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Discussion Starter #1
Hey there guys:

94 Thunderbird, 4.6 here.

(Detailed post, I suspect this may help others with similar problems with their Keyless. If solved succesfully, the capacitor might be the only culprit and may save some OEM modules out there from being junked)

A few days ago the keyless entry system stopped working.

Symptoms:

No Keyless
No alarm light on dash
No chime
No power locks initially, then after some wire checking and disassembly of door panels, only the "lock" function seemed to be restored by pressing door switch, but when the unlock button is pressed, no actuators response is heard/seen and all four corner lights and dash lights come on (!!!! weird huh?) for as long as I keep the button depressed.

What I've checked already:

*Batteries ok (car's is less than a year old, cranks with full power... FOB battery also OK)
*Checked all fuses. All OK, including the two in-line on the looms that run to the modules in the trunk... all fuses in engine bay too.
*Checked wires on driver door (by rubber boot between door and body) for crimped, broken, worn, shorted wires: Nothing wrong there.
*Checked connectors and cables on door switches. All seem fine, corossion/wiggle free. Solid.
*Checked connectors behind the fuse board, under the dash. Took connectors off and checked, including pins and blades, they're all spotless, firm and gunk free.
*Checked trunk Modules: inspected connectors and and also visually inspected inside the keyless and alarm modules' PCB's for burnt, loose, desoldered components. Nothing I could see with the naked eye. Both were spotless. Except for a 4-wire (flat, kind of old "flex" type) connector that hooks up the two PCB's in the Keyless entry module, it had a slight tar-like mark on the white paper insulation, (see pictures) like the ones you get from a burning cigarette on an ashtray. (More on that below)
* Disconnected the under hood light harness, one of the leads could have been touching the other and shorting... I won't really know until I replace the capacitor.


I had a similar issue some 2 years ago and I swear I had checked all the fuses and they were all fine, and then on a second check after some frustration, I found a 10 amp fuse blown. Go figure!

Exact same thing happened this time around. First fuse check yields nothing! and some more checking and cursing after disassembling stuff and I find exact same fuse blown again (Fuc&% man!!! :boom:)

CURRENT PROBLEM:

I replaced the fuse as I did the first time but this time it blew after the horn sounded twice as if the alarm had been armed AND triggered, then silence and a slight burnt electronics smell. Checked fuse: Blown again, but smell didn't come off of it, but somewhere else:

Checked the the trunk and took the Keyless entry out of the big black plastic cover and gutted it. Indeed, the smell was coming from inside the module... but, strangely, nothing seemed obviously burnt. Except for the flat "flex" connector... this time it was a just a tad darker than before, but still, nothing seemed to have burnt that badly in order to let off that amount of stench and brownish mark on the white jumper (see picture)

Whatever had burnt, had to be close then.


Did a continuity test on all four wires and they worked fine but two of them were shorted (!!) and that was very strange... Why would designers use two wires to connect different copper tracks if they were connected in the end?

It turns out a ceramic capacitor nearby was shorted and seemed alright to the naked eye, but upon very close inspection (using my phone's camera on highest zoom, I was able to see a hairline crack (see picture) and the ceramic capacitor was, indeed, shorted. Disconnected one of the capacitor's leads and the flex connector wasn't shorting anymore.

I'll buy a replacement capacitor once I find the value of this part but:

QUESTIONS:

Do you think this failed capacitor was the original source of the problem or the victim of something else that killed it?
Are these caps (ceramic ones) known to fail often? ... I know for a fact that electrolytic caps do fail often (they bulge up, leak and fail)

Any comments on this greatly appreciated.

Sergio Salinas.
 

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That's not a capacitor.

It's a Metal Oxide Varistor - a surge protector.

Yes, they can fail that way; usually when excessively high voltages come into the circuit. Or marginally high ones for years then getting hit with a slightly higher one.

Here's a link to a write up on them: https://www.tdk-electronics.tdk.com/download/531268/8954d4a78154a9da5c70a7119fa03e86/siov-general.pdf

It's been YEARS since I had to read one; I THINK that's 100V, but I could easily be wrong (hey, Grog6, you know off hand or have a reference handy?)

RwP
 

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Discussion Starter #3
That's not a capacitor.

It's a Metal Oxide Varistor - a surge protector.

Yes, they can fail that way; usually when excessively high voltages come into the circuit. Or marginally high ones for years then getting hit with a slightly higher one.

Here's a link to a write up on them: https://www.tdk-electronics.tdk.com/download/531268/8954d4a78154a9da5c70a7119fa03e86/siov-general.pdf

It's been YEARS since I had to read one; I THINK that's 100V, but I could easily be wrong (hey, Grog6, you know off hand or have a reference handy?)

RwP
Timely post! ... I was trying to find info on what I thought was a ceramic cap... Thanks!!... I'll read up.
 

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Actually, it looks more like an inrush current limiter. An NTC resistor.

It limits the current when cold, and loses resistance as it warms up.

Like the units that used to run Degaussing coils in TV sets, WBW.

I have a keyless module here somewhere; let me find it and I''ll see what it is.
 

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Discussion Starter #5
Actually, it looks more like an inrush current limiter. An NTC resistor.

It limits the current when cold, and loses resistance as it warms up.

Like the units that used to run Degaussing coils in TV sets, WBW.

I have a keyless module here somewhere; let me find it and I''ll see what it is.
The PCB does label the part as "MOV1", (you can see the "MO" on one of the pics) I missed that out of ignorance and assumed it was a capacitor, RalphP correctly identified it... I figure my local electronics store will have them, will pay them a visit tomorrow and report, will buy at least three cause I don't know if something else in the wiring or circuitry caused it to burn or it simply quit working out of 24 years of taking the pounding from a bad voltage regulator (changed it a few weeks ago) ... I just need to read the values, and haven't had luck on the web yet.
 

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If it's labelled MOV, that's it.

The part number was leading me to it being an inrush limiter.

Ford uses "HOuse Numbers" a lot, which mean nothing outside ford.

It will be set up to start limiting at ~18-20V, as it's there to limit load dump and loose wire kinds of things.

You don't want it to come in below 16V, as the alternator can output that.

A good identifier for the voltage is the ratings on the electrolytic caps; if they're 16V, that's the voltage I'd get.

I'd do 18V at max.

Most automotive chips can handle 24V without blowing.
 

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Discussion Starter #7 (Edited)
If it's labelled MOV, that's it.

The part number was leading me to it being an inrush limiter.

Ford uses "HOuse Numbers" a lot, which mean nothing outside ford.

It will be set up to start limiting at ~18-20V, as it's there to limit load dump and loose wire kinds of things.

You don't want it to come in below 16V, as the alternator can output that.

A good identifier for the voltage is the ratings on the electrolytic caps; if they're 16V, that's the voltage I'd get.

I'd do 18V at max.

Most automotive chips can handle 24V without blowing.

That's really useful to know now. I went to the only two electronic components stores in this shitty town today and couldn't find anything remotely close to the values I need (I was going for somewhere close to 18v) All parts available were above 250V, with some going up to 44Kv :surprise:

I guess I will have to order a few online and I'm really, really hoping that a supplier in Mexico will have them in stock, I don't want to spend $50 US to import and ship a part that costs one buck :frown2:

I have found some equivalent parts from TDK...

Will report how everything turns out once I get the part

Thanks for now!!...

:smile2:
 

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Does Digikey or Mouser electronics ship there?

IDK where you are, but there should be equivalents in most countries.
 

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Discussion Starter #9 (Edited)
Does Digikey or Mouser electronics ship there?

IDK where you are, but there should be equivalents in most countries.

I live in Cancun.

I found Mouser's website for Mexico.

Here's the search results applying three filters:

a) "18v Varistor Voltage"
b) "14mm" (mine is 15mm)
c) "MOV"

https://www.mouser.mx/Circuit-Protection/Varistors/_/N-5g3h?P=1yzt5irZ1yoakqvZ1yzvqm3

That gives me 8 hits, but the bottom 5 are no longer in production.

The site shows different voltage values to filter varistors by so it's kind of confusing for me, I used "varistor voltage" but I have no clue as to whether that was the correct parameter to filter by.

This series seems to be the one but, as I said, the technical data seems all confusing to me and I dont really know which one I should go with...

https://www.mouser.mx/datasheet/2/240/Littelfuse_Varistor_LV_UltraMOV_Datasheet.pdf-586789.pdf

Sorry for being a pain... lol...

The screen shot below show lines 9 and 10 from the data sheet posted above with what I consider 2 suitable candidates... agree?

Green arrow is 14mm body and orange is 20mm
 

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