blazing928

Hi, long time since original post, https://rennlist.com/forums/928-foru...ohm-range.html, with Xmas, then school holidays in OZ is for me all of January, testing etc has now lead to this post;

My original post was flawed as my fingers are slower than my brain...hmmm
Thanks for the replies, I've investigated most of them.

the issue is that the CCU [climate control unit] is not telling the heater solenoid to close the heater valve when the temp slider is at full cold, or when you switch on the A/C to full cold.

When you switch the CCU unit to OFF, the heater valve closes [correctly], but on ALL other settings, the heater valve opens again.

so in my original post #1 I got this detail very wrong , I'm very sorry!!

The vac circuit is all working correctly, even with a very small slow vac leak from the heater solenoid.

I have replaced the external temp thermistor,[ouch$$] as I couldn't get reliable figures.
for future reference a new unit shows at ;
-13c 1442Ohm [in the freezer]
-8c 672ohm,
23c 584ohm,
43c 360ohm

I have tested according to the WSM [terminal 13 & 14 on the HVAC controller], and I believe the regulator circuit is good with the values of;
564 ohm at 18c
1586 ohm at 27c
1704 ohm at 30c
so I get a 1000Ohm change

So when testing as per the WSM "Checking Operation Switch",
what I don't get is this; [terminal 4 is the earth]
TEST 8; term 8 & term 4 - NO battery voltage - goes to term 9 on the stepper motor
TEST 10; term 9 & term 4 - NO battery voltage - goes to "circulating air flap" solenoid
TEST 13; term 12 & term 4 - NO battery voltage - goes to heater valve solenoid

Question; Which one is 'circulating air flap' & also what is 'impact wall' solenoid?

Also as a result the setting/stepper motor also doesn't get told to close the flap that directs the air blend from heater & a/c core = so always hot water flowing into the heater core equals HOT temps,[grrr] see above term 8

I have traced these wires & tested connectvity, the setting motor or stepper motor wires all connect correctly, no broken wires.

My theory is that, the relay inside the CCU when closed, which it does when you turn on the A/C switch wasn't sending power to term 8.
Also term 12 should also send power to activate [close] the heater solenoid, when you slide the temp slider to 18c, to close it and thats not getting power either. Note the A/c turns on.

I had my CCU unit looked at locally, they replaced some diodes, but this didn't fix the problem.
I have tried another CCU unit and that doesn't fix it either. There is a difference in the units, my very early 02/87build date 87S4 unit has a 928.573.060.06 [may not be original, but it used to work]and the other unit i have tried is a 928.573.060.08

I have read about the relay inside the CCU failing, mine has a clear cover -not blue or yellow, but it does work, the A/C compressor turns on and when tested the multimeter shows its connecting [I get a beep] from the multi meter.

So where do we go?
the vac system is ok, the a/c switches on & off etc [needs a gas- but the rear a/c is cool- so I know it works], when I trace circuits with my multi meter I seem to get connectivity. Even if I physically push the relay contacts it still doesn't power up the heater solenoid or move the stepper motor. I haven't replaced the stepper motor, its been pulled apart & cleaned, but I feel that the issue is either still with the CCU or somewhere I have a broken wire, yet when tested the wiring is good.
Im just not getting power from terminal 8,9,12

Any ideas what I've missed????!!!

thanks Nigel

blazing928

well an update...

I moved by hand, the stepper motor to the closed position,

then when I move the CCU from full cold to hot, the stepper motor moves!! and the heater valve closes too.....but it doesn't work the other way...hot to cold, so I know think the stepper motor is faulty.

I have read about getting the 4 transistors replaced, so Ill give that a try & see if it works.

StratfordShark

Reading your first post I was thinking that everything has been checked, so the comparator circuit for setting motor likely at fault. Then I read your 2nd post!

Let us know the outcome of testing motor/changing transistors.

The nice thing is these are old school circuit boards with acres of room for the components, and thick copper lands on the PCBs so you needn't be too delicate desoldering the original transistors.

Do you know how to test trasistors using resistance range of multimeter?

blazing928

Thanks, buying the 4 transistors today, all of $2, so I'll practice soldering skills at work!

No, I don't know how to test the transistors. Do they need to be removed before testing?
The person who replaced sone diodes on the CCU board said you had to remove a diode to test....

StratfordShark

Whether they can be tested on the board depends on the circuitry around them, so to be absolutely sure unsolder the component under test or snip leadouts and resolder later.

The transistor for test purposes behaves like 2 diodes joined at common terminal (the base). When you know transistor type (should be easily visible on body) you can check pinouts so you know which is base, emitter, collector.

Measuring resistance between collector and base (and emitter/base) you will see it conduct one way but not the reverse, if OK. Finally should see near infinite resistance between collector and emitter.

Whitesands

Hi,
Take a photo of circuit board just for reference.

Replace one transistor at a time,

Make sure you solder the transistors at the original height otherwise the circuit board won't fit properly.

It's an easy fix, I've replaced the transistors in my original and my spare one, both worked fine.

blazing928

FIXED Yay, yee haa

Well after about 40 hrs at least of testing, guessing the fix was the settings motor transistors.
One out of the 4 was wonky of BC 327 & BC337
They DO need to be removed to test, as the results when soldered are different to not.

I did get 10 diodes changed in the CC head unit, whether they also contributed is really unknown, but for future reference if the test above aren't working, despite what the workshop manual states you need to make sure the settings motor is working first. The workshop manual said replace HVAC controller if some tests aren't correct.

In hindsight the test on the settings motor that wasn't working may be an indication to fix the transistors, or at least try to get a known working unit before checking the HVAC controller settings.

I have retested the tests that weren't correct & now all good.

A billion thanks to the rennlist knowledge base

now just the weird pod auto lights to fix...
https://rennlist.com/forums/928-foru...ts-in-pod.html

regards Nigel

Whitesands

Well done mate.