Code#15 and #28 on 93z

[lt1redz]

i have a 93 4-a lt1 with 189000 miles on it.according to shoe box #15 is-engine coolant temp sensor circuit and #28 is quad-driver module #3 circuit-fan control relays-a/c relay. the #28 was corrected when i replaced the temp sensor with a napa #ts4052 sensor.[not oem]. [background] had a cylinder missing when cold but would stop missing when the engine would start warming up about 2 miles on the road.i replaced the plug with a used a/c plug.no missing now. now it is hard to start when cold.when it finally starts i run it to 1500 rpms and about 30 to 45 seconds later it smooths out,but get code #15.i cleared the code by pulling the fuse and also disconnecting battery cable..no help. after engine warms up,no problem starting even after 8-10 hours later. after i reconnected the battery i noticed the fans would cycle on and off during15 minute warm up to operating temp. the only mods i have is ram air,flowmaster cat back and fan switch mod. any help would be appreciated.hope and pray it is not the opti as this is a daily driver.

[Joe 1320]

i have a 93 4-a lt1 with 189000 miles on it.according to shoe box #15 is-engine coolant temp sensor circuit and #28 is quad-driver module #3 circuit-fan control relays-a/c relay. the #28 was corrected when i replaced the temp sensor with a napa #ts4052 sensor.[not oem]. .

Make certain that you did not use teflon tape or any kind of sealing agent on the threads of the coolant sensor. This needs to make a firm and clean contact to the block as it's the source for the electrical ground for the sensor. Teflon tape or sealant WILL cause erratic readings.

[lt1redz]

i used teflon tape.i'll remove the sensor and remove the tape.i'll let you know the result.

[Injuneer]

Hard to understand how replacing the coolant temp sensor would "fix" DTC 28, since the coolant temp sensor is not part of the fan power relay circuits. It only connects to the ECM.

Are both fans turning on when they are supposed to (each fan has its own relay and the relay connects to a separate driver in the Quad Driver module)? That's all the ECM looks at - what is the voltage on the relay control circuit, and is the voltage correct. If you have access to a Tech 1 scanner, it will tell you the status of all three relays driven off the QDM #3, and tell you which relay circuit is setting the code. But fixing the coolant temp sensor did not fix DTC 28.

Installing a fan switch in the fan control circuits can cause the DTC if the wiring is not correct.

DTC 15 means the resistance on the circuit is very high.... typically caused by an open circuit. Did the temperature sensor you bought have two pins? And you replced the temp sensor in the water pump housing, not the one in the driver's side head - correct? With the key on, pull the harness connector off the coolant temp sensor (water pump housing). Measure the voltage across the two pins in the harness connector. Should be 5V. Then measure the resistance across the two pins on the coolant temp sensor. Should read per the table of resistance vs. temperature shown on Shoebox's guide to checking temperature sensors.

http://shbox.com/1/4th_gen_tech2.html#ect

[lt1redz]

Thanks for the help. Looks like the problem are the fan relays.I changed the right sensor and it has two pins.The voltage on the meter is 5 volts. The fans are coming on at almost to the red. Actually the small mark below the red. I swapped relays to get the fans working. Another day I'll remove the temp. sensor and check the ohms reading in a pan of water using a thermometer.Until then I reinstalled the fan switch. NO more code #15 so far.I have to wait until the AM to see if it starts hard..Thanks again.

[FTA1995]

for my one wire sensor in the drivers side head i used teflon, but i carefully scored some away at the peak of the thread to ensure a good ground AND a good seal. Ground the peaks seal the valleys.