Joe W

I've competed a 4.8 swap into a 68 chevy truck. I'm using a Bully Dog to monitor everything. I also have a backup mechanical water temp sensor on the passenger head that's completely isolated and just used for reassurance.

Coolant temp on the BD monitor reads much cooler (60 degrees) than the mechanical reading when the truck is running but if I turn the key off the BD monitor temp instantly switches to a higher temp that matches the temp of the mechanical gauge exactly.

The truck has only been ran in the garage and no temp related codes have been set. My fans wont kick on because of the lower reading and I'm sure my fuel trims are adjusting for a cold motor when in fact its at temperature.

Does anyone have a suggestion as to why the temp is switching?

Hotrodnova66

Try taking factory sensor out of the head, hold a lighter under it and watch your monitor and see what the readings are doing. Might be a bad sensor or wiring problem to the sensor

Joe W

Thanks! I was not trying to hijack your thread just trying to drum up some conversation on the issue and provide you issues I'm seeing/having addressing a high idle. I appreciate your post and will try the test with the key off and on. It just seems odd that it switches temp like it does when the engine turns off. I'm wondering if it has something to do with the 12v switched signal feeding the pcm (what else is changing)? I don't think it's the bully dog because when I was having a custom HP tune it was doing the same thing in his tuner software. I have not seen anything post out there that have had this problem! I hope it's something real simple! I will start at the sensor and go from there. Good luck tonight! Don't forget to check the pcm connections when doing your test!

jimmyblue

iTrader: (11)

Does Bully Dog know which of the possible temp
sensor styles you are using? Thermistors come in
a variety of temperature curves. Not only that, if
the PCM is meddling by throwing current down it,
and Bully Dog wants to do the same, maybe the
two are combining to whack the reading.

Joe W

I'm not sure if that's the case or not but will keep it in mind as I work thru it. Isn't the OBD2 just receiving a calculated value from the PCM that it receives (in voltage) from a stock ECT sensor. The bully dog is a Watchdog. Its just monitors things and has the ability to read and clear codes. my thought is it would be no different that hooking it up in any other stock truck?

Joe W

I replaced the ECT sensor and it still does the same thing. It reads low with key on or running but correct when the key is off. My E fans will not come on unless I unhook the ECT. I'm not seeing anyone out there that has had this issue. Not sure if its the bulld dog or my vehicle. I'm going to call BD on Monday and talk to a tech but I would really like to figure it out this weekend!