Hi guys, hope you can help troubleshoot my temp gauge. It was staying pinned key on or off. I took the gauge out hoping the needle was stuck but it wasn't. I'm reading 5K ohms between the sensor and ground. Is this the right value for a 1988 LB9? Maybe someone put the wrong one in? The upper blade socket in the dash is ground, the right one is 12 volts and the left one is the 5K to ground as at the sensor. I removed the resistor from the gauge and it reads around 80 ohms the gauge itself reads around 50 at those 2 pins. The gauge is kind of lifeless when connected properly if you move it manually. If you turn the key on it just kind of hangs in the middle. I tried connecting ground to the bottom copper colored terminal on the lower side of gauge and it seems to come to life as it goes to 0 when the key is on. The pictures show the upper terminal bolt is silver and the other 3 are copper, I was wondering if this is significant. Is this the wrong gauge? If not why are there 4 terminals and only 3 blade sockets in the dash? Any ideas? Thanks, Jack.

 

I'm assuming you did all the basic checks first, checking the sensor unit connecting wire, the sensor in the head, connections, etc., right?

Only time I had a similar issue turned out to be the sensor, itself. In curing that problem, I learned that the sensor and the gauge have to be matched to each other, so if the sensor is the incorrect one, the problem persisted. Auto parts stores often sell the wrong sensor as evidenced by me and by other posts I've read on other Forums.

On my previous L98, 86 Vette, if the Coolant Temperature System CIRCUIT fails, it should set a Code; either 14 or 15 but this is for the Circuit, not just the sensor.

Are you seeing a Code?

I hope someone with an engine like yours will chime in to help.

Jake

West Point ROCKS! Natin's TOP COLLEGE per Forbes Magazine!! Graduation Day Parade 20 May 2010!!!

 

OK, you guys are not paying attention. Being determined I kept troubleshooting this gauge issue, post Xmas decompression I suppose. If you notice the orientaion of the resistor on the back of the gauge is horizontal. Some Einstein put it back together incorrectly when they were troubleshooting I suppose. I put the resistor back to vertical between the silver top terminal and the bottom copper colored one. Now the gauge pins when the sensor lead is grounded. I have a spare sensor from my TA and put it temporarily in place of the one in the engine using jumper leads. Key on it starts low, 50 degees or 7 o'clock position. I heated the sensor with a heat gun and sure enough is slowly crept up as I heated the sensor and came down whle cooling it.
Now the question is a matter of resistance. The sensor from the TA shows 400 - 500 ohms at room temp. The one in the car shows 4500 ohms, seems a little high. Anyone know what a good sensor should read? Thanks

 

Are you sure you have your meter set on the correct scale?

Jake

West Point ROCKS! Nation's TOP COLLEGE per Forbes Magazine!! Graduation Day Parade 20 May 2010!!!

 

Yes. I also verified the sensor is grounded to block, so that reading is from the pin on the sensor to the body of the sensor as well as the block ground. Do you know what the sensor should read when it's disconnected from green wire.

 

Problem is I have a 96 Vette so my system is different. I have both FSMs for my car but info in them probably won't apply to your system.

That's why I am hoping someone with a gauge like yours would chime in. I'm still with you though.

Jake

West Point West ROCKS! Nation's TOP COLLEGE per Forbes Magazine!! Graduation Day Parade 20 May 2010!!!

 

I see that the range on the gauge pictured is 100 to 260°F like mine.
The temperature sender's resistance in my driver's side head is approximately 1365 ohms at 100°F and 55 ohms at 260°F.
FYI
The Temp Gauge Sender (Head) and Cool Temperature Sensor (Intake) are independent circuits of which only the latter is monitored by the ECM and are not interchangeable.

 

FYI, that fixed it. I had it outside yesterday and ran it a while to get the Stablized gas through the system and checked the gauge operation. after all that poking around and resetting the needle position and installing th resistor correctly it looks like it's really close. According to the gauge the fans come on around 210 degrees and go off around 180. Close enough for me.

 

That's a 30° difference, makes it difficult for the engine to maintain it's operating temperature.

 

I don't know what it's supposed to be, just pointing out that the gauge seems to respond correctly within resonable expected temps. I haven't driven the car yet, only high idled it in the driveway, but I'm assuming the ecm is working properly and regulating the temps as programmed. Both fans do come on and off at different times and the engine felt reasonably cool, no hint of overheating. I'll eventually get an infrared temp sensor to see what the head temps are and compare it to the dash gauge. Only 3 months to spring...

 

What should your engine operating temp be? I don't know. But, I would look into it.
In April I did a 9 minute data log of a road test from start up to turning off the logger.
the last 1.5 minutes of the data were on the freeway at an average of 70mph using a 170° T-Stat in a high compression TPI 6.4/Liter 350 SBC with aluminum vortec (fast Burn heads) top end. After analyzing the data I decide to change to a 180°F T-Stat (haven't done that yet.)

FYI - My temp gauge agrees with the head temp sensor, but varies +/-10° from the CTS.
The month before I do emissions testing i change to a 195° and change the fan set points to 220°/225°.

The average temp for the run was 184° starting with 186° warmed up ending with 180°, the fan set points are:
On @ 194° no A/C
Off @ 188° no A/C

Max/Min CTS temps were 198°/176°.
The average CTS temp was 184° and the standard deviation was about 14°.
Max/Min MAT temps were 127°/108°. The average MAT temp was 120° and the standard deviation was about 11°.
The fan cycled 5 times before I reached 70mph and 10 seconds after that the fan turn off and stayed off for the last 1.2 minutes of the log.