Need help identifying a capacitor from a car ECU - No visible markings - Page 1
Forum section: electronics / beginners

author: ladui
author_rank: Newbie
timestamp: October 10, 2025, 06:01:36 am
content: Hello everyone,I'm trying to repair a car ECU that shows a fluctuating system voltage (between 6V and 13V in diagnostics) even though the battery voltage is stable. I suspect a faulty capacitor on the main power supply line.The capacitors in question are a small, yellow, ceramic capacitor located right next to the main power input pin (pin 12) and ignition (pin 13) of the main controller chip. There are no visible markings or codes on it.I've attached a close-up photo of the capacitors (pin 12 is constant + and pin 13 is ignition + ) and an overview of the board. I've checked the resistance with a multimeter and pin 12 capacitor goes for 0.8MΩ and pin 13 capacitor goes with 2.8kΩ. The ecu was under water so from my little understanding I suspect this could be the problem for the fluctuating system voltageCould anyone help me identify its likely value? Any help would be greatly appreciated!Thank you in advance.

author: inse
author_rank: Super Contributor
timestamp: October 10, 2025, 07:38:47 am
content: Those caps look fine and do not seem to have suffered from moisture.There is probably some moisture or corrosion in the battery voltage sensing circuit.In the easiest case it’s a high ohmic voltage divider at the micro controller.Maybe there is some conductive residue that disturbs the measurement circuit or the ADC input.Scrub the board with a toothbrush and pure IPA and see if this improves.The connector or wiring loom did not suffer from the watering?

author: Sorama
author_rank: Frequent Contributor
timestamp: October 10, 2025, 08:08:35 am
content: @TS,I don't agree with your analysis.Those caps won't make the power rail unstable.I suppose these caps are in parallel so you could always (after verifying) remove those caps and measure the voltage again.Anyway, those caps are not sensitive to water.

author: ladui
author_rank: Newbie
timestamp: October 10, 2025, 08:11:16 am
content: I've already scrubed it multiple times using IPA same result. Also I've tried now to measure the capacitor, the pin 12 gives me 104nF but the one on pin 13 return nothing. Sould I try to desolder it? Is this a 10n ones?Edit I've also found this circuit scheme.

author: ladui
author_rank: Newbie
timestamp: October 10, 2025, 08:41:01 am
content: The wiring from pin 12 looks coroded

author: Sorama
author_rank: Frequent Contributor
timestamp: October 10, 2025, 09:01:24 am
content: Quote from: ladui on October 10, 2025, 08:41:01 amso what?won't have any impact.again, forget about those caps--> irrelevant.

author: ladui
author_rank: Newbie
timestamp: October 10, 2025, 09:21:52 am
content: Quote from: Sorama on October 10, 2025, 09:01:24 ameven thought the capacitor from pin 12 returns 104nF and the one from pin 13 nothing?

author: Sorama
author_rank: Frequent Contributor
timestamp: October 10, 2025, 09:27:21 am
content: Quote from: ladui on October 10, 2025, 09:21:52 amif you want to measure them, you will have to take them out.

author: CaptDon
author_rank: Super Contributor
timestamp: October 10, 2025, 01:44:41 pm
content: I am guessing the battery voltage was still present while the ECU was under water? As evidenced by the corrosion on several pins of the connector there will also be corrosion where the connector is soldered to the board and I would suspect any of the SMD resistors could have damage. We actually had problems with SMD resistors failing in various ways when exposed to sulfur compounds found in fuel oil. Those resistors did not receive direct exposure to liquid fuel, only to the vapors found in the engine room. You may have a totally unreliable module no longer worth repairing. It may suffer additional unpredictable failures and leave you stranded.

author: Gertjan
author_rank: Frequent Contributor
timestamp: October 10, 2025, 04:40:29 pm
content: Quote from: ladui on October 10, 2025, 06:01:36 amI had a similar problem with water damage. Scrubbing with IPA did not help enough.I had to remove the IC's to clean under them. That was where the troublesome deposits were.regards, Gertjan.

author: ME
author_rank: Frequent Contributor
timestamp: October 13, 2025, 12:55:47 am
content: why do all pics say content not viewable in your country?

author: BradC
author_rank: Super Contributor
timestamp: October 13, 2025, 04:40:20 am
content: Quote from: ME on October 13, 2025, 12:55:47 amBecause they're hosted on imgur and your government is crap. Don't take offense, ours is crap too and I'm sure in the not too distant future shit hosted on imgur will be unavailable to us also. This is why I host my own images. Third party image hosting services are crap.

author: Infraviolet
author_rank: Super Contributor
timestamp: October 13, 2025, 08:08:14 pm
content: ME: get a VPN, Proton VPN gives a pretty trustworthy free tier of service for if you only need it occasionally. Mullvad is the one usually considered well regarded for paying customers....Also, even if it weren't for lunatic governments, it's bad practice for forum posters to rely on external image hosts like that, an external compnay could one day just pull out of doing image serving (I think one of the big ones even did that some years back) or could go bust.Also, just to guess, the capacitors in question here. It sounds like they're being used for a decoupling application? In theory then any replacement which can handle the loads placed on the originals would be good enough, over sizing would usually not be harmful and would usually help suppress the ripple voltage further.

author: Gyro
author_rank: Super Contributor
timestamp: October 13, 2025, 08:21:49 pm
content: Quote from: Infraviolet on October 13, 2025, 08:08:14 pmYes, that was when Photobucket stopped free image hosting. We lost all sorts of valuable stuff. Imgur will surely go the same way. Just don't do it!

author: Haenk
author_rank: Super Contributor
timestamp: October 14, 2025, 04:41:00 pm
content: Quote from: ladui on October 10, 2025, 09:21:52 amDid you check for resistance as well? It might be dead short, or there might be a short up the path.Other than that, the visible corrosion is not great, might have damaged a couple of components, when moisture was present and unit powered on.