2010 Accord 1 ohm of resistance in ECU/PCM ground

Bob Kontak · Bob Kontak's Garage

Is this resistance enough to cause problems? It seems top me it would be.

Things like the fuel pump are controlled by battery voltage and the ECU will tell the fuel pump when to run with another set of wires that are very small gauge. So my fuel pump is not running but I can get the fuel pump to run with a direct 12 V from a battery to the fuel pump terminals.

I just have no experience with how much one ohm of resistance can disrupt the workings of the ECU.

1990C4S · 06-22-2023, 09:01 AM

How are you measuring that? Your meter is not likely capable of anything near that resolution.

My guess is that you should look elsewhere.

Is there a fuel pump relay? Look there. Is the coil getting 12V, and is it working?

Bob Kontak · Bob Kontak's Garage

Quote:

Originally Posted by 1990C4S

How are you measuring that? Your meter is not likely capable of anything near that resolution.

My guess is that you should look elsewhere.

Is there a fuel pump relay? Look there. Is the coil getting 12V, and is it working?

I am measuring it from ECU wiring harness connector B1 and B10 as well as C48 and C44 to ground G101. There are three connector ports but the four wires mentioned go to engine ground G101 which is near the injectors. Other things like the transmission range sensor and the OBD port signal ground go to G101 and are not working properly.

My multi meter has (as it's smallest) a range of 20 ohms. What would 0001.0 mean?

Help me out with above first and I will test fuel delivery after I get the engine put back together. Deal is the OBD code is P0685 - read back when it did not start except for one time - like last December. P0685 is ECM/PCM Power Relay Circuit Malfunction. ECM relay was replaced as well as the battery. Two big culprits for that code. I am also replacing battery cable ends as they are skunky.

I have access to Snap-On's shop key pro site which gives me access to schematics (maybe a curse rather than a help) and internet "people" said the G101 ground point and associated circuits can contribute as well.

Here's the trouble shooting guide for the fuel pump. I don't fully get it but I can take it one piece at a time. Posting this as I have invested some time into this and I just want a bit of guidance.

https://www.haccord.org/fuel_pump_circuit_troubleshooting-896.html

Long story short can someone help me with the one ohm resistance question or confirm that I am full of crap and my MM can't read one ohm? It's not an expensive meter but it zero's out just fine on other grounds.

1990C4S · 11:18 AM

I would bet that the resistance of 1 ohm is not relevant, you have a decent ground. So yea, full of crap.

Troubleshoot in a logical order, if the ground is really the issue you will get there.

From the link you attached, does that relay (step 5.) pull in? Is there voltage Pin4 to ground?

If there is 12V then the relay should pull in, which I assume powers the pump.

If it pulls in and the pump doesn't run, then you need to troubleshoot the wiring, since you say the pump works.

Bob Kontak · Bob Kontak's Garage

Quote:

Originally Posted by 1990C4S

From the link you attached, does that relay (step 5.) pull in? Is there voltage Pin4 to ground?

OK. I'm on it. I did some testing but not methodically.

Stay tuned.

Thanks.

1990C4S · 06-22-2023, 11:50 AM

If the ground is really the problem, you will measure 10 or 11 volts somewhere and the device will not be operating. But I say that's not going to be the case.

The Accord instructions are a bit confusing, but the logic is correct.

Bob Kontak · Bob Kontak's Garage

Quote:

Originally Posted by 1990C4S

So yea, full of crap.

Better to ax a stupid question.

Zeke · 06-22-2023, 04:42 PM

I can hardly ever read zero Ohms with the leads supplied with various meters.

asphaltgambler · 06-22-2023, 05:14 PM

You need to a voltage drop test, ignore the 1 ohm resistance

fastfredracing · 06-22-2023, 06:00 PM

Id be sniffin' around the main relay Bob . If you need any wiring diagrams , let me know

Bob Kontak · Bob Kontak's Garage

Quote:

Originally Posted by asphaltgambler

You need to a voltage drop test, ignore the 1 ohm resistance

Im going throught the fuel pump diagnostic first. That was my first thought in February.

I have access to diagrams Fred. I also bought another multi meter for a second opinion.

FWIW the resistance starts at zero and over a couple seconds climbs to 1.0 ohms. The other ground wires stay at zero that do not terminate at the G101 ground on the engine. However, I'm doing the fuel pump test before anything else but Im slow because of the hip replacement.

Thanks all.

john70t · 06-23-2023, 07:25 AM

https://www.rapidtables.com/electric/ohms-law.html calculator

1 Ohm= 13.5 Amp drop, theoretically, which might be enough to prevent worn windings/bearings from turning.

Motors do not like running on reduced Voltage.

Voltage present does not equal flow necessarily.
Higher loads and Amps will be most affected by resistance, up until the point of connections and wires overheating/melting.
The ironic things is the multi-meter uses very little Amps to internally test for resistance.

I also suggest replacement of the FP Relay first for preventative maintenance.
Once had a Bosche that would randomly overheat and leave me stranded in traffic situations.

asphaltgambler · 06-23-2023, 07:29 AM

^^^So the voltage drop test is the only accurate way to test the circuit under load as it operates.

john70t · 06-23-2023, 07:41 AM

"As it operates" would be the key words. Good point.

Voltage being present on the grounding side of the FP might indicate a load/resistance further downstream.

1990C4S · 11:50 AM

Quote:

Originally Posted by john70t

https://www.rapidtables.com/electric/ohms-law.html calculator

1 Ohm= 13.5 Amp drop,

I'm not sure what you're calculating there, but if 1 ohm drops the motor current by 13.5 amps at 12 Volts, then the motor's effective resistance is about .56 ohms, and it draws 24 amps normally, and 7.5 amps with 1 ohm added in series. I don't thinks that's the case here.

Regardless, he is a long way from worrying about a 1 ohm resistance measured on a non-calibrated meter.

Edit - I guess you looked at 13.5 Volts across 1 Ohm? That's not the case here. You need to look at voltage drop across the 1 ohm wire when it's in series with a running motor. If it really is 1 Ohm...

john70t · 06-23-2023, 05:09 PM

You're right. The mathematic side didn't seem to jibe and not my strong suit but it could make a difference. ty.

1990C4S · 04:08 AM

No need to tell an engineer he's right.

I think Bob will work through the troubleshooting chart and find a bad relay, or a broken wire.

Bob Kontak · Bob Kontak's Garage

I've been out of it since last Thursday. Sorry for the slow response.

Under dash relay is not pulling in and is not receiving 12V when key is on.

I've done lots of work but it's cumbersome to articulate when the factory manual trouble shooting guide is kind of lean as to the basis of why vs a Scotty Kilmer video. lol

There is a ground that goes from the ECU to the under dash relay that is supposed to have zero resistance but it has resistance of a few ohms.

Long story short it appears to be isolated to the fuel pump circuit which provides me a better chance to diagnose.

Spark plugs fire when starting.

More as I get my head wrapped around it.

1990C4S · 06-27-2023, 03:10 PM

In the link you posted, is A or B not pulling in?

john70t · 10:04 AM

Relay will have 4 prongs at least i think.
Two for the signal/control path. Put 12V across that in correct direction and it should click shut the load circuit.
Two for the load path. The bigger prong connectors. That should be infinite or 0 ohm across.
There may be another from ECU etc. This may be 5 or 9v. IDK. Caution.

Wiring chart and research before. Above advice from an idiot. Changing it is probably due.