55 belair with 4.8 fan problem

[badbowtie]

I have a 1955 belair with a 4.8L with a turbo setup on it which has been like this for several years never having a problem till yesterday. I started getting hot so I shut it off and started checking things and noticed the electric fan was not working. I pulled the 30amp fan fuse and it was blown replaced it and still nothing. So I went to the laptop and used hptuners to command the fan on and it turned right on and about 3 seconds later shut of. I manged to get home as long as I stay on the interstate and no stop and start traffic the car runs 190 or less. So tonight I started checking things. I know the ecu supplies a negative signal to the relay to complete the wiring to make the fan run on my 85 terminal of the fan. I am only running one relay and a single 16" electric fan. So I unplugged the relay and took my test light and put one end on power and the other end on my ground wire coming from ecu. Using hptuners again commanded the fan on and my test light light up and 3 seconds later test light went out. I am not sure what is causing the ecu to stop sending the ground signal. I could sure use some help Thanks in advance.

[JHPerformance]

First off I would start with the basics, make sure you have good solid ground connections at the PCM and main harness. If you're fairly certain it isn't a wiring problem, and the PCM is dropping ground, not sure which PCM pin you're using for your fan trigger, but the 2nd fan pin also controls the HVAC door. You have to change the settings in the tune to fan #2 under type fitted tab. But if you're using fan 1 trigger then this should not matter. I would see what amp the fans are rated at, and make sure you have a good gauge wire to support it and that something is not shorting out and popping your fuse and of course double check the wiring. Some relay's will not hold up to the amp draw on certain fan motors. Not many but I have seen it before.

[badbowtie]

Well for a update I guess I dont have a ecu problem that I thought I did comes to find out in the vcm scanner if you go to command the fan on it only runs for 3 seconds then shuts off. So something in the new version is different than it used to be in previous version.