Engine fan control output

[DVT873]

(05-04-2025 )Hamish Wrote:  Good point to check the voltage being delivered to the ECM - I guess I had assumed it wouldn't even fire up if voltage was that low. Will certainly check that next before doing anything else

If your built after Jan of 2007 you may have another module in the mix. January 07 and up is S08339 and found at this site http://bodybuilder.navistar.com/General/BBcircuitIX.htm
Or just search for International Body Builder wiring. Prior to 07 they were 8600's as far as manuals went.
I have not tangled with a fan speed module but a quick look at the wiring shows some grounds. It looks like the body computer gets involved also. If it is newer tha Jan 2007. Post the VIN and I can see the build info.
Prior to 2007 it is just on/off from looking at the manual.