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Temperature Control Noise (Thumping)


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1994 LS400 - original owner - 220,000 miles

Six months ago heater stopped working, A/C cooling OK. Heating just blows cold air. About a month ago A/C started making quite loud thumping noise when temp set below about 72. Thumping about twice per second. Threshold for thumping lowers (below 72) as time goes by until only thumps at "Max Cold" - still no heat.

I could find nothing similar in Forum. Is it a damper motor?

Any ideas?

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Your heater may have stopped working due to the heater control valve failing, which is located towards the top middle of the firewall in the engine compartment.

This happened on my previous 92 LS at about 180,000 miles and it ended up being a very easy fix. I believe the replacement valve I purchased online was actually for a 1991 Toyota Camry- but it was a direct fit, much less expensive than the factory Lexus part, and was very easy to install.

Maybe take a look at yours and see if it is leaking. All the coolant that leaked out of mine over time stuck up the cable that opens/closes the valve- which is pushed out/pulled in by a motor inside the dash when the temperature is adjusted.

For the first couple of weeks after I installed the new valve, I still had to adjust the cable manually from under the hood in order to change from cold/warm air. Doing this enough times eventually freed up the cable to where it moved on its own when the temperature was adjusted and the problem was solved.

I'm not sure about the thumping noise- and not totally sure the heater control valve is the source of your heater not working- but it's worth starting with.

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TA in KC -- after posting I found the 2000 LS400 service manual source someone posted here and down loaded. A quick perusal and I located the section on the heater control valve service/testing. A quick check of mine shows that it is stuck in one position and doesn't move at all with various temp settings. Looks like that is the place to go to get my heat back. Thanks

1990LS400 -- thanks for the info. If heater control valve doesn't stop thumping, I think I'll locate the servos and and try disconnecting them one at a time to see if I can locate a bad apple.


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Update -

Well, the heat control valve is OK. Can be moved by hand and does control heat in cooling and heating modes. Problem is wire that moves the valve lever apparently isn't functioning, which is a major deal. I can push/pull it by hand but it won't move on its own when temp control is changed.

I assume now that servo probably is OK, but that motor driving wire is "dislocated" from wire and that is the thumping I'm hearing - motor spinning around with no resistence. Problem is getting to the motor/servo which is not so easily done for the common car owner with no special tools.

Anyone got any advise?

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