jonny2k

*****UPDATE: Problem Solved!!! See below Post #12 for answer*****

I still can't isolate this problem and am asking for assistance again...I'm hoping the title change may help!

My dad's 1992 LS400 has been having a heater issue for the last year but really only causes problems in the winter. The heater only blows warm/hot air if the car is driving over 40mph but as soon as he comes to a stop or speed drops below 40mph, the air becomes cold again. It's really a problem for my parents (they're in their 70's now) during the winter up here in the PNW and I want to try to figure out the problem and fix it before the next winter.

First off *I USED THE SEARCH FUNCTION* and found some useful information to begin the diagnosis. I began by starting the car from a cold engine and set the temperature to 70* and watched the heater valve to see if the valve would move as the engine reached operating temperature. As the engine reached operating temp I noticed the valve never moved and the fan continued to blow cool air, so I added a couple degrees at a time until I reached 84* and it STILL didn't move nor did the air get any warmer. However...as soon as it reached 85* the valve moved ALL the way up and the air was blowing hot!

Now...does this mean the HEATER VALVE is bad, or does this mean the SERVO MOTOR under the dash is bad?? There was no coolant leak around the valve and doesn't appear that there is any corrosion around the cable either.

I'd like to replace the correct part(s) but because my folks are on a fixed income, I'd prefer to hear from those who have fixed this issue before I start replacing random parts.

I appreciate any help and offer many thanks in advance from both my father and myself!!

~Jonny

steve2006

Accurate diagnosis is difficult without the car in front of you but given the symptoms I would check the outside temperature sensor inside the engine compartment mounted near the fender bracket,these have been known to fall off with corrosion.
If is not in the correct position the car heater may think the outside temperature is warmer than it actually is and not turn the heater temp up.
I am going by the fact that at 45mph the heater does work and the airflow around the sensor at that speed may be enough to give a correct reading.
There is another interior temp sensor mounted inside the heater assembly but not so easy to get to.
Have a look for the exterior sensor 1st it will be easy to spot if hanging off.
With the water valve working to a point I would think that it is OK along with the servo and the AC ECU but as I said the temperature sensor if incorrectly located may give a false reading and give the problem you have.
If it is the sensor it is an easy and cheap fix to put it back.

deanshark

Man, these cars are so complicated. I have a similar problem with my heat. (I should say, no heat) I think mine is the heater core though. I have no heat but when I drive for about 15 minutes it comes out warm, not hot, if it's on low. If I turn it up it just blows cold. I'd also like to hear a little more diagnosis if any and appreciate any input. Winter will be here soon enough.

Thnx.

jonny2k

Steve,

Thank you for your insight and suggestions. This temp sensor you mention is not something I'd considered, and in fact did not know was even present! However the only thing that makes me suspect this sensor may not be the culprit is when I was testing the valve, I had the hood up and the ambient temperature was constant. The ONLY time the valve would move (and therefore provide hot air) was when the temp control was set at the highest setting, 85*. At 84* and lower, the valve would drop all the way to the lowest/coldest setting....in other words, it shut off.

I'll look for this when I am at my folk's house over this coming weekend. Thanks again for the input!

lexuslspro

jonny2k,

First off use the search feature( joking)

Did you consider the thermostat?

My Volvo had the same problem...was easily solved by replacing a cheap thermostat.

billydpowe

iTrader: (1)

Not to rain on your parade, but in my 60+ years of working on cars, (I am76) I have run across symptons like this..... low coolant flow until the rpm's are 2000+. try this, without driving, run the rpm's up to 2k and see if you get any warm air... to see if your coolant is moving thru the heater core. it could be restricted, or even have a faulty "air diverter door" (old time term), have seen them where a pencil fell down the defroster vents and kept the door from operating... BUT check the coolant movement through the hoses, they should be hot to the touch... after warm up. We once had a waterpump that was faulty and would only move coolant at high rpm's, the vanes (little fan) that pushed the coolant along were eat off on the ends...
none of this may help, but we are trying..... keep us posted.

jonny2k

Actually yes, it was one of the first things we checked. The engine gets up to temperature and maintains just fine which would indicate the thermostat is working, and hot air will indeed blow as long as the climate control is maxed out at 85*....any lower temp setting though and it closes off the heater valve completely resulting in cold air.

Tried this too and the system seems to be full, and I may try and "burp" the system to see if there's an air bubble but I honestly don't believe low coolant is the issue. Low coolant wouldn't keep the heater valve from moving until the climate control is set to the highest temp (85*) would it? The problem seems to be still coming from somewhere within the whole climate control system...I just need to isolate it w/out throwing unnecessary parts into it.


Thanks again for all the input guys...I really appreciate it. I'll let you know how things turn out after I look at it again this weekend, but I'm happy to hear any more suggestions before then!!

JimsGX

Jonny2K,

There's a cabin temp sensor, I believe its down by the drivers right knee behind the lower panel (has slotted grooves in the panel where its located). You should check its operation.

If I understand correctly how it works, when you turn the climate control up to the max of 85 degrees, the climate control ignores what this cabin temp sensor is telling it and opens the heat valve all the way to provide max heating. The sensor may be bad or the wires may have gotten disconnected from it if someone took the panel off.

How the overall system and this sensor would react to increased speed / RPM is unclear to me however when it comes to the heater valve opening.

Just a guess...

deanshark

Hey Jim, I have some kind of sensor hanging under my dash in the area where you mentioned. I never traced where it goes but it was usually just stuck to the bottom of the dash. Part of it is still stuck on. Is this the sensor you're talking about? It's not behind any panel though. There is just too many sensors for this heating system.

JimsGX

Sounds like the sensor I'm talking about. It's the "Room Temperature Sensor". Do you see where the slotted vents are in the drivers side kick panel? That's where it is. The sensor location is in the center diagram of this pic... The next is how to test its resistance.

jonny2k

Wow...never would've thought of that!! Great stuff..thank you Jim!!

jonny2k

So I thought I'd tie up this thread by letting everyone know how it was resolved.

First, I tried and tried but couldn't find a used heater valve or servo motor available to try to fix the problem. I didn't want to buy new ones because they seemed to be operational and therefore would appear to just be throwing money at the problem w/o knowing if it was the real culprit. I rechecked the coolant level and though it was a little cloudy, it was full. Absolutely baffling.

I had kept my eye out for people parting 1st gen Ls400's both here and locally, constantly on the lookout for the heater valve/servo motor combo. Then about 2 weeks ago my father called me because while driving he'd heard a loud "POP" and then steam drifting from the engine bay. He'd pulled over and was waiting for AAA to tow the car to our mechanic friend. It was here that we'd identified the problem...one that I didn't think of.

The radiator had blown a hole and when it was removed, it weighed 3x more than a stock radiator! It was absolutely clogged with gunk and wasn't allowing coolant to flow freely at all. Amazing how a heater core requires coolant to create heat in the cabin, huh?? When the new radiator was installed with fresh coolant the heater "magically" began working at all temperatures again!!

Just to let everyone know, there was never any overheating of the engine during this period when the car wasn't generating heat from the climate control. The temperature gauge was always reading right in the middle so it never occurred to me that the radiator was the problem!

Anyway, I wanted to let my fellow CLers know how this story turned out...primarily because if anyone uses the Search function in the future perhaps this will aid them in fixing their problem.