schibsted

After having trouble with no heat in my 944, and a HCV that always had hot water flowing i decided to remove the unit and take it apart to see what was wrong with it.

So my problem with the unit was that there was no change when i turned the temperature dial.. The mixing flap never opened to let hot air in, but the
heater control valve always had water flowing...

After spending a good two weeks on creating schematics for the entire circuit i found that one chip controls both the hcv and the mixing flap
movement (IC 4). I did some measurements and it turns out the chip was dead.. Got the new LM2902N chip yesterday and after 3 years of having the car the
mixing flap now will move as it should, and the hcv will close when the mixer is closed.

I see that theres companies that repair these units in the US, but they weren't interested in sharing any info with me when i asked so thats why i made this.
Hopefully this can be useful to some of you.

Needless to say i have also gotten quite familiar with the circuit so if anyone has questions about measurements etc. i will do my best to answer.
I have never drawn schematics before, and i'm quite sure there will be mistakes in here, but for most repairs this is more than enough.
See attachment for the schematics. I have also added datasheets for the ICs. The unit i fixed is not an AC model, but i would think most of the circuit is similar.

Heres some pictures of the project





EDIT: Opening the schematic in a browser gives crappy quality. With a proper reader they are allright
Had to remove the datasheets to keep under the size limit.
EDIT2: edited title to include CCU

Jay Wellwood

Right on - thanks for sharing!

Mach944

Very nice, thanks

Hans

THX!

aussie944cab

Thanks for sharing

951Dreams

Awesome work!! What's your next project to share? LOL

KAROZZA

Guys, I'm going this procedure right now but can't find the IC4(LM2902DP) mentioned by Shibsted, not even online.

Anyone know where I can buy the IC online?

PaulD_944S2

You can use an LM324, LM224, or LM124 depending on the temperature range you want.

See links:
http://www.alldatasheet.com/view_dat...hword=LM2902DP

http://pdf1.alldatasheet.com/datashe...S/LM2902D.html

This link is TI (Texas Instruments) and it appears that they can provide samples.
http://www.ti.com/lit/ds/symlink/lm2902.pdf

This is On Semi and they show current samples:
http://www.onsemi.com/PowerSolutions...ct.do?id=LM224

What package are you looking for?
2902DP shows SOT-108-1

BTW, LMx24 is one of the most popular quad op amps around, you might even find it as a pull from a used circuit board.

KAROZZA

Hi, many thanks for the response.

Just to make it clear, I do not understand these electronics as much, only a basic idea.

But, the original post had exactly the same issue as I have & so I thought, after testing everything else, I have concluded to change the same IC mentioned.

The problem, being so old, I think it is no longer available!

So, back to your suggestions, if I use one of these; LM324, LM224, or LM124, which one is closest to the LM2902DP? What do you mean by temperature range?

Lastly, what would be the precise code I would have to ask the electronics store; would I still use the DP after the numbers?

schibsted

Hey. Good to see this is useful for others

I forgot to mention in the original post that i ended up buying a LM2902N which has worked just fine to this day. There are some listed on ebay:

http://www.ebay.com/itm/2-x-LM2902N-...oAAOxyhodRzKJf

If you cant get this one, as Paul said any one of the ones he lists could be used as a direct replacement.

KAROZZA

This definitely is useful & appreciate you sharing.

I never would have established that IC could be the culprit. If I manage to get it fixed by replacing the IC, I shall follow up on this.

In fact, I actually have two control panels, one for AC & the other normal heating for non AC car, both are giving me the same issue as you mention above. I hope to repair both.

Cheers again for the reply.

Justin McMickens

Man I could really use your help for some reason the solenoid that opens and closes allowing vacuum to the heater control valve is losing power intermittently causing the heat to come on (for a couple seconds) then when it gets power again the air goes right back to being nice and cold. I cannot figure out what gives/takes away power from this solenoid. I tried swapping the cab air temp sensor, relay, ccu nothing helps same symptoms. Tends to occur when slowing down/about to come to a stop then it cuts right back on. Please help.

KAROZZA

Have you tried another solenoid?

schibsted

Edit: Just saw you said you changed CCU so disregard my post in the other thread. The CCU powers the solenoid and takes in input from multiple devices. It could be your Mixer chamber servo that is bad(Look at pin 2 IC4). Have you checked all the 3 temp-sensors?

KAROZZA

Replaced IC4 in CCU & tried it out on the car & still mixing flap or Heater Control Valve do not operate

Tested all the temp sensors with multi meter & all seem to change reading with temperature difference