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Old 02-22-2000, 06:28 AM
Join Date: Jan 2000
Location: Maryland. USA
Posts: 180

HVAC won't maintain a set temperature.

Ran through job 601 (Quick Test without tester) and zero faults. Works like a champ. BUT, it doesn't evaluate "quality"

Moved on to job 609 (Testing ambient, in-car and potentiometer). They all check within limits.

Checked In-car sensor for air flow pulling through. It checks okay.

I'm left with job 611 (Balancing of sensor chain). I suspect it will check okay since it simply measures the three resistors in series.

So, assuming I'm stumped to here, is it:
Amplifier (any quick checks)
Regulating Valve (only 3 yrs old) (any quick checks)
Or, what else?

Obviously I don't have the tester. I was thinking of locating the pin that sends control voltage from the amplifier to regulating valve and then run a "quality type check to see what voltages are being sent. I am aware that the amp can send out of spec voltages that can damage the reg valve.

Thoughts and help very much appreciated.

80 450 SEL
86 280 GE
81 Vovlo
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Old 02-23-2000, 04:53 PM
Join Date: Jan 2000
Location: Maryland. USA
Posts: 180
Part 2 to this saga.

Seems after I ran the afore mentioned checks things are now okay. What gives? corrosion build up on the electrical contacts of the plug? Running the system full cycle those few times freed something up in the regulating valve?

Thoughts please.

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Old 02-23-2000, 08:56 PM
Aaron's Avatar
Join Date: Aug 1999
Location: Cincinnati, Ohio
Posts: 1,935


You will know if your amp is fried if it opens up the center vents when they're supposed to be closed and vice-versa for cooling, defrost, etc..A rebuilt one is only about $70 from VDO of North America. What exactly are your symptoms with the ACC? I have had to deal (unpleasantly) with many of these first generation systems (hate them to death). Perhaps I can help you out. P.S.--I assume you did check the aspirator tube behind the dash (foam padded tube) to make sure it isn't split or broken.

Aaron Greenberg
MB technician
Precision Motorcars, Cincinnati, Ohio
'67 250SE Cabriolet
'77 450SL
'80 300SD
'85 380SE
'89 420SEL
'93 300E 2.8
'72 Alfa Romeo 2000GTV
'73 Triumph TR6
'74 Jensen Interceptor Mk.III
'81 DeLorean DMC12
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Old 02-23-2000, 10:23 PM
Join Date: Jan 2000
Location: Maryland. USA
Posts: 180
Aaron, Thanks for responding.

I ran through all of the lengthy checks that are mentioned in my first post. Your additional info has helped me establish that the amplifier is not bad.

What makes it troublesome is that everything is now okay.

My symptoms were, I would set a temperature and it would not hold. For example it was a cold morning. We headed out for an hour drive and everything heated up nicely, but it didn't stop heating. So we ended up turning it off and on as we needed it. I checked all of the fuses and they all check along with the two that were added later in the 116's life. I even turned the fuses with my fingers because of the corrasion potential with those darn fuses.

Like I mentioned in the earlier post everything is working. With my luck it's an impending failure.

Thoughts on the way to check out the regulating valve?

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Old 02-23-2000, 10:26 PM
Join Date: Jan 2000
Location: Maryland. USA
Posts: 180
Aspirator tube? Is that for the cockpit air draw across the in-car temp sensor?

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