Toyota Corolla temp gauge problem

NVC4785

MEMBER
TOYOTA COROLLA

Please help me isolate whats wrong w/ my toyota corolla's temp gauge. The temp gauge quickly rises way above the full scale in a span of 5-10 seconds after turning on the ignition.I already replaced the temperature sender but still has the same response, what else could be wrong?

Thank you very much

6 Replies

MMPRINCE3000

MECHANIC
133 POSTS

The gauge itself could be bad or you may have a coolant leak somewhere.
Pressure test the cooling system to eliminate that as a cause. Hook up a scanner to see what the ECM is reading as the coolant temp.

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Thursday, March 1st, 2007 AT 6:02 AM

NVC4785

MEMBER
2 POSTS

Tnx 4 the advise sir. I'm sure there's no leak in the cooling system. Cooling fans and thermostat valves also are functioning well.I'm thingking of abnormally high voltage being read by the gauge.I took some reading and the voltage rises from about 9v-12v in just 5-10secs. I also checked the gauge, I found a couple of 1w 27ohm resistors and a 3v zener diode sticking on the gauges' body, a simple ohmeter reading confirms the resistors are good, but the zener diode shows some abnormal (almost short)readings. Could this be the faulty component? But i'm thingking, if the zener is faulty, temperature should rise instantly and will have no delay. But anyway, i'll try and replace that. I also found out that the gauge has 3 connectors, one for the B+ supply, the other from the the temperature sender, but do you have an idea where the other wire is connected? (Body ground maybe? I'll appreciate it if you can give me any diagram on how the temp gauge is connected to the cars's electrical) and what is an ECM? Thank you very much sir for the help you provided.

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Friday, March 2nd, 2007 AT 5:10 PM

MOONWITT

MEMBER
3 POSTS

I know this thread is pretty old, but it is the only one that matches my problem I am having with my 1998 Corolla. I think you are on to something with the zener diode on the gauge. You said it was a 3V zener diode. I have located it and would like to replace it and see what that does to the reading. Do you have an exact part number for the diode? I can only find 3.3V zeners online. Thanks

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Tuesday, September 7th, 2021 AT 10:56 AM

KASEKENNY

MECHANIC
18,907 POSTS

Thanks for updating this post. I am not sure we can provide a part number because this is not an "approved" repair as Toyota would just tell you to replace the entire part. They would not allow people to go in and replace these as they figure more would get damaged.

However, if you have the capability of doing it then you should be able to use most any diode.

I just did a Google search for 3 volt zener diode and this came up.

https://www.amazon.com/3-volt-zener-diode/s?k=3+volt+zener+diode

Clearly you can source them from other places but they are out there.

I am posting a couple guides that I doubt you need but others may visit this page just like you did that may need a little more info on testing these types of circuits.

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-to-check-wiring

Please let us know how you make out with this and let us know if you have other questions. Plus you never, know that this person may respond. I would think not as it is almost 15 years old but you never know.

Thanks

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Tuesday, September 7th, 2021 AT 7:53 PM

MOONWITT

MEMBER
3 POSTS

KASEKENNY1. Thanks for the info! I know the engine is not overheating as I can read the actual temperature with an OBDII reader. I have replaced the sending unit, and tested it in hot water to ensure the proper resistance readings. It's a fairly simple circuit so I believe the gauge is bad. I know how to solder, so I'll give this fix a try. I'll let you know how it turns out.
Thanks again.