ATC Sensor Diagnosis?

[jaywish]

Hi,

I got the no AC and hot air blowing out of the defrost going uphill. I figure it is a leak. I check and it seems the ATC sensor has a leak. So I get a NOS unit from rockauto. The new one leaks also. So either I am testing them wrong or they both have problems.

I apply vacuum with a pump to the small vac fitting and place my finger over the larger vac fitting that supplies modulated vacuum to the blend system. I pump up the vac and it goes down pretty quick. The modulation function seems to work when the lever is moved.

Am I doing this right?

If I am correct and they are both bad, has anybody rebuilt and or opened one of these units?

[gadget73]

test it with a vacuum guage hooked to the servo side of the sensor, and the engine (or an electric pump) supplying the vacuum. With it set to hot, it should pass no vacuum. Set to full cold, it should pass basically full manifold vacuum. In between is some value in between. THey won't hold vacuum perfectly from a hand pump just due to the imperfect seals and vacuum regulation sort of deal that they do. Normally when it fails, you get no heat because its leaking vacuum through.

The heat going uphill is usually either the soup can, the TBL leaking, or the check valve under the hood at the vacuum tree not acting like a check valve. I suspect your air temp sensor is fine, and the issue is under the hood.

If that stuff checks out OK, you can try applying vacuum to the system directly, but its not going to hold perfectly. Even when its working exactly right it bleeds down. There is a spec in the book somewhere for it that it has to hold vacuum for an amount of time, or not lose more than so much in so many minutes but I don't honestly know what it is off the top of my head. It should hold for several minutes though before losing it completely. If it drops off in seconds, something is broken. That test you kind of want an electric pump for, or you want to get creative with clamps to let the engine suck it down and then isolate the ATC from the source to watch what it does. Your hand would probably fall off before you got the thing pulled down to 15" or better with a hand pump.

[jaywish]

Hey,
The soup can and the check valve are both holding up fine, both have been fixed or replaced. What is the TBL?

The hand pump (Lisle) can make 25inches. You say to connect to the servo side. I can try that.

Holding the sensors in your hand and applying vac to vac port while pluging the servo port, both units lose all vacuum in about 10 seconds or less regardless of the lever position. The vac modulation function does work though if you test for that.

With the atc sensor cut out of the system and applying vac on the engine side of the firewall vac line to the system (without the soup can) it holds vac for about one minute twentytwo seconds. With the sensor installed the hold time is about 40 seconds.

Ever open one up?

[sly]

Thermal Blower Lockout (the thing on the heater hose line).

[jaywish]

Thanks will check that.

[gadget73]

minute and a half seems low, and under a minute is way low. Mine will hold for a couple minutes. I'd be surprised if its more than 10, and perhaps not more than 5 but its definitely more than you're getting.

hand pumps will make enough vacuum, its a matter of volume. If its leaking down to nothing in 40 seconds, you're gonna have to overcome that in addition to drawing down the entire system. Gets tiring real quick trying to hold it if its leaking at that rate.


The sensor you usually check with a guage on the servo side. It won't let you apply vacuum to it, or not anything that holds, but it will show the vacuum level changing with temperature position. The servo itself just needs to hold vacuum if you pull it with the pump.

The TBL is tee'd into the main vacuum source. Its commonly a leak point. Anything leaking there will make the whole system act dumb. Plugging the lines off will work pretty well as a test.

I've never opened one of those sensors up, but the last I looked at one, it seems to me it was plastic welded together or otherwise assembled in some way that didn't really look like it would come apart in one piece. I know what ought to be in there though. Basically its a sheet of rubber and a fair sized bimetallic spring. The temperature control arm operates a cam that adjusts tension on the bimetallic bit. Essentially the spring warps and seals or un-seals the vacuum ports on the regulator. The more it un-seals them, the more vacuum passes through and the colder the outlet air is. Fully sealed passes no vacuum and you get heat. The typical failure is the bimetallic spring loses tension, or the rubber petrifies and it constantly passes vacuum to some amount. I suppose if the body was cracked or something it could cause a vacuum source leak though.


Related thought, if you have the smog pump still, you might be able to cheat the system out if it refuses to be fixed. The smog pump includes a small vacuum venturi to assist with cruise control vacuum under high engine load. You could probably re-direct that into the climate control system to provide some extra vacuum. I don't know how much it pulls, but it may be enough to make it go.

[jaywish]

Great idea on the smog pump.

On the sensor, So with the servo port plugged you think the sensor will not & should not hold vacuum applied to it's vacuum port?