302 FI shows codes 21, 24 when cold
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New User
Joined: Nov 2003
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302 FI shows codes 21, 24 when cold
I have an '86 F150 with a 302 FI. When the outside temp drops below 40, the truck will shows codes 21 and 24 (ECT and ACT voltage out of range)when it has sat overnight. What happened last winter is that once it dropped below 0, it would not start - it would flood. I replaced the ECT last winter with no change, and I have recently checked both sensors. Both are receiving about 4.57 V, and the resistances of each this morning (35 degrees outside) was about 81 k ohms. A few weeks ago, they were about 55 k ohms (65 degrees outside). The voltage reading I got from the ACT this morning was 3.73 V when connected.
Does anyone think I should try replacing sensors again? Also, if the computer is reading an error from a sensor, does it go to a programmed value instead? I seem to think my next step is a new EEC module.... but I'm not ready to dish out the dough yet.
Does anyone think I should try replacing sensors again? Also, if the computer is reading an error from a sensor, does it go to a programmed value instead? I seem to think my next step is a new EEC module.... but I'm not ready to dish out the dough yet.
Lead Driver
Joined: Nov 2002
Posts: 9,557
Likes: 44
From: Concord, NC
I don't think there is anything wrong with those sensors, and likely nothing wrong with the EEC. I get those codes when the truck has been sitting for a while too, in warmer temperatures than that. Double check that all your wire connections are solid and aren't corroded. Check the idle air control valve to make sure it is working properly.
Elder User
Joined: Sep 2004
Posts: 941
Likes: 4
Replacing the PCM (EEC) usually doesn't fix the problem.
Check all your grounds. Battery post, chassis grounds on radiator support, PCM near left hood hinge, battery cable to frame, battery cable to block. Offsets on the grounds may create an error voltage which throws off what the PCM thinks it sees.
According to the Probst book, the sensors should be about 37K ohms at room temp (20 degrees centigrade). The Dali Design material says 58K ohms max at 50 degrees. The resistance goes down with temp, so your sensors are reading on the high side if you see 55K ohms at 65 degrees.
You might buy a couple of resistors at Radio Shack in the 50 to 60 K ohm range, and use them to check the accuracy of your ohmmeter, before condemning the sensors.
VREF seems a little low
Any voltage offset due to high resistance can throw off
Check all your grounds. Battery post, chassis grounds on radiator support, PCM near left hood hinge, battery cable to frame, battery cable to block. Offsets on the grounds may create an error voltage which throws off what the PCM thinks it sees.
According to the Probst book, the sensors should be about 37K ohms at room temp (20 degrees centigrade). The Dali Design material says 58K ohms max at 50 degrees. The resistance goes down with temp, so your sensors are reading on the high side if you see 55K ohms at 65 degrees.
You might buy a couple of resistors at Radio Shack in the 50 to 60 K ohm range, and use them to check the accuracy of your ohmmeter, before condemning the sensors.
VREF seems a little low
Any voltage offset due to high resistance can throw off
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