1999 C5 gauge showing incorrect temp

[Vette Dreemz]

Could really use some help with this problem. The problem started suddenly with the temp rising to 255 at a steady rate from cold within 2 miles. Took it to my mechanic and he cannot find the problem. Put his pyrometer to various points and got different readings at around 30 degrees cooler. The coolant has been power flushed, the thermostat is opening and the water flowing, changed the sensor etc. He is currently testing resistance values, but basically we are stuck. The computer is being fooled by something, perhaps a wiring problem. Could it perhaps be the computer itself? Really strange. Any help would be greatly appreciated. Thanks guys and gals.:confused;help

 

[dadaroo]

OK, first pull your DIC codes and post them.

 

[Vette Dreemz]

OK, first pull your DIC codes and post them.

OK, the codes are as follows:
10PCM no codes
28TCS 1 code (C1255H)
40BCM 1 code (B0507H)
58SDM 1 code (U1064H)
60IPC no codes
80RADIO no codes
99HVAC 1 code (U1160H)
AO-LDCM no codes
A1-RDCM no codes
A6-SCM 1 code (B2860 H C )
B0-RFA 3 codes (U1096H / U1064H / U1016H )

My mechanic feels that this needs to go to Chevrolet for a resolution. Again, have a funny feeling it is the computer itself. After the last go round, I got the car back and now the remote does not work at all and it is brand new from Chevy dealer. Strange. This is on top of all the little pain in the neck problems including the EBCM module tanking and then finding out that they cannot be repaired nor are there new ones available from GM, which to me does not make any sense at all. I just bought this car with the understanding that the only problem was the windshield wiper motor needed replacement. But "AS IS" means just that. Oh well, it is only money. But by far the most important problem facing me at this point is the overheating. Again when I picked up the car today and with his again telling me that regardless of what the gauge is saying, the car is definitely not overheating. Took it on the freeway and drove it for about 10 minutes with the gauge reading at the first red hash mark and the display reading, I believe, "Coolant temperature over limit". It was several hours ago, so I not absolutely sure as to what the display reading actually said, but it quit giving me temp readings after 255. Thanks for your input as to what this might be.

 

[dadaroo]

Didn't see any codes related to your temp issue. I am surprised your guy didn't recommend replacing the temperature sending unit. They are inexpensive and easy to do.

 

[Vette Dreemz]

Thanks again. The temp sensor was replaced to the tune of $32.00 including labor. He has checked grounds, resistance in the wiring, battery corrosion under the battery box. Honestly, everything he could think of. The wife does not like him due to his attitude, calls him Mr Wonderful, but he is really a good guy and a great mechanic. Still can't believe GM would abandon a critical part ( sorry, now I am talking about the EBCM module for the early C5's) from a car they designed and one that is not all that old. Anyway if I find out what it is, I will let you know. You can add it to you knowledge base. Thanks again

 

[dadaroo]

Here is the design info on the system.

Factory temp settings are low speed on at 226 degrees and off at 219 degrees. High speed on at 235 degrees and revert to low speed at 226 degrees. As stated both fans run at low speed and high speed and fans are commanded off by the PCM when the vehicle speed exceeds 35mph.

You can force low speed operation at 185 degrees by turning on the AC. High speed operation with AC on is still 235 degrees.

The 2 wires from the sensor go to the PCM C1 Blue connector. The Yellow wire goes to pin 74 and the Brown to pin 80. It could be a loose connection there.
Here is the connector. Pin 1 is shown in the lower left and 80 is in the upper right.

 

[LLC5]

Could really use some help with this problem. The problem started suddenly with the temp rising to 255 at a steady rate from cold within 2 miles. Took it to my mechanic and he cannot find the problem. Put his pyrometer to various points and got different readings at around 30 degrees cooler. The coolant has been power flushed, the thermostat is opening and the water flowing, changed the sensor etc. He is currently testing resistance values, but basically we are stuck. The computer is being fooled by something, perhaps a wiring problem. Could it perhaps be the computer itself? Really strange. Any help would be greatly appreciated. Thanks guys and gals.:confused;help

Coolant temp's rising to 255*F in 2 miles suggests combustion hydrocarbons in the coolant or a air lock in the cooling system. I would consider having your tech doing a cylinder leak down test looking for any air bubbles coming out of the cooling system in each individual cylinder, and doing a test for hydrocarbons in the coolant. The 30*F difference in temps with his pyrometer is probably normal, as it is not measuring the coolant temp inside the engine where the sensor is. So, if your outside engine temp is being measured at 225*F (255*F -30*F = 225*F) in 2 minutes of driving from cold, that is not normal and an engine problem might exist. I have seen this kind of an issue with blown head gaskets and cracked cylinder heads, keep in mind that I am not saying this is your issue, but verifying that is not with a leak down test and hydrocarbon test eliminates that as a possibility. Good luck with it.