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My 1986 truck is running rough and has fault codes that come and go. I think the electronic engine control module may be the culprit. I removed it and one of the capacitors has odd markings on the side. See picture below. It does not have a bulging top and I don't see fluid leaking. All of the other capacitors look fine.

Is it likely that this capacitor is faulty? More generally, would it make sense to replace all the capacitors since they are almost 40 years old? The truck runs fine in general, but has fault codes and rough idle that come and go. I've checked everything else I can think of. I don't really want to mess with the EEC module unless there is a good chance that the capacitors are indeed faulty.

Update from Comments:

The vehicle is a 1986 Ford Bronco with the 302 engine and EFI. The fault codes are KOER fault code 13 (low RPMs at idle) and code 32 (EVP sensor low / out of range). The issue is rough idle when the engine is hot, but runs smooth when the engine is cold. I replaced EVP sensor, still failing. Manually tested sensor and it is fine. Checked vehicle grounds and also disconnected the EEC and tested ground on pins 16, 20, 40, 60. Everything good there. Checked EEC signal return on pin 46 with signal return on EVP sensor connector and it is good.

The only other odd thing is that if I test EVP sensor connector VREF against EEC signal return on pin 46 I get 1,300 ohms resistance. Shouldn't I get no connectivity at all here? Does this indicate a possible short somewhere? This is with the EEC disconnected, so just testing EEC signal return at EEC connector against VREF at EVP sensor connector.

image of capacitor

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4 Answers 4

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The one in your image seems to have some burn marks so I'd be suspicious about it.

Get yourself an LCR component tester for about €$£10. This will allow you to remove each capacitor, check its capacitance and its equivalent series resistance (ESR) which can deteriorate with age.

enter image description here

Figure 1. Screengrab from LCR tester review on EEVblog video.

You might as well replace them all while they're out but using the meter might help confirm that there was an actual component problem. You could also compare the ESR of each old and replacement component.

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It may not be a bad idea to replace the capacitors, but make sure you buy automotive-grade parts rated for 10,000 hours minimum at 100C or more, from recognized distributors like DigiKey, Mouser, Newark, Allied Electronics, RS Components.

Those capacitors will not be cheap. Forget about getting them from AliExpress, Amazon, Alibaba, or eBay. Any "savings" will be illusory only. You do get what you pay for here, and not paying will cost you time and aggravation. That's a trap that many beginners fall into. In general, unless you have lots of experience, buying parts from trusted sources will be less painful.

I don't really want to mess with the EEC module unless there is a good chance that the capacitors are indeed faulty.

They may be out of spec, but whether that's the sole cause of your troubles is not a given. I'd start checking connectors and wire harnesses, in bench-like conditions - meaning that you look at the pins with a hand-held microscope, with good lighting, to really see in what condition they are. Visual inspection of connectors that old without optical aids is a misnomer.

Is this capacitor faulty?

You won't know until you pull it out and measure it using an ESR or RLC meter (not a multimeter).

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It appears to me that hot wax or flux or similar has dripped onto the capacitor melting the plastic wrap over the aluminum can. There is some waxy looking material protruding over the capacitor (red circle). I doubt that this would render the capacitor defective. That does not rule out that it is faulty. Replacing all the electrolytics (testing as you go} as suggested in other amplifiers is a good idea.

enter image description here

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    Or somebody accidentally touched it with a soldering iron in the factory.
    – Michael
    Feb 5 at 10:05
  • Thank you. That is some glue on the outside of the board, it does not interfere with the capacitor connections.
    – fdubs
    Feb 7 at 5:25
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contrary to what others have written here, I would not go randonly replacing electrolytics unless:

  1. there is evidence that they have been subjected to high temperatures over a sustained period, which could be the case if they are in a sealed box that runs hot, or are situated next to a hot device such as a heatsink which causes them to dry out.
  2. there is other clear evidence (like a meter reading or visible bulging and leaking).

I have serviced quite a few electronic devices over the years, and I have not noticed electrolytics to be any less reliable than (say) diodes, transistors or resistors except where heat has been involved. In many cases I have seen good quality equipment over 50 years old which high voltage electrolytics that measured and performed perfectly.

Especially on older equipment, there is always the risk that you substitute an inferior component, or inadvertently damage a PCB track or something while removing a perfectly good one (all too easy on older single sided boards). I would consider old solder joints, crimp connectors, electro-mechanical components like relays and switchs, and so on, to be more likely first suspects if you are not able to directly track down a root cause.

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  • Well, there was the time of the capacitor plague. And I think that's what's in the head of the people thinking about capacitors. But that was in the 2000s, so well after that unit was build. And there are solid electrolytic caps, which behave quite differently...
    – Arsenal
    Feb 5 at 15:54

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