Imo000

I might have this!

nuc

When I got my '84 Euro, for some reason a PO had taken this out. I found one from a scrap yard and put it in, very easy 5 min. job.

OP, I posted on your other thread a listing on fleabay. ****** it up, new they're over $200. Ouch!

GregBBRD

Mixing motors can/do fail. Everything on a car can/does fail.

I have a dedicated test board that we can put any piece from the system on and test it. Your rebuilt control head spent about 1/2 an hour, being tested on this board, before it was shipped to you....if it came with a fuse and instructions on how to install the fuse. If you call and get a mixing motor from 928 International...but tell them that you want it to be tested, before they ship it, I'd be glad to take my test board over there and make sure it is good, before it leaves. Easy.

Interesting that the heater control valve is open. I'm not convinced that you don't still have a vacuum leak, but I'd get a mixing motor that works into the system, before I wasted too much more time trying to figure it out....always replace the known bad component before you start digging any deeper.

BIMMERMIKE

Thanks for your help Greg, yes it did come with fuse link and instructions. I will call tomorrow and ask for a tested mixing motor.

I checked all lines at the solenoid bank, no leaks and all lines held vacumm.

blown 87

If any one learns nothing else from Greg Brown, or any other mechanic, let it be this.

Mrmerlin

setting motor is held in with 2 screws,
one holds a bracket that holds the wire harness, .
If your crafty swapping out the transistors is easy , someone on here has posed the part numbers and the pictures of the offending parts .

Bill Ball

Transistors...

https://rennlist.com/forums/928-foru...w-working.html

StratfordShark

Great thread find Bill.

And these boards are not like opening up an ipod. Will be so easy to desolder and replace those transistors if my base collector mumbo jumbo confused so no testing necessary.

Use a desolder pump (looks like metal bodies syringe) or desolder braid. You will need small 15 or max 25W soldering iron to melt old joints. When soldering in replacements apply iron tip as fast as you can consistent with nice shiny joint - semiconductors don't like heat.

smiffypr

First test is to force the arm on the setting motor to the cold position. then set the lever to cold and turn on the ignition. Move the temp slider to the right and the setting motor will probably move. Move the lever back to cold and the setting motor probably won't return. It means that one of the four transistors has burnt out. Rather than diagnose which one, just replace all four, they're only a few cents each. 2 BC327 and 2 BC337.

Smiffy

Cosmo Kramer

This could be my problem too, mine doesn't work and all of a sudden starts blending properly after driving for like an hour. Both temp sensors are good just like Mike's.

Calgary Ole

My motor would get stuck near full hot but would move with a little pressure with a finger. I bought a used motor and all works well now.
Really easy to remove once you wiggle under the pod on your back, two screws and pop the control arm off then two electrical connections.

Lorenfb

"the unit is a rebuild from 928 int, also with previous HVAC unit I carefully inspected the temp slider touches micro swithces."

The majority of so-called bad CCUs (about 75%) have no problems. Many/most mis-diagnose
the 928 climate control system and start by wasting money on a rebuilt CCUs. The majority of CCU
real problems relate to the A.C. compressor not working. The case described in this thread
is not that, so most likely buying a rebuilt CCU was a waste of money.

"Mixing motors do go bad."

Actually, that's not the case. And the unit is better described as a servo motor which when
understood implies that it requires feedback in order to function properly, i.e. without any
correct feedback a servo system goes open-loop. This results in the motor being fixed at
one extreme in rotation or the other extreme. So as with the case of the CCU, the servo
rarely fails and is also mis-diagnosed as a problem source.

"There are a couple of transistors that go bad."

Another bad assumption! So to start suggesting that part of the troubleshooting effort
is to start replacing transistors is mis-guided. Since doing that may well damage the
servo unit. The servo is easily tested by supplying +12 and ground and momentarily
grounding and un-grounding the feedback input, i.e. the temperature sensors.

The most common problem for full heat and no control is an open circuit of the outside
temperature sensor. A test of the sensor at its location (generally near the alternator)
doesn't assure that it's properly connected to the servo unit. That sensor combined
with the inside temp sensor and the variable internal resistor of the CCU are key
elements for the servo unit to function properly. Anyone of the three being "open" will
result in the servo unit stuck in one position. Moving the temp slider back and forth
will result in the servo's arm moving from one extreme position to the other when
the feedback loop is "closed".

Bottom line: Shot-gun troubleshooting wastes money and time and can result in additional
damaged/problem CCU system elements as exemplified in this thread.

smiffypr

That is not an assumption, it is a fact.
It would be a bad assumption to assume that it is the only problem they have, but in my limited experience it is the most common. I have fixed four of these units, and in all cases it was one blown transistor.

Smiffy

GregBBRD

Hope this helps shed more light onto the problem with this vehicle.

BIMMERMIKE

I appreciate all the input, after doing all the tests I believe the mixing motor has failed giving constant full heat.
Once I have a know good motor I will report back.