AHK repair

[pchotard]

Hi

My 1991 850i is one of the non CSI to be equipped with AHK system.

After unplugging the four connectors on the AHK module located in the trunk (see this thread for other module pictures https://www.bimmerforums.com/forum/s...&highlight=AHK)

PXL_20220728_144849255.jpg

I found several pins of the 2 black connectors which are broken (12 pins connector and 18 pins connector):

IMG_4454.jpgIMG_4455.jpg

I was able to found only one of this black connector on the internet.
For the other one, i simply reuse undamaged pins of the other connector and put them on it.

I put everything back in place and then on the MID, i got this error message : "R/AXLE FAILSAFE MODE"
Good sign !! The communication has been established again.

Next step was to install INPA on an old laptop equiped with a serial port and using also a BMW INPAS / Ediabias OBD and ADS interface (for example https://www.one-stop-electronics.com...ads-interface/ )

And then, read AHK module errors and clear them. Unfortunately, i didn't take a picture of the amount of AHK errors, but there was at least more than 50 errors stored !! Probably due to the fact that several AHK module pins broke in the time and generated misbehavior.

Now, the "R/AXLE FAILSAFE MODE" error message disappear on the MID !!

Then i decided to read AHK live value :


As you can see in the above video, the AHK pressure is fluctuating from 140 to 160 bars every 8 seconds and we can hear the hydraulic pump making noise when AHK system regulate the pressure.

What is surprising me, is that the AHK pressure is fluctuating rapidly, whereas the rear steering wheels are not moving.
Does it means that the AHK accumulator is dead ?

I decided to remove the hydraulic command block located under the spare wheel.
IMG_4273.jpg

To ckeck if the AHK accumulator is dead or not, i measure how far is the membran inside the accumulator using a vernier caliper.

PXL_20230429_095338581.jpg

[pchotard]

PXL_20230429_095350419.jpg

The vernier caliper indicates 3 cm, so this accumulator doesn't seems to be bad ? Is someone can confirm this ? Did i proceed correctly to check the AHK accumulator ?

[ArnoNym]

I'll do a test run once my job is done (Banjo Bolts and more) and will report back with results. But that will take a few more days.

[pchotard]

I'll do a test run once my job is done (Banjo Bolts and more) and will report back with results. But that will take a few more days.

Ok no problem
I am interesting to compare the AHK behavior on your E31 regarding the period of pressure régulation (8 seconds in my case).

[ArnoNym]

Well, my vehicle is finally running again and I was able to measure the time between the pressure accumulator charging processes. In my case, that's pretty much exactly 12 seconds.

[pchotard]

Well, my vehicle is finally running again and I was able to measure the time between the pressure accumulator charging processes. In my case, that's pretty much exactly 12 seconds.

Hi , thanks for the feedback .
I am still wondering why the pressure accumulator charging process is needed as frequently when the AHK system is not active ( i mean the rear wheel stay straight)

[ArnoNym]

Is the AHK seminar working material available to you? If not, please PM me with your email address. Unfortunately, I only have the document in German. Still, maybe that's better than nothing.

On page 10 of this document, there is talk of "pilot oil consumption (zero flow rate) at 120 degrees Celsius, at 160 bar". This pilot oil consumption is given as 0.25 l/min. However, it will hardly be possible to convert this pilot oil consumption into seconds/per accumulator charging cycle. One does not know exactly what volume of oil leaves the pressure accumulator during the 160-140 bar cycle.

[pchotard]

Is the AHK seminar working material available to you? If not, please PM me with your email address. Unfortunately, I only have the document in German. Still, maybe that's better than nothing.

On page 10 of this document, there is talk of "pilot oil consumption (zero flow rate) at 120 degrees Celsius, at 160 bar". This pilot oil consumption is given as 0.25 l/min. However, it will hardly be possible to convert this pilot oil consumption into seconds/per accumulator charging cycle. One does not know exactly what volume of oil leaves the pressure accumulator during the 160-140 bar cycle.

Hi ArnoNym

Thanks for pointing this document, i found it in my E31 archives directory . I will read it carefully !!