rws021193

Hi, I have a W203 1.8 with the following issue:

The key fob doesn't open the door either by RF or IR. Once placed into the ignition, it isn't recognized, preventing the ignition switch from turning.

The previous owner mentioned the car was fine until his key fob got slightly wet in the rain. I've cleaned the fob's electronics with alcohol and reflowed all solder joints using a soldering iron and flux. The key's red LED flashes when buttons are pressed, and IR light is visible on my smartphone camera, but the car still won't respond.

Next, I took the NEC microcontroller from the possibly faulty key fob and transferred it to another known working W203 key fob. The newly prepared key fob would flash the red LED when a button was pressed and emit IR, but it still wouldn't unlock or start the car.

I don't have an RF tester to check if the key emits a signal at 433 MHz. I've learned that I can measure the signal received by the antenna with an oscilloscope. If I detect a signal, it would indicate the fob isn't likely the problem.

What should I investigate next? Should I look into the ignition switch module? I've read that even when just trying to unlock the car, the ignition switch module verifies data from the antenna to confirm if the fob is authorized. A faulty ignition switch could potentially cause symptoms where the car won't respond to the fob for unlocking or starting the ignition. Could someone please confirm that?

I've checked the battery, which is fine. I can lock the car from inside using the button, but unlocking requires pulling the door handle.

Any suggestions would be appreciated. Thank you.

112233

I would agree that it looks like a fault in the EIS. I already fixed one with a lot of luck, because the fault was temperature dependent. I had a fault in the voltage regulator chip. And I reflowed a lot. It is worth a try. No fun to take it out.
Don't you have a 2nd key?

PS: An other thought, is is possible that the key is not recognized as valid anymore, so if you transfer the mc, you transfer the problem too?

rws021193

Thank you for your response. I think I’ll end up taking the EIS apart. I fix laptop motherboards for a living, so maybe I stand a chance of winning this battle if it’s truly the EIS at fault (assuming it doesn’t require programmers or software I don’t have). Unfortunately, I don’t have a second key.

You’re right that the microcontroller within the key fob could be the issue. But how would I even check that? I don’t have any diagnostic tools or software. I just bought this car because it was very cheap, and I thought I’d give it a go with what I have.

I’ll probably just take the EIS out, try to find the problem, and if I have no luck, it might end up being scrapped.

112233

In Germany there are a lot of companies fixing EIS problems. You must sent them the key, EIS and the steering wheel lock, or buy a used EIS+keys and you own EIS to klone the data.

Petethepug

If all else fails there’s a work around for a cell phone based starter & key until you get a diagnosis.

https://www.midcityengineering.com/d...t-key-starter/