Increased revs when A/C kicks in

[rich1068]

Now I'm LED to believe this is a "feature" of the LS400. That as the A/C kicks in the revs rise considerably, most noticeable when stationary or at low speed when the car almost drives itself. 

My LS has spent much of its life outside the official dealer network, was there ever a fix? Specifically a software fix that may be available at a main dealer? Owners must have complained at the time.

It's not a major problem but it is an annoyance. I've taken to switching the A/C off when I'm driving around town but I'm not entirely happy with that solution.

[ajchetcuti]

Now I'm led to believe this is a "feature" of the LS400. That as the A/C kicks in the revs rise considerably, most noticeable when stationary or at low speed when the car almost drives itself. 

My LS has spent much of its life outside the official dealer network, was there ever a fix? Specifically a software fix that may be available at a main dealer? Owners must have complained at the time.

It's not a major problem but it is an annoyance. I've taken to switching the A/C off when I'm driving around town but I'm not entirely happy with that solution.

The worst part is when you are braking in traffic and the AC kicks off, the brake force increases as the revs drop and the car abruptly stops [emoji1787]. I've raised this before and I think unless something new comes up as an idea, that's just how it is...

 

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[Malc]

I'm sure technology has moved on a little since 1997 .........  but I have read of your issue and no one has ever suggested a " fix "

just something to live with methinks

Malc

[Linas.P]

Not sure how much is "considerable" increase in revs, but for all cars I ever had revs would increase by 100, maybe 200 revs when AC is running in stationary car. I just assumed it is normal. Although, it seems that 4l V8 should not feel much of challenge from running AC compressor even when idling.

[cruisermark]

I noticed this issue more so on my mark 3 than mark 4

But I used to leave my AC running all year round so I never noticed any of this on/off.

What was more noticeable was the thud when sifting from P to D on cold engine and then there was the raw engine sound first thing in the morning for a couple or miles or so.

The LS430 is more refined in this respect so non of these exist

[Herbie]

There can't be a 'fix' for something that's not faulty.

AC coming on means more load on the engine, ergo a rise in rpm.

No one can defeat the laws of physics.

[rich1068]

13 minutes ago, Linas.P said:

Not sure how much is "considerable" increase in revs, but for all cars I ever had revs would increase by 100, maybe 200 revs when AC is running in stationary car. I just assumed it is normal. Although, it seems that 4l V8 should not feel much of challenge from running AC compressor even when idling.

I can't remember exactly what he said but Dave at Lex-Tek said something about the issue being ECU/software related and was always a thing even back in his day at Lexus Sheffield. You're correct that A/C often has an impact on engine revs but I've never had it to this extent. For example, it can mean the difference between the auto box holding you on a hill or not!

3 minutes ago, cruisermark said:

I noticed this issue more so on my mark 3 than mark 4

But I used to leave my AC running all year round so I never noticed any of this on/off.

I've left the A/C running on every car I've had since 2003. I think this is the first time I've turned it off! Even with the climate on the pump doesn't run continuously, it's just when it kicks in and out.

[rich1068]

3 minutes ago, Herbie said:

There can't be a 'fix' for something that's not faulty.

AC coming on means more load on the engine, ergo a rise in rpm.

No one can defeat the laws of physics.

But I've never had a car that does it to this extent. You can feel the car trying to pull away when you're waiting at the lights! Turn the A/C off and it stops.

[Herbie]

4 minutes ago, rich1068 said:

But I've never had a car that does it to this extent. You can feel the car trying to pull away when you're waiting at the lights! Turn the A/C off and it stops.

Has it done this all the time you've owned the car or just started?

Could be a sign of the compressor on its way out if it's a new symptom.

[rich1068]

Since I've had it. But that's not long. As I say, it seems to be a "feature". Some of the LS owners above seem to corroborate. When I originally mentioned it to Dave he almost finished my sentence for me.

[Razor61]

I disagree that it’s a feature and nothing can be done about it. I think the book says idle should be 600 rpm with aircon off and 650 rpm with it on or something around that. Certainly not high like 800 - 1000 or even 1100 as mine was.
Have a read of the posts below but here is a brief summary. I had the cutting out issue and erratic and high idling. My aircon compressor then went kaput, it was the clutch assy on it, but while the aircon was kaput I replaced the ecu coolant temp sensor and the idle was spot on after at 600rpm and didn’t cut out anymore at all. But when I fixed the aircon and used it all the time, the idle went wonky again. High idle, erratic speed which was annoying when sat at traffic lights etc etc and engine running on when driving at low speeds when it shouldn’t. I think if I turned the AC off the idle went ok, it’s a while ago so can’t remember exactly. Anyhow, I replaced the ecu for an updated ECU and everything is as it should be. I checked with Lexus UK that the updated ECU I bought was correct for the car first. Apparently in the USA there was a recall to change the ECU for the cutting out issue.

 

 

[Razor61]

Forgot to add the topic about the ecu temp sensor.

 

[rich1068]

Cheers Paul. Further investigation and reading required I think.

I'll post up a video in the morning of what is happening. Though I should add that I wouldn't describe the idle as erratic and it certainly doesn't cut out. I found plenty of references to both while searching but not much that described my situation.

[Razor61]

👍 your issue is the same as mine after I fixed the aircon (nowhere near as erratic as when I bought the car) and was sorted by fitting the updated ECU. Thinking back it was constantly high more than erratic with the ac on, turn ac off and the idle settled down to normal. Also my definition of ‘erratic’ may be ‘pretty steady’ to someone else.........

Regarding the cutting out, I didn’t have any issues with that for quite a while after I bought the car and thought mine wasn’t affected...................then it happened. Not saying you are going to get the issue though.

[ColinBarber]

5 hours ago, Herbie said:

AC coming on means more load on the engine, ergo a rise in rpm.

No one can defeat the laws of physics.

Laws of physics would state the opposite, if you put more load on the engine it will slow it down - which is what you see when you put an auto into Drive with your foot on the brake because of the load the torque converter puts on the engine.

When the AC comes on the engine ECU opens the throttle, either directly or via an idle-up valve, to not only compensate for the load which would otherwise reduce the rpm, but actually increases it so that the compressor operates at a speed sufficient to give good cooling.

Early LS400 had a standard of idle speed of 650 +/- 50 and  750 +/- 50 with the AC on (and clutch engaged).

The MKIV standard idle is 750 +/- 50 and 800 +/- 50 with the AC on (and clutch engaged)

[Razor61]

11 hours ago, ColinBarber said:

Early LS400 had a standard of idle speed of 650 +/- 50 and  750 +/- 50 with the AC on (and clutch engaged).

My car idles at 600 rpm with AC off and around 700 rpm with the AC on and fluctuates slightly around that depending if the clutch is engaged or not. So 100 rpm (ish) difference which fits in with the 'standard'. I thought I'd seen an idling standard table in the workshop manual for 'AC off' and 'AC on' but I've looked again and can't find what I thought I'd seen. But I did find the stuff below. It seems with the AC 'on' 800 rpm in neutral should be the highest. If it's significantly above 800 rpm then it's not in 'standard' and you will get issues like I had and Rich has, would anyone agree with that?

My car did peak at 1100 - 1200 rpm periodically but not all the time after I fixed the AC. All fine and dandy now though.

Idle 650 +/- 50 with AC off, as mentioned above.

Then this in the aircon section, 900 rpm idle seems high to me (does say approx. 900) but I'm not a mechanic, just a humble 'diy mechanic' jocky😁

 

[Herbie]

12 hours ago, ColinBarber said:

Laws of physics would state the opposite, if you put more load on the engine it will slow it down - which is what you see when you put an auto into Drive with your foot on the brake because of the load the torque converter puts on the engine.

When the AC comes on the engine ECU opens the throttle, either directly or via an idle-up valve, to not only compensate for the load which would otherwise reduce the rpm, but actually increases it so that the compressor operates at a speed sufficient to give good cooling.

Yes, sorry, my fault for not being thorough.

I was assuming that I didn't need to state that extra load will slow the engine so the ECU raises the revs to compensate, so I skipped a step

[rich1068]

Right then...

Firstly, thanks to Paul for the info and PM'd research re: replacement later ECUs.

Secondly, I'm not convinced I have an issue with the temp sensor but rather how the ECU manages the running of the A/C compressor. As Paul points out a later ECU may solve this.

I've been observing the car's behaviour today while driving around town. Stationary idle isn't too much of a problem. The pull experienced due to the increase in revs is possibly more noticeable due to it being a 4.0litre V8! What is more concerning is the pull felt at slowish speeds in traffic. 20-odd mph on a slight downhill or level road and the car is quite capable of taking you over the 30 limit when the A/C kicks in. Turn it off and this behaviour stops.

[rich1068]

While I've got you all, not entirely related but I intended to reset the ECU as I had once or twice on the GS.

Instructions in the tutorials state

1. Make sure the ignition is turned off

1. Remove the engine bay fuse box cover

1. Remove the 20Amp/25Amp EFI fuse

1. Remove the 15Amp ETCS fuse

1. Wait 30 seconds

1. Re-insert the two fuses

1. Install the fuse box cover

I have a 20amp EFI 1, 10amp EFI 2 but I don't have a 15amp fuse marked ETCS. Could anyone confirm both on my '96/'97 LS?

[Razor61]

38 minutes ago, rich1068 said:

What is more concerning is the pull felt at slowish speeds in traffic. 20-odd mph on a slight downhill or level road and the car is quite capable of taking you over the 30 limit when the A/C kicks in. Turn it off and this behaviour stops.

That is exactly what I experienced but you describe it better I think, the car shouldn't be driving forward or speeding up by itself without you pressing the accelerator.

I didn't think I had an issue with the temp sensor, it was a case of 'I'll give it a go and see what happens' based on the info I found. The biggest thing that convinced me to replace the temp sensor was the comment below. In his case the sensor was knackered but didn't throw up any faults, in other cases the sensor was working but not giving the right signal to the ECU to a greater or lesser degree. I'm not saying changing the sensor is a magic cure by any means.

"After studying ECU theory, I found out the Coolant Tempurature Sensor is the "MAIN SENSOR" for the computer. This sensor has more control over the computer than any other sensor. To make a long story short, my COOLANT SENSOR was telling the computer it was -20 degrees farenheit(thats minus 20 degrees) and no check engine light. Cost of new sensor was 50 dollars. I now get 23 miles to the gallon higway with a light foot. I now save 650 dollars a year in gas driving 12000 miles a year and I can feel the 250 horsepower when I open it up. If you have an old LS400...CHANGE THIS SENSOR!!"

[Razor61]

45 minutes ago, rich1068 said:

I have a 20amp EFI 1, 10amp EFI 2 but I don't have a 15amp fuse marked ETCS. Could anyone confirm both on my '96/'97 LS?

I did the same thing and iirc couldn't find the ETCS fuse either. 20amp EFI 1, 10amp EFI 2 sounds about right when I did it.