babadioum

Intro: engine overheated, blew gaskets, ended up rebuilding the whole engine while at it. All sensors AND temp sending unit are new. Now done.
Problem: RI reads indicate proper temps for the fans to kick in so the system works fine: 1st at 225 and 2nd at 240, BUT the temp gauge indicates overheating (when there is none) even at actual 225: the needle is already in the red zone when the 1st kicks in so my gauge is overreading by about 40 F.
Assumption: When the car overheated, it must have damaged the temp gauge circuitry and its accuracy; the problem must be the faulty gauge unit and/or circuitry since the system is working exactly as it should per RI temp readings of the upper radiator hose and the block behind the temp sending unit.
Question: what do I do other than replacing it with an external gauge?

OrangeBird

iTrader: (1)

Since you overheated the engine , and now after repairs the gauge reads high , you very likely cooked the gauge's sending unit VS the gauge itself . Think about it , the gauge itself wasn't subject to excessive temperature , but the gauge's sending unit WAS heated way beyond it's normal operating temperature . A new sending unit will fix the gauge back to it's original functioning (which wasn't a precision measuring instrument to begin with) .

PS , the gauge does indeed have it's own sending unit , VS the one for the ECM and the one for the radiator fan , so what's happening with the ECM and fans has no bearing on what the gauge will be doing .......

babadioum

Thanks for the reply, but ALL sensors and sending units, including CTS and the temp sending unit (single green cable to dash) are new. There is always a possibility that I got a lemon, but then again

OrangeBird

iTrader: (1)

Sorry I missed that you replaced the sender , just curious , did you try the old sender ? Yes indeed new parts can be factory defective so here's two ways to check things out ;

The service literature states that the sender is 1365 Ohms at 100 degrees F , and it's 55 Ohms at 260 F . Run the engine till it gets to normal operating temperature and use an Ohm meter to measure the sensor's resistance to ground . You'd be looking for something above 55 Ohms (which is 260 degrees F) it'll be somewhere around 60 to 80 Ohms at proper operating temperature . Or , you could buy a couple of resistors at your local electronic hobbyist store , get one at 1356 Ohms and one at 55 Ohms (Or as close as possible , you won't find those exact values but you should be able to come close enough , you'll likely find a 50 Ohm and a 1500 Ohm as standard ratings . These are very cheap , less than a Dollar each) and connect the 1365 between the green wire and ground . With the high value resistor in place the gauge should be at the bottom of the cold area and with the low value it should be at the top of the hot area .

With the way the "gauge check" function pegs the gauge at each startup and the gauge survives it , I'd be highly surprised if one instance of the engine overheating would damage the gauge .....

GeneralDisorder

iTrader: (1)

OrangeBird has you covered on the specs. I just came to say that "new" parts for these cars are often inferior to the originals and it absolutely could be a bad "new" (Chinese) sending unit.

But don't load the parts cannon till you do your testing.

GD

babadioum

No, it was fried Thanks for the advice, I'll try. I agree that a single instance normally should not be enough for the damage, but then again, old instruments

babadioum

I agree with the parts comment Regarding the ohms check, high resistance won't tell me much since it works fine and accurate when cold, the low res will put it smack to the end of red line so it won't tell me much either, what I'd need is a res value for 220 against which I should check the reading. Anyhow, perhaps best if I just go on a spending spree and bury 6-7 bucks on another one )) The one I have is an AC Delco worth 25 bucks so if it turns out faulty, I'll try to return it.