bf mkII fault U0121

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Joined: Sun Aug 02, 2009 9:16 pm
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Re: bf mkII fault U0121

Postby VX L67 Getrag » Wed Jul 31, 2019 5:51 pm

It's strange I thought the VIN was in either block1 or block2 but doesn't seem to be in there & wonder if they do have that info in there at all & whether linking is an issue.

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Joined: Sun Aug 02, 2009 9:16 pm
Location: Bayside, Melbourne, Victoria

Re: bf mkII fault U0121

Postby VX L67 Getrag » Fri Aug 23, 2019 7:34 am

OK so a bit of an update... I was pretty sure the speed signal is taken from the front left & that had a different size wheel to other 3 so I changed it & made no difference.

I got another ABS module & programmed info from my other abs module & still the exact same issue.

The other strange thing is it seems to not happen as much after driving for more than 20 minutes, if I do a reset at that point sometimes the speedo stays working for 10+ minutes, but when it does fail the 3 lights flash on the dash(brake fail/TCS & ABS) the speedo stays on that speed for 2 seconds & then drops to nothing.

The other weird thing it does that I haven't figured out yet is from the rear right especially at lower speeds there is a chatter type noise & not sure whether that is the diff, driveshaft or speed sensor/abs issue?

Posts: 2444
Joined: Sun Aug 02, 2009 9:16 pm
Location: Bayside, Melbourne, Victoria

Re: bf mkII fault U0121

Postby VX L67 Getrag » Thu Aug 29, 2019 1:00 pm

I just noticed when the 3 lights flash on as it does that the engine has a misfire for a split second!

I hadn’t noticed it before as it was running on petrol but I’ve just fit LPG to it & it’s a lot more noticeable on it!

So it has to be something electrical like everything is dropping out for a split second, but not as bad when it’s warmed up!

Posts: 2444
Joined: Sun Aug 02, 2009 9:16 pm
Location: Bayside, Melbourne, Victoria

Re: bf mkII fault U0121

Postby VX L67 Getrag » Sun Sep 08, 2019 11:04 pm

O.K. so I have changed the complete diff carrier & has fixed the noise from that area but nothing else has changed.

It seems weird & 99% sure it's in the TCM but in saying that when the 3 fault lights come on the dash & then speedo stays for 3 seconds & then drops to nothing are ALL incorporated with the ABS & when looking at data it's the only module that completely drops out when the issue occurs.

Although I have used a different module with the same issue & all 4 wheels now are the same & have IDENTICAL speed signals... I may have to look closer at the harness's for this controller!

Posts: 2444
Joined: Sun Aug 02, 2009 9:16 pm
Location: Bayside, Melbourne, Victoria

Re: bf mkII fault U0121

Postby VX L67 Getrag » Tue Sep 10, 2019 4:06 pm

So as I thought it was a wiring issue now I pulled everything apart around the ABS module & down the back near where firewall meets inner guard the wiring harness goes through there from the ABS module.
I found the out shieth ripped apart & looked there & found a purple wire with black trace(twisted with red wire & 1 silver dot) that had a cut & bend in it & thought I'd found my issue.
When I wiggled it there didn't sem to be enough free movement to cause issue but repaired it anyway & thought I'll do a few more checks while it's apart...

When I pulled the sliding cover off the ABS plug I followed the main thick wires & thought it was weird the thickest wires(yellow & a black) go to heater & the yellow is the main power to fan motor & other wire from fan motor is brown.
I then followed the black thick wire from the ABS module & it went to the heater module & where I found that other brown wire from fan motor & both of these where they go into heater module had been really hot.

I'll tweak them so they make good connection & see if it happens again as to whether it's just that someone hadn't connected it properly in the past or if there is something in HIM making them overheat.
Just sounds completely STUPID to me as why have Ford used the ABS & heater module to carry the highest current for fan motor, it's just asking for issues!

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