Key fob repair for 31 year old car - Page 1
Forum section: electronics / repair

author: SolderSucker
author_rank: Regular Contributor
timestamp: August 30, 2025, 04:58:28 pm
content: I'm trying to repair a key fob for a friend - he's been restoring a 1994 Jaguar XJS for some years and, while the key fob used to work erratically, it stopped functioning some years ago.I've taken a look at the fob, repaired some obvious faults (corroded battery spring and the momentary button) and installed an MN21 battery.Now when the button is pressed on the fob the car beeps but does nothing else, so at least the fob is doing something.What's strange is that the handheld diagnostics tool that he has (which can show an RF signal) shows nothing when the fob's button is pressed, even though the car itself is obviously recognising something (hence the beep).The diagnostic device has a function which will recognise RKE or RF, and I guess the fob uses RF (there's no evidence of it being IR because there's no IR LED).The encoder/decoder chip on the fob's PCB is the ML145026DI'm not at all familiar with RF, particularly in devices that are 31 years old.Also, as this fob doesn't have a microcontroller, there's nothing to program.Any ideas please?

author: BILLPOD
author_rank: Frequent Contributor
timestamp: August 30, 2025, 07:58:17 pm
content: Good Morning SolderSucker, There's a new one for sale on Ebay for $34 USD.https://www.ebay.com/itm/135289842975?var=0&mkevt=1&mkcid=1&mkrid=711-53200-19255-0&campid=5338590836&toolid=10044&loc_physical_ms=105222&customid=5eb2f21d781b11eecf529aae2ebfae18&gclid=5eb2f21d781b11eecf529aae2ebfae18

author: mikerj
author_rank: Super Contributor
timestamp: August 30, 2025, 08:49:28 pm
content: Quote from: SolderSucker on August 30, 2025, 04:58:28 pmAre you sure it isn't theML145026, the Landsdale Semiconductor clone of the MotorolaMC145026?It's possible that a keyfob that old uses a frequency that a modern diagnostic tool does not cover. These old RF encoders generally use pins to set the address and data value of the outgoing frame, the fact that car receives it suggests the modulated RF output is working and the address in the frame is set correctly. Possibly you have an invalid data value, caused by a short or open on one of the data pins.Cloning another keyfob is as simple as copying the configuration values on the address pins, often set by a series of solder pads on the PCB.

author: SolderSucker
author_rank: Regular Contributor
timestamp: August 30, 2025, 09:31:11 pm
content: Ah, sorry, I made a typo - yes, it's definitely the ML145026D (I've found PCB images online which instead show the MC145026D as being used).Thanks for all of the extra info - if I still don't have any luck getting it to properly work with the car then I guess one option is to replace the encoder, assuming that I can find one (I've not looked yet).

author: SolderSucker
author_rank: Regular Contributor
timestamp: August 30, 2025, 09:32:45 pm
content: Quote from: BILLPOD on August 30, 2025, 07:58:17 pmThat may end up being the best option if I can't get this old fob to work, so thanks for the link.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 30, 2025, 11:17:37 pm
content: I know nothing about this particular car or fob, but on many of similar vintage the car is programmed to recognize the FOB. Presumably stored in some sort of non-volatile RAM. It seems quite possible that over the years this has been corrupted, and the car may need to be reprogrammed to recognize the key. On some vehicles this can be done by the owner by shorting the right pair of pins, or doing the right dance on various buttons and pedals, on others it can only be done by someone with the right factory gear. It would be worthwhile searching the net for how this car is programmed.

author: Fraser
author_rank: Super Contributor
timestamp: August 30, 2025, 11:35:23 pm
content: To add to the comment by CatalinaWOW,The key fob may have been faulty but so too could the cars security system unless it has been proven good with another key fob. If the car system beeped, I would expect the security system to be acknowledging a correct code but it is not producing the usual control function output to whatever it drives…. central locking, immobiliser etc. Some car security systems contain backup batteries (in case someone disconnects the car battery) that can leak over time, causing damage to the system.Fraser

author: Benta
author_rank: Super Contributor
timestamp: August 30, 2025, 11:55:25 pm
content: This is a very primitve remote unlock system, transmitting at 434 MHz.Coding is hardwired with a fixed code.Forget your pal's diagnostic thingy, it obviously doesn't work (probably tuned to 868 MHz, which is the standard today).Fob and receiver apparently work correctly, as the car reacts. Which means the fault is in the car. Either the door lock solenoids are fried, or the driver stage for same. Apart from that any wiring fault could be in play. Wouldn't be unusual in a British car. First place I'd look is the wiring bundle between A-pillar and door.

author: wilfred
author_rank: Super Contributor
timestamp: August 31, 2025, 12:27:45 am
content: When buying a replacement keyfob online make sure it is the same frequency. An American one will be on a different frequency to an Australian or European one.Also since you said the car audibly responds and it operated intermittently then maybe the fault is with an actuator where old lubricant is gumming things up. Or in the case of door solenoids if the old car had a marginal battery then a combination of things may have contributed. I'd work my way in from both ends and you seem to have a working fob at least.

author: SolderSucker
author_rank: Regular Contributor
timestamp: August 31, 2025, 01:13:04 pm
content: Thanks for all of the suggestions, I'll do a deeper dive using them as a basis for investigation.

author: Kwakerman
author_rank: Regular Contributor
timestamp: August 31, 2025, 07:40:27 pm
content: My old and long gone 1996 Land Rover Defender (possibly a similar alarm) had the alarm 'computer' a '10AS system ' IIRC in the dash, it also had a battery back-up sounder (BBUS) under the bonnet but also an immobiliser box under the passenger seat. That box has a couple of relays in it that were prone to getting dry joints and causing various issues. I think one relay was the starter motor isolator, cant remember what the other did. The box under the seat was riveted closed but you could drill them out and inside was a plastic box with the relays. I just resoldered and reassembled and all was ok. Not saying the XJS has the same alarm but they are both from the same manufacturer and of similar age.Put a picture of the fob on here and I will tell you if it is similar to my old one.

author: Kwakerman
author_rank: Regular Contributor
timestamp: August 31, 2025, 07:50:11 pm
content: A bit of googling seems to indicate the XJS could have the 10AS system (made by LUCAS), I also seem to remember there was a re-synchronisation procedure to carry out when the battery was changed. It was something like ignition on, drivers door open then press the lock (or unlock?) button 5 times until the indicators flashed.Hopefully attachment works, if not try googling 'xjs 10as alarm'

author: FIXITNOW2003
author_rank: Regular Contributor
timestamp: September 01, 2025, 06:23:40 am
content: here is some note for the land rover version

author: Kwakerman
author_rank: Regular Contributor
timestamp: September 01, 2025, 08:30:44 am
content: Yep, that sounds familiar, I think the door open bit I mentioned above must have been for one of my other cars.

author: SolderSucker
author_rank: Regular Contributor
timestamp: September 01, 2025, 09:24:31 am
content: Quote from: Kwakerman on August 31, 2025, 07:40:27 pmThanks, here's an image of the exact same fob that I found online:https://i.ebayimg.com/images/g/uaoAAOSwk4VfkBcQ/s-l1600.jpgBTW, thanks for the PDF link.

author: SolderSucker
author_rank: Regular Contributor
timestamp: September 01, 2025, 09:26:05 am
content: Quote from: FIXITNOW2003 on September 01, 2025, 06:23:40 amThanks a lot, I'll have a read.

author: darkspr1te
author_rank: Frequent Contributor
timestamp: September 01, 2025, 09:46:20 am
content: Try checking these pin for the listed voltageshttps://www.the12volt.com/installbay/alarmdetail/1205.htmlThe fob works on 318Mhz so your tester should pick it up.The alarm also uses two separate fuses and both are required for the car to function, I have not found a diagram of this, only forum posts referring to the same. F11 in the cars internal fuse box is ment to the main one (without the alarm wont work at all) and F15(only 1 reference to this, could be wrong) in the engine bay controls starter/fuel/doors/remote start etc.often on older alarms of this type would have main module and secondary module , the main module receives the signal and then will send a signal to secondary module to trigger windows, doors, fuel relay and starter Enable. They are often powered separate too.hope this helps.darkspr1te

author: Sorama
author_rank: Frequent Contributor
timestamp: September 01, 2025, 10:12:35 am
content: The issue is obviously not the fob itself.

author: SolderSucker
author_rank: Regular Contributor
timestamp: September 01, 2025, 10:34:22 am
content: Quote from: darkspr1te on September 01, 2025, 09:46:20 amThanks, that's also very helpful.

author: iJoseph2
author_rank: Regular Contributor
timestamp: September 01, 2025, 11:29:52 am
content: I'm just going to pop in to say have you checked the basics, specifically the voltage on the main car battery? (And while you're at it, check the connections especially the earthing points).Many years ago I had a 1994 BMW E36 and the locks wouldn't always work, and at one point the actuators stopped half way on unlock and got the locks stuck. Problem was the main battery having low voltage and being somewhat old.If the main battery is low voltage then why? Is there a parasitic drain? You say restoring an old car .. so make sure you start up the individual ECU's correctly when you reconnect the main battery .. usually put the key into position 2, then off, lock car, and do it again a second time. Should get the ECU's into normal operating state and will shutdown correctly when locked.

author: iJoseph2
author_rank: Regular Contributor
timestamp: September 01, 2025, 11:38:48 am
content: Quote from: Benta on August 30, 2025, 11:55:25 pmAnd on a 1990's BMW, the boot lid wiring.I had electrical gremlins in my car that was caused by the wires shorting in the boot lid flex. And my parents too on their car.