ECU Voltage

And thanks again for the input! Much obliged

 

Here is more.


As you can see, pin 2 originates from the fuel pump assembly and goes to ID2



Replace the resistor first, then reinspect. Worst comes to worst you or I (or both), will have to re- review these schematics again and determine what the hell.

Regardless... I think the resistor is the best course of action to start with. However, it is possible you have two separate issues.

 

@Scalleywag dont think you mentioned anywhere how you're measuring the alternator/battery voltage? I would assume a quality meter directly on the battery posts, but just wanted to confirm. Ive never actually put a meter on my batteries, but my ScanGauge II very consistently reads about 0.3V higher than one of my GMRS radios. Which is hard wired to the main battery.

I dont know for sure if the SGII reads the voltage from the ECU via OBDII, or if it has a voltage sensing circuit within itself. But either way the ECU reports higher, not lower

 

Good point, I'm measuring ECU voltage of a Innova 3160G scan tool via the OBDII port. Battery voltage is being measured with a known accurate multimeter. The meter does great on voltage, just not ohms for some reason.

 

Sadly pulling the fuel pump relay and the resistor made no difference. The voltage is still there.

 

You still have 11.6 volts with the relay and the resistor out?

If you pull out both, that circuit should theoretically be dead. I'm starting to wonder if you have a wiring issue somewhere down the line.

Check pin 4 at the pump plug and let me know if it has voltage as well.

Has the truck been hit at all? Do you wheel it hard?

 

Yep,

Correct, with both the relay and resistor connector pulled the voltage doesn't move at all, stable to the tenth of a volt. There is no voltage present on any of the other pins with the key out, connector F-19 has room for 5 pins but is only configured with 4 pins.

I just got the truck two months ago, I don't do much off-roading at all less some very "green laneish" stuff during hunting season. Maybe the prior owners did, I don't see any evidence of that though like gouges in the tank or a bent skidplate. No lift and I'm on stock 16" Toyota wheels.

 

Okay so if that is the case, what is going on here is pin 2 and 3 run the fuel pump sending unit assembly. Pin 2 is power and Pin 3 is supposed to be ground. The variable is run through the sender unit which is simply a resistor. Pin 4 and 5 run the fuel pump. If you need to have 4-7 volts at pin 2 of the pump plug and you are still getting voltage bleed through after eliminating the resistor and the relay then you need to disconnect the instrument cluster. If the voltage bleed through goes away then you have a bad instrument cluster. If the voltage bleed through proceeds then it's definitely a wiring issue.

First, start simple however, disconnect the 7.5a gauge fuse at the fuse block. It may be the interior fuse block, which I believe it is. See if the voltage goes away entirely. If it does, cool, put it back in and move to the instrument cluster gauges. Disconnect them and see if voltage bleed through goes away. If it doesn't then you have an issue in the wiring. If it does, it is highly likely your gauges are the issue.

 

You have the pinout and its labels correct for F19. Pulling the 7.5a Gauge fuse did not alter the voltage at F19 pin 2. I was already in the process of pulling the cluster when I saw that you replied so I'll report back when its out and I can check C9 pin 8, the other end of F19 pin 2!

 

What? It didn't eliminate the voltage?

Oh crap... It should had. I wonder if you got bad wiring somewhere in the harness. Like two wires fused together or something. Check at the dash cluster and let me know.

 

If the wiring behind the dash hasn't been tampered with I'm leaning towards a chafe in the harness somewhere or maybe a bad diode. The plot thickens!!

 

I would have to agree with that logic. You may have to manually Trace down the wiring within the harness itself, but luckily nothing major really runs all the way that far back so if you have a wiring issue it will be easy to trace.

 

Finally some relief, when I disconnected C9 the voltage was gone. 0.0v now. I have no voltage on C9-4 or C9-8.

All the solder joints look good on the back of the cluster, do capacitors or something inside the clusters die and bleed?

Voltage comes right back when I plug the cluster back in.

 

I did find the main loom of wires under the steering wheel zipped EXTREMELY tight. Putting substantial pressure on the quick disconnects. The zipties were regular hardware store white ties and didn't look like OEM work to me, I cut the ties off and am wiggling (the wires ) now looking for a change in voltage at F19 pin 2

 

Removing the ECU-B fuse or the Short Pin either one will remove the voltage from Pin2 of F19. So there is the power supply side, now to figure out what in the heck runs off of those two circuits!?

 

Wow, you got a lot of help. Very cool. I didn't read it all. I'm curious, did you get the OBD2 voltage the same as the battery? How?

 

Cleaning the battery terminals (to include disassembling the positive terminal and completely cleaning) and the main ground by the negative cable has me within .3v now. While monitoring the stray voltage on pin 2 of F19 I found that removing the drivers door switch removes the voltage. Pretty strange if you ask me, Im going to dig a little deeper and see if I can isolate the specific source.

 

Ok, this may be lengthy but here are my observations so far.
These voltage readings were found with the whole pump assembly disconnected, I probed connector F19 pin 2 from the front (Not backprobed).

With the drivers door open pin 2 (connector F19) has 11.60v present at all times key on or key off, close the door with the key off and the voltage will drop to 0, but only on the first closing of the door after being open for 1 minute or more. If you reopen the door (key off) the voltage will reappear and reclosing the door will not affect the voltage until the door has remained closed for one minute at which point the voltage will drop to zero again (key off).

With the drivers door closed (and the key off) the voltage drops to zero after exactly 1 minute every time. I completely removed the door switch and it made no difference, the exact same results were observed. Interestingly the voltage on the door switch wire is exactly the same voltage as pin 2, 11.6v.

When I reopen the door after a minute, and the voltage is re-applied to pin 2 I can hear something in the instrument cluster click and I can visually see the tachometer needle flicker about 2mm worth of travel.

Turning the ignition at any time, key on or key off will cause the voltage to be present again at pin 2.

Until I ran the test specified in the FSM I had no idea that there was constant voltage present on this terminal. Maybe I'm reading the manual wrong and I should be back-probing the terminal with the fuel sending unit in the circuit? The way the FSM reads it looks to me like the test should be performed with the pump assembly disconnected.

It seems like something in the ECU-B circuit and/or the instrument cluster is causing this stray voltage, any ideas on what circuit would be energized at all times when the the key is on, and at all times when the door is open with the key on or off, but also stay energized for 60 seconds after closing the door?

 

What do you guys think? Test 1 part (C) test for 4v-7v with connector F19 unplugged or plugged in?

 

I’m looking for any insight here, does this sound like a bad Combination Meter or a bad ECU-B?

I can put the truck back together and drive it as is but I’d love to find the issue, I’m fairly confident that my truck isn’t the only one with this issue. I’m seeing numerous other posts about fuel gauges that won’t go below half, a quarter or an eighth of a tank. It'd be great to find a solution for those who want their gauge to work!