1990 Bronco 5.0FI engine with AOD transmission. About a month ago the high idle while cold stopped working and the check engine light came on and stayed on. When I start it it will idle (barely) until warm and then idles good and even a little high. I checked the codes and the only one is code #51 "Engine coolant temperature (ECT) sensor-signal voltage is too high" I had replaced this sensor, located at the thermostat housing, last spring due to a cracked plug and never had any other problems with that code. I thought that maybe that it had already gone bad and changed it again yesterday. Still the same code #51. Why would it still show too high voltage, what should the voltage be, and what could be wrong? HELP!!!

 

I don't have the exact answer for you but someone will. Did you reset the computer code after you replaced the piece this time by unhooking the negative cable for at least two minutes?

 

I would carefully check the wires going to the sensor as well.

 

According to Haynes, you can turn on your ignition but not start. The temp gauge should read cold. Remove the wire from the temp sensor and ground it. The gauge should read hot or past hot. This indicates a bad sensor. If the gauge doesn't move then it maybe the gauge or the wiring. This might help desiding if your sensor is still good. Also it warns not to leave the jumper to ground on any longer than necessary. Hope this helps.

 

i have a 1991 bronco that is doing the same thing. i replaced the sensor and it didn't help. this is a weird deal. it will start fine but won't run. i have tried everything to get it to run. i can rev up the engine and keep it running for a couple of minutes but it is not driveable. as soon as you let off the gas it dies. i am getting error codes 51q

 

i have a 1991 bronco that is doing the same thing. i replaced the sensor and it didn't help. this is a weird deal. it will start fine but won't run. i have tried everything to get it to run. i can rev up the engine and keep it running for a couple of minutes but it is not driveable. as soon as you let off the gas it dies. i am getting error codes 51 and 54 which are ect and act which i have replaced both. i am starting to think it is in the computer.

 

The sensor for the gauge and the ECT are two different animals. The ECT sensor will trigger a "stored" code that will require the computer (PCM) to be "cleared" via the battery disconnect method already mentioned above.

I'll second the idea that you may have a wiring issue from the sensor if clearing the PCM memory doesn't correct the problem.

 

I cleared the computer by removing the battery cable and also with the code reader. I keep wondering if it is a computer problem also. When it stopped having a high idle (like the choke is on with a carburator) while cold is when the check engine light came on and stayed on. Doesn't the ECT sensor tell the computer when the engine is cold and that's what tell the computer to speed up the engine until it warms up a little? It also is very sluggish and is using a LOT of gasoline, about 6 MPG.

 

llane915, any luck with yours yet? I'm still stuck with the code #51 ect sensor and it will not idle high when cold. Code says the ECT Sensor voltage is too high. Anyone have any idea what the voltage in and out of the sensor should be?

 

gov, greystreak was dead on with the guage being a seperate circuit and sender from the ect circuit and sender. The two have nothing in common. If you replaced it and you are assuming it is a good sensor then you have a problem with its signal circuit to the pcm or the pcm itself. The pcm being less likely but still a possibility.

Relating to your idle issue, i think you have a good grasp on the circuits opperation. If you ect is reading high voltage it is telling the pcm the vehicle is fully warmed up. Needless to say on a cold start there is a different calibration to run the engine consisting of a higher iac count, more air for the high idle, and a higher pulse width, more fuel for the cold start. In your case your "barely idles" is probably caused being the pcm is commanding enough of an iac count and enough fuel for a warmed up engine. This would also cause poor performance when cold being the engine is not getting enough fuel. Once the problem is fixed when you start the engine you should get the normal high idle progessively dropping as coolant temp rises.

Dtc 51 reads as pegged high coolant temp, pcm reads this as engine is running at 254 degrees. Check the schematic and find the signal circuit to the pcm. Use a meter and check for continuity to ground with the pcm unplugged. Im guessing that circuit is going to be shorted to ground. You need to either trace that circuit and find where the wire is pinched and grounding or where it is chafed or cut and grounding out. If that circuit is not grounded and you are sure the sensor is good you have a pcm thats grounded at the ect signal circuit but again, this is unlikely.

Hope that helps.

 

Joboo- Hey do you work for a Ford dealer?You seem to know quite a bit- I've been at mine since 11/77 and am our PTS coordinator.

 

Lol, Gm 15 years, Trans and Drivability as bread and butter, and everything else when i ran out.

<<<<-I have been breed as an anti ford guy, though, i do own 2 of them

 

Small dealer- rural??I run a custom in house exhaust shop and do a lot of GMs- never owned one though.Some things are basic the same though- GM or Ford huh.

 

Currently 28 tech shop, auto side. Not sure about the body side. Ive allways worked gm, some small dealers, some larger, ford work was allways done for my own stuff or friends.

 

Joboo is right on with prob. Temp. Sen. is a negative coefficient sensor. That means as temp goes up resistence goes down. High volt indicates no resistance, therefore short to ground most probable, check for continuity to ground.