Video: How to test the radiator fans on your C4 Corvette

[Rob]

From the video author: In this video I show you a simple and easy way to test your cooling fans on your 1984-1995 C4 Corvette (OBD1 cars). 1996 and up corvettes are OBD2, which cannot use the paper clip trick.
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[geekinavette]

What he's done there is put the OBD1 ECM into diagnostics mode...one side result of that being it turns the cooling fan system on. It should be noted that the entire cooling fan SYSTEM must be operating properly to use this as a "fan test". For example if a relay is bad, or the ECM's relay driver is bad...the fan itself may be perfectly ok, but it will NOT turn on.

This "paper clip" method also allows you to check basic functionality of the ECM and retrieve any stored trouble codes. It flashes the CHECK ENGINE (or SERVICE ENGINE, or whatever text is used) indicator in a manner that displays any stored codes.

You should ALWAYS see a code 12, which means "No Distributor Reference Pulse"...in other words the engine is simply not running.

This code will "flash" like so:

One short "blink", followed by a pause, followed by two short "blinks". This will repeat two more times (each code is displayed THREE times), then a longer pause, then any other stored codes are displayed in the same manner.

If there are no stored codes, then it will just repeat the code 12 three times over and over.

Another thing this diagnostic mode does is fully close the IAC valve, which is pretty useful for doing throttle body adjustments (which is NOT part of normal maintenance, only if modified, damaged...etc). At least it did on the '84...I've heard that all OBD1 ECMs do this, but I have not tested that myself.

So, what happens if you start the engine while in "diagnostics" mode? Not much, everything works pretty much as normal, but the CHECK ENGINE light now shows you O2 sensor cross-counts. It will flash every time the O2 sensor voltage crosses the programmed rich/lean thresholds. (only when up to operating temp and in closed-loop, of course)

 

[Hot Rod Roy]

This paper clip trick won't turn on the radiator fan in an '84. The fan is not controlled by the ECM in the '84.

 

[geekinavette]

This paper clip trick won't turn on the radiator fan in an '84. The fan is not controlled by the ECM in the '84.

Quite true!

It will do the other stuff, but right, not the fan.