Voltage Drop / Difference (1 Viewer)

[florida95fzj]

Does anyone know what the typical voltage difference is when comparing the battery and the ECU's voltage when read via OBDII?

I have been fighting a no start situation. After a lot of maintenance it started last month (once) but never again. My new battery is between 12.5-12.6 but the OBDII reading is only 12.2V. I also put a heavy truck jump starter on as well.

The fusible links are new and I cleaned all of the connections. I hooked a timing light up to plug #1 and no spark. The starter is cranking and it sounds good. I am concerned the voltage is too low at 12.2V. I originally thought I had the distributor off a tooth but it has started since then.

 

[South Texas 97]

From several OBDI development sites,
"Control module voltage is the voltage supplied to the ECU. It isn't battery voltage but is usually close enough when the vehicle is running."

I am curious as to how you tell there is no spark by hooking up a timing light? Perhaps as it cranks over, you are not getting the flash from the timing light?

Check your grounds and check there is 12 volts to the high voltage transformer (Igniter)

 

[florida95fzj]

For whatever reason, I see about a .5V drop in voltage measured at the ECU compared to the battery. But, that wasn't making a difference with my problem.

My problem ended up being a slightly bent pin on the distributor control connector.

I had the timing light on wire 1 and the light stretched inside to the dash so I could see it. I normally would have held the wire against some grounded metal part in the engine bay but I didn't have a helper. When I had a good connection, I could see the light fire.

 

[george_tlc]

Well, you've resolved your starting issue, so that's good news.

How are you measuring the voltage and what pin on the ECU to come to the conclusion it is 0.5V less than at the battery. I presume you're using a decent multimeter to measure the voltage at the battery. What are you using to measure at the ECU?

cheers,
george.

 

[florida95fzj]

Well, you've resolved your starting issue, so that's good news.

How are you measuring the voltage and what pin on the ECU to come to the conclusion it is 0.5V less than at the battery. I presume you're using a decent multimeter to measure the voltage at the battery. What are you using to measure at the ECU?

cheers,
george.

Yeah, it's good to have that behind me.

I shouldn't have said I was measuring at the ECU itself. I was reading what an ODBII scan tool was reading from the ECU. I saw 12.2V and it concerned me. I chased down every ground, cleaned every connection up, etc...

I still have one electrical problem to chase down and maybe I find a bigger issue. I had to the pull the fuse for the electric door locks because it was locking/unlocking itself continuously from time to time.

 

[george_tlc]

Yeah, was fishing to see if you were going to say your OBD reader. I wouldn't trust what it is displaying without measuring with a decent meter. It's not like the reader is some kind of precision voltage measuring instrument...

You could find the pin on the OBD connector that provides nominal 12V and measure it to a good ground with your meter. That would be much more trustworthy.

Good luck with the rest of the fault finding. Electrical issues can be mighty irritating to chase down, especially if intermittent.

cheers,
george.

 

[florida95fzj]

Yeah, was fishing to see if you were going to say your OBD reader. I wouldn't trust what it is displaying without measuring with a decent meter. It's not like the reader is some kind of precision voltage measuring instrument...

You could find the pin on the OBD connector that provides nominal 12V and measure it to a good ground with your meter. That would be much more trustworthy.

Good luck with the rest of the fault finding. Electrical issues can be mighty irritating to chase down, especially if intermittent.

cheers,
george.

So I was originally thinking the reader was just passively reporting a voltage measured by the ECU. But, if the OBD reader is actually taking the measurement itself, I wouldn't trust it for sure. I didn't occur to me that was a possibility. Good point.

Thanks