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Old 07-26-16, 04:48 PM   #1
98LSFan
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Default Possible Bad Gauge??

So I have a 1998 LS400 with 170,000 miles on it. It has been babied and garaged. It is in fantastic shape. One morning this week I started the car, drove about a mile and noticed that the temperature gauge was just under the red line. It normally hangs out just under the halfway mark. I tuned around and drove home as not wanting to run it too hot. Here's what I checked on to determine the issue. Water pump, timing belt, thermostat, all radiator hoses and new coolant was just done 12,000 miles ago. This check out normal. Then I thought that it might be the sensor/sending unit indicating the wrong temperature to display on the gauge. I was going to just buy a new one and change it out, but then I thought of hooking up my OBDII scan tool to check the engine temperature. Sure enough the engine temperature read normal through the real-time scan tool. Here's my logic. If the scan tool shows the correct temperature as read from the ecm computer, it must be getting the temperature sent to it from the sensor correctly which means that there isn't a reason to change out the temperature sensor as a possible fix. I believe that I can rule it out. I think that only leaves me the instrument gauge itself that could be the culprit in this issue. Also, I think that this is the part I need. CLICK-HERE

If this is the issue, would it be better to just buy the gauge and change it out or send the instrument cluster in somewhere to be rebuilt? Also, occasionally the tach sticks at zero so a good slap on the top of the dashboard above the gauge fixes it. Maybe a rebuild of the instrument cluster would fix this too.

Please help me figure out this dilemma. I try to keep everything on the car working at 100% so even if the car isn't running hot, I want the gauge to report the correct temperature.
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Old 07-26-16, 04:51 PM   #2
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ECT and gauge sensor 2 different things, could be a bad gauge sensor
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Old 07-26-16, 05:39 PM   #3
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Thank you for the quick reply. So the ECT and the gauge sensor are two different things? I know of the temperature sensor located at the bottom of the radiator on the drivers side. Two wires. I unplugged the clip and the temperature read the same but the cooling fan went into high mode. That was just a test to see if the gauge changed. Can you please expand on the ECT? (not familiar with that acronym, sorry). So did I find the gauge sensor or the ECT plugged into the bottom of the radiator and what is the other on you mentioned? Thanks.

This is the sensor located in the bottom of the radiator on the drivers side.


Last edited by 98LSFan; 07-26-16 at 05:54 PM. Reason: Add Photo
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Old 07-26-16, 06:19 PM   #4
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ECT is engine coolant temp sensor, it tell ecu the engine temp for fuel and spark control. On older ones anyway there is a separate smaller sensor for the gauge, and is most likely your problem. I looked it up on Oreillys parts and it was listed for $25. or so, so 98's must have the same deal. The bottom of radiator units are for the electric fans as far as I know.
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Old 07-26-16, 06:27 PM   #5
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So if my real-time OBDII scan tool shows correct operating temperatures, you think its the smaller sensor that reports directly to the gauge rather than a faulty gauge? Any idea where it might be located? I'm thinking that it is located on the engine side of the thermostat. At least I think it should be. I could not find another sensor than the radiator one. Most threads online show the older engine with great pics etc. For reference I have the 1UZ-FE engine. Also, when I turn the key to the on position in the morning the gauge reads high, but the scan tool shows ambient air temperature as the engine temperature so I know it's not overheating.
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Old 07-26-16, 06:41 PM   #6
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The scan tool reads from the ECT, not from the gauge sensor..
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Old 07-26-16, 06:44 PM   #7
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Ok thanks. So I think I'm at leat not going to overheat the engine. Do you know where the gauge sensor might be located? I appreciate your help!!
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Old 07-26-16, 07:58 PM   #8
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If its like the old ones it will be near the ECT up front on the water manifold near the thermostat. See my pics on Help full pictures?
http://www.clublexus.com/forums/ls-1...-pictures.html
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Old 07-27-16, 12:00 AM   #9
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I know dicer is kindly trying to help you, but you are partially mislead by him. It seems to me that he doesn't know well enough about Gen 2 and 2.5 models.

A 98 doesn't have an independent temperature sensor for the gauge. The ECT signal (Only the DC level) is sent to the ECU and processed by the AD converter there. The converted digital signal is used both for the engine management and the temperature gauge. The engine management uses the raw digital signal. But the temperature gauge can't accept it. Because the gauge needs the signal through the bus line. So the raw digital signal is converted to the bus signal and sent to the gauge. The gauge has a microprocessor and it converts the bus signal to be used by the gauge.

Since your engine is not overheated and the OBD-II reading is good, your problem is caused by the area where the raw digital signal is converted or after that. You need to analyse the bus signal to distinguish the problem. If you are not OK to analyse it, swapping the gauge is one choice to distinguish.
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Old 07-27-16, 03:33 AM   #10
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An air bubble will also cause periodic issues with the temperature rising dramatically only to be resolved by revving the engine hard. This is due to water pump cavitation if the bubble finds its way into the pump, or air lock if the pump isn't spinning fast enough to move the bubble around.

Bleeding the system with distilled water to top it off is quick and easy (especially if you have a "no-spill funnel" aka bleed funnel.).
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Old 07-27-16, 11:59 AM   #11
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Quote:
Originally Posted by Yamae View Post
Since your engine is not overheated and the OBD-II reading is good, your problem is caused by the area where the raw digital signal is converted or after that. You need to analyse the bus signal to distinguish the problem. If you are not OK to analyse it, swapping the gauge is one choice to distinguish.

Yamae,

Thanks. How difficult would it be for me to analyze the bus signal. Might there be a video online with instructions, or is it something that I wouldn't get if I didn't already have some knowledge of that?

Also, Did I list the correct gauge in my previous post for the part I would need to replace? It's this one http://www.lexuspartsnow.com/parts/l...83-50010.html?

How difficult is it to remove the gauge cluster and swap this out?

By the way what is the correct temperature supposed to be for the car. According to the OBDII real-time scan tool yesterday it was 105 degrees out and I had the A/C on and it was holding between 198.5 and 204.0 degrees. I'm thinking the temp should be 200???

Thanks
98LSFan

Last edited by 98LSFan; 07-27-16 at 12:04 PM. Reason: Ask additional question
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Old 07-27-16, 12:01 PM   #12
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PureDrifter, Thanks. I'm thinking it's not an air bubble, because as I said it already reads high first thing in the morning before I even start it up. The computer on the real-time OBDII scanner shows ambient air temperature for the engine temp and it goes up correctly once the car is started.
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Old 07-27-16, 07:01 PM   #13
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It's not easy to analyse the bus signal for Sunday mechanics. It requires an equipment to observe the bus signal. It also requires some knowledge and the skill to perform. That's why I suggest you to do a swapping for the distinguishment.

The part you showed by the link is only the needle part and it's not enough to distinguish. Because the microprocessor board is not included in that.

The water temperature should be around 185-190 degrees F at a normal running. It goes up some more at very long up-hills but 200 degrees F is a bit higher than normal.

Regarding the swapping, there are some threads about it in the Forum. This may be useful.
http://www.clublexus.com/forums/ls-1...999-ls400.html
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