thoughts.

 

So, when I tested the voltage on the number 2 pin of the F19 connector on the fuel pump assembly instead of finding 4v-7v with the key on, I have 11.6v with the key completely out of the ignition switch? Voltage is bleeding onto this circuit from somewhere! That would explain the higher than normal reference voltage to the gauge! But where the heck is this stray voltage coming from. Stay tuned as I wiggle wires and start digging deeper.

 

I found 11.6 volts while conducting test 1(c)

 

I'll need a wiring diagram to be able to help you, I have access to them, but I'm a little tied up at the moment. I'll get back to you in a bit.

 

Roger, thanks. I've gone ahead and tested the ECU power and ground supply, both are OK. Cleaning grounds and battery terminals now.

 

From what I can gather, you have two things leading up to pin 2 at the connector, the fuel pump resistor, and the relay.. now based on the age of the truck, I am going to assume it is the relay which is sticking, but given your symptoms, it actually seems to me that you have a failing resistor.. they have failed before albeit not that often, but they do. Replace that thing and recheck your issues. If it continues it wouldn't hurt to swap out the relay as well.

 

Ok, that’s a start. My diagram 05-06 FSM shows the sending unit wiring going to the instrument cluster (combination meter)

I didn’t realize the connection for the fuel gauge passed through the fuel pump relay!? I’ll check it though, if I can get the stray voltage off this circuit and just the reference voltage on the circuit when the key is on I have no doubt the gauge should work.

 

I'm not basing things off the fact that your fuel gauge is wonky, I'm basing it off the fact that you indicated that upon checking pin voltage at pin 2 it should had been 4-7 volts but you were getting 11.6 which is battery voltage. Meaning the relay is juicing the pin direct voltage without a resistance to drop the voltage to indicated desired voltage. Almost as if the resistor is allowing direct voltage to pass through.

Remember the fuel gauge is just a symptom of the issues, not the direct cause. You treat the cause the symptoms go away with it.

 

Hey, I’m not second guessing you! Just processing it. I totally agree. I just like to check all the factors I can to isolate the problem, I work slow and some might call me “slow” all together!

I was under the impression that the fuel pump resistor ran the pump itself at varying speeds depending on the load on the engine. It makes sense that there has to be a resistor or voltage reducer to step down from 12v-14v at the battery to the 4v-7v specified in the manual for the sending unit. I just figured the ECU sends the edited voltage signal.

I’ll check it as soon as I get home and report back. I’d be elated if the stray voltage disappeared by pulling the relay. Even more elated if a new resistor and relay fixed my problem.

I’ve been down the rabbit hole of tearing harnesses apart looking for a break and that’s not my idea of fun.