Jump to content

Capacitor on ECU Power Supply


cj.surr

Recommended Posts

I have a smaller dry cell battery that often drops to about 8v when the starter is first engaged. For that short moment (fraction of a second), the ECU will power off and lose connection to my laptop or dash. Is there a recommended size of capacitor that I could add to the +12v power going to the ECU to prevent it from shutting down during this scenario? I would assume a diode would also be needed to prevent the capacitor from discharging backwards into the system. 

Thanks

Link to comment
Share on other sites

This can also happen if the grounding to the engine isnt quite up to size. Remember that in most setups the ECU grounds to the engine block or intake manifold. If there is an undersized or corroded ground anywhere between this contact point and battery negative it can cause the voltage at the ECU to drop even if the supply side is good.

Link to comment
Share on other sites

I think the ECU itself is powering off. I did an ECU log and the log didn't show anything from before the engine starting. So I think the ECU shut down and lost that part of the log. 

Can't be ground in this case, my ECU and engine harness is grounded at the chassis. 

So, to size the capacitor, I'll assume the ECU is consuming less than 2A, since I'm not switching anything on the high side. 

Link to comment
Share on other sites

What ECU do you have?  The wire-in ECU's dont drop out until below about 6.5V, the ADC's start getting inaccurate about 6.8V.  Some of the plug-in ECU's wont go that low as the main relay control circuit will cut power to the ecu before then - in this case it could be fixed by a small change to the wiring.

You dont want a cap or any other type of filtering on the ECU batt input as that measured voltage is what your deadtimes and dwell is based on.

Link to comment
Share on other sites

5 hours ago, Adamw said:

What ECU do you have?  The wire-in ECU's dont drop out until below about 6.5V, the ADC's start getting inaccurate about 6.8V.  Some of the plug-in ECU's wont go that low as the main relay control circuit will cut power to the ecu before then - in this case it could be fixed by a small change to the wiring.

You dont want a cap or any other type of filtering on the ECU batt input as that measured voltage is what your deadtimes and dwell is based on.

That's a good point, I figured there would be a reason I didn't want a cap on the supply. 

It's an Xtreme wire-in. I didn't think battery voltage would get down to 6.5v, I'll have to check if there's any significant voltage drops in the starting system. It may be difficult because of how short of a duration it occurs in.

I have the ECU and harness powered off the starter terminal, so it is subject to the voltage drop of my starter cable, which is fairly long. I may be better off running a separate power supply from the battery to the engine harness. 

Link to comment
Share on other sites

I had this same issue and pretty much asked the same question.

The issue turned about to be my power wire routing which was set up just like you did. I wired from the trunk battery, to the starter, to the power distribution.

I separated the feeds (you shouldn't need nearly as big a wire) over the winter when i did a bunch of wiring changes and it resolved any dropping issues during startup (I think I was also having starting issues in cold weather as well due to the same issue).

FYI I'm running a tiny lithium battery in my car.

Link to comment
Share on other sites

I have tried powering the ecu from the starter motor terminal, and never had any success.  It has always resulted in a big voltage drop caused by the huge transient currents drawn by the starter.  Running separate feeds for the ecu and fuel pump should minimise the voltage drops.  

The situation is often compounded by small, race batteries which in my experience only add to the difficulties. 

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...