My Led Water Temp Gauge Drops Down to 75 Then Back Up Again

1. 

   Canyon Jeff [OP] Well-Known Member

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   Jeff

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   98 Tacoma V6 3.4, TRD 4x4 5-speed 100k miles

   ...Well, at least I think its random...

   So this has happened to me a few times in the last month while driving on the highway. The temp gauge is typically right in the middle when my rig is all warmed up, but suddenly it will just jump up to the red line real quick and then fall back down to the middle, and then a few seconds later jump about halfway up and then fall right back down. It will go on like this for a few minutes, just jumping to random spots in between the middle and the red, and then it will eventually quit and it won't happen again for days.

   It always jumps UP. it never drops below the mid line (which i read elsewhere might indicate the sensor is losing power). The speed at which it jumps is variable: some time super quick, other times a bit slower. But it doesn't move as if the engine is actually overheating: you know, slow and steady. HOWEVER, today, for the first time, it did do a little bit of creeping as it climbed, but then it would just fall back down again. The problem doesn't SEEM to be related to speed or RPM's, except today, the times that it crept was during times I was accelerating. But then some times it will spike when I let my foot off the pedal. I'm trying to find a pattern but there doesn't appear to be one... So I am really confused...

   I searched this forum and other forums and have read about problems people have had with their temp gauges, but nothing quite fits this description.

   Thoughts? Thanks!

2. 

   Canyon Jeff [OP] Well-Known Member

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   SO, I've had some more time to think about this problem, and I guess I'll add some of those thoughts here.

   I don't know a whole lot about electrical wiring... really, nothing... BUT I'm thinking the problem here is the ECT sensor. But I am not sure if the problem is the sensor itself, or the wiring going to the sensor...

   Here is what I do know. My truck has a ton of other minor PITA problems that I have been slowly resolving. One problem that I resolved recently is the well-know vomiting front differential breather hose. Before I got around to fixing that, a whole lot of shit had gunked up the electrical wires that run along side the breather hose. I don't know what those wires are or what they do, BUT I did notice that this problem began almost immediately after a particularly large gear oil vomiting event.

   Another thing I noticed a LONG time ago was that the jiggling those wires sometimes caused the temp gauge to rocket all the way up to the red line when the engine was turned OFF completely, no key in the ignition or anything... jiggling those wires some more would cause the gauge to drop back down. That was DEFINITELY an electrical issue, and I am wondering if these two problems are somehow related... but how...?

   I don't even know where the ECT sensor is on my truck, and its getting late here so I am gonna try to find that tomorrow morning. Anyone wants to chime in in the meantime, be my guest!

   thanks guys!

3. 

   DrZ Well-Known Member

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   There is a coolant temperature sensor for the ECU/EFI system and a coolant temperature sensor for the gauge. Two different sensors.

   I believe the temperature gauge in the cluster has two solenoids in it. When the ignition is on, one solenoid has constant power and pulls the needle down. The other solenoid receives current through the resistance of the temperature sender gauge and pulls the needle up.

   So, if the first solenoid loses its connection to ground, then the needle will go up. If the sender gauge (or its wiring) shorts out, then the gauge needle will also go up.

   So, check the wiring to the temperature sender. I believe this sensor has only one wire attached to it (as opposed to the ECU temp sensor which has 2). Another possibility are the nuts that hold the gauge onto the back of the cluster are loose or have some slight corrosion, and since the nuts provide the electrical connection, this could cause the problem.

4. 

   Canyon Jeff [OP] Well-Known Member

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   Jeff

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   98 Tacoma V6 3.4, TRD 4x4 5-speed 100k miles

   Thanks for the reply, DrZ.

   I looked at a post from yotatech and found where the temperature sender for the gauge is located. It looks hard to get to, all sorts of stuff in the way...

   http://www.yotatech.com/f160/overheating-216563/index2.html#post51528530

   As for the cluster, will I have to take the whole dash apart inside? never taken that journey before...

   Any advice on exactly what to check for on the sender itself? Do you have a wiring diagram? I don't. But I am assuming there are several different places to check the wiring for issues.

   Thanks again!

5. If I were you, I would find an OBD2 scanner that you can monitor live engine data with (like a Scangauge, or some other hand held unit). Check the scanner's coolant temp readings vs what your temp gauge on the dash is showing to verify that the dash gauge really is giving wonky readings.

6. 

   o0oSHADOWo0o Just lurking in the darkness

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   Shadow

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   Just a few LEDs...

   Have you tried replacing the thermostat? If it is original or old it could be sticking.

7. 

   2toys Well-Known Member

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   I'm experiencing the same problem after changing the head gasket. I can see temp on a scan gauge is normal, but temp gauge spikes randomly. I too am thinking it's an electrical issue, maybe ground since it's a one wire sensor. I just can't put my finger on it.

8. 

   mwrohde Well-Known Member

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   I nearly overheated an engine when it was cold once. I'm guessing one of the radiator hoses froze, preventing coolant flow. That could explain why "only when it's freezing".

   Random spikes - I'd replace the thermostat, too.

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   2toys Well-Known Member

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   What are the three grounding points. There is one from firewall to head. One near battery to fender. Is the other on the intake manifold under the service plug?

11. 

    TacomaJunkie8691 1999WineTacoma

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    FrstGenTacomaLover

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    My old Tacoma does the same thing, but only does it when it is completely warmed up, and you go to start it again. The temperature gauge will suddenly move toward the red, and then drop a few seconds later. It does this randomly, and there is not any consistent pattern to it doing it.

12. 

    Snowy Is neither here nor there

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    Connor

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    Stock-ish

    This isn't a swing of actual temperature, its a quick gauge sweep up and down randomly. My truck does it too even though the temp stays rock solid at 190 on a data reader. Definitely a ground issue somewhere that I don't feel like running down.

13. 

    TacomaJunkie8691 1999WineTacoma

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    FrstGenTacomaLover

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    Yeah Snowy,

    I completely agree with you. It is not worth my time to run it down. If it consistently did it, I might convince myself to run it down, but not some sporadic issue.

    Take it easy,
    Paul

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    AugustusJack Well-Known Member

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Source: https://www.tacomaworld.com/threads/temp-gauge-spikes-randomly.359872/

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