You may have read my thread about my broken T housing bolt in my 85 Z51 Auto. Got that squared away. Drove the car about 60 miles without any problems other than a Code 14, coolant sensor signal voltage low. I followed the trouble shooting section of the FSM and the solution was a faulty temp sensor. I broke the original so the one that caused the code was a new replacement. I replaced the pigtail also. So this code was set with a new sensor. I purchased another sensor from O'reily's and still getting code 14. Diagnostics shows if you get a code 15 the ecm and wiring is ok. I get a code 15 when I should. I've checked the ground, voltage and sensor ohm's and all are within specs. Still a code 14. I have 4 sensors, the broken one, the first one I installed that I thought was bad, the replacement that is currently installed and I purchased a another one just in case. All have ohm readings between 2.1 and 2.5 k. Which if my math is correct is 2100 to 2500 ohm. According to the chart that falls within a temp reading of 70 to 100 F. I also checked the diagnostics for a code 15 and it passes. Here are some pics of the FSM pages and the ohm meter reading of the sensor that is currently install. I'm stumped. I can answer any question and will try just about anything. Appreciate all of the help here.

TIA,

Joe

 

When I had my 91 that started stalling when it reached closed loop pulling codes it showed a code 15 showing a CTS issue. What symptoms does your car have?? After CTS replacement car ran fine again. I also tend to remember it was pulling about 5 volts which I believe is correct voltage. Have you checked for voltage ??

 

As I stated above code 14 will not clear even with a new sensor. Symptoms of code 14 is cooling fan runs continuously and runs lean causing hard cold start. I checked the voltage as I stated and it was 4.6 volts which is within specs.

Joe K

 

Have you tried running it with CTS unplugged and if so how did that change operation. With CTS unplugged it should be rich when starting cold, but what happens after it reaches full temp?

 

That is part of the diagnostics that I did above. No changes still code 14.

Joe K

 

I'm just curious, how are you checking the resistance? Back probing the sensor while connected? Unplugging the pigtail and checking? Back probing the ECM harness?

 

On the car right now, in the photo posted,I disassembled the pigtail wires and taped my test leads to them. I checked the others on the bench using my old broken connector and also just touching the lead in the switch. Ohm results as posted 2.1 to 2.5 k.

Joe K

 

So that all seems in order. You get the code 15 connector off. You said you get 4.6 volts key on on the yellow? And you have a reading to ground on the other one? Just trying to make sure I have that right.

 

Yes to all.

Joe K

 

Okay... that is really strange. Have you tested at the ECM connector by back probing to see if it's an issue while plugged in? I had an issue with an oxygen sensor where it was fine unplugged (tested and ohmed out fine to the ecm) but however it twisted the harness once reconnected would ground out. I'm starting to suspect that this may be your issue... You don't have any means to check the data stream do you?

 

I have a scanner and plan on seeing what info it shows. I'll post the results.

Joe K

 

Ok codes cleared, scanner connected, key on engine off and cold. Coolant temp 290 degrees, well above the 275 that triggers code 14. Pigtail disconnected coolant temp -19 degrees. That's what is expected and would set the code 15, which it did during earlier diagnostics. Scanner doesn't have voltage but I checked it manually. Coolant sensor connected, key on. Power wire to engine ground, 5v. As expected. Power wire to sensor ground .6xx volt. Extremely low voltage. Voltage at op temp specs 1-2 volts. That's set code 14, but not with a sensor that ohms to spec which it does. Continuity to ground with sensor disconnected good. No continuity to ground through the power wire. Key off of course. So I don't believe I have a short. Car had no issues prior to me breaking the original sensor.

Joe K

 

I may have said this but ohms with everything connected and key off 2.4 k or 2400 ohms. As expected with a good sensor.

Joe K

 

Things are indeed a bit odd since all your readings seem right on the money. If your scanner can log everything you can post it and I'm sure someone here will comment on anything that doesn't seem right

 

So I think I'm going to pull the ecm, find those 2 wires. Clip them and attach wires and temporarily run them out the window or whatever to the sensor. If I still get a code 14 at least I can be sure I checked everything thoroughly. Then the only thing left I can think of is the ecm. Though I may just drop the cash for a ecm and just install it and see what happens. That's going to be the quickest and easiest way. Any other advice will be appreciated. Thanks for the help. I'll post an update. May be next week

Thanks again!

Joe K

 

Use a paper clip tocle pin them from the harness at least. Then you don't have to deal with splicing under the dash.

 

Agree, pull the pins from the ECM, don't cut and splice. You can get a bag of pins for your jumper wires pretty cheap.

 

That's a floating ground, a faulty power wire with excessive voltage drop, or the pullup resistor inside the ECM has failed and cannot provide enough current causing code 14. You should have 5V power to ground wire, that reading should match power to engine ground exactly, continuity tests suck in that they will pass even if a single strand in the wire is connected, a voltage drop test will not. With how old these cars are, you very well could have broken the wire internally enough to pass a continuity test but not enough to properly conduct.
Don't depin the ECM connector, plug in the CTS, pull the ECM plugs and measure resistance between C10 and D13, it should read exactly the same as what the CTS read when measured before, if it does, ECM is faulty, if it does not, the harness is faulty. If it reads higher suspect bad connections, broken wires, if it reads lower, there is a short.

EDIT: Check ECM grounds, there is one that goes to the inner most t-housing bolt, coming off the injector harness, but check the rest as well.

 

The bolt I broke was the one on the housing. I've posted a pic. Can I move that ground to somewhere else? Though I don't see how it couldn't be a good ground. Going to check it momentarily.

On the voltage drop the FSM shows operating temp voltage should be 1-2 volts. I took that to mean on the sensor ground wire. I'm getting .6 v key on. So should be 5 even through resistor (sensor)?




Thanks!

Joe K

 

Ok, 0.6V with the connector plugged in says bad wiring or bad ECM, I missed that it was with the sensor plugged in. Without the sensor, you should have 5V, with the sensor plugged in, the voltage will drop the hotter the sensor. The last check would be to check resistance at the ECM connectors across the sensor, as mentioned above, if that does read normally it's, unfortunately, that means the pullup resistor in the ECM has failed or has a cracked solder joint. If it worked normally before the t-stat mishap, then I'd lean towards internally broken wires though.
That ground looks to be in good shape, I'd assume its functional since the wire or ring aren't broken.