The check engine light came on in my 1989 4wd truck. It has the 4 cyl motor. I pulled codes, and they are 51 and 21, I looked up these codes in the owners manual, and 21 is the Oxygen sensor signal, and 51 is the switch signal. I have no clue where to start, and what is the switch signal? Any help would be great.

Thanks

The switch signal is the Idle Position Switch.The only way that code 51 will be set is when you have the terminals TE1 and E1 shorted together (while checking codes) and either: A) the idle switch contacts are not closed and/or B) the shifter is in any position other than P or N (on automatic trans vehicles). Fix the Oxygen Sensor first,and then check the IDL contacts.The easiest way to do this is to jumper the check connector while it is idling and see if the RPM drops and the ignition timing returns to the base timing setting of 10 deg BTDC.If it does,clear the codes and forget it,you may have bumped the throttle or accel pedal while testing.If it doesn't,you may need to check the throttle cable for proper slack when released,clean the throttle body,or adjust the Throttle Position Sensor.

Thanks for the reply. How do I clear the codes? How do I adjust the TPS? I will check what you said to do. Should I replace the oxygen sensor? I have another question. Back in the summer, The idle was a little low on the truck, and when you turn on the AC, it wouldn't compensate for the draw on the motor, and then the AC would cycle on and off, and I found that if I held the throttle open a little, the AC would stay on. So I just adjusted the Idle up alilttle witht the set screw. And it worked fine all summer. Then these problems just came up. Thanks for the reply!

Stefan

OK, here is an update: Today I was crawling around under the truck and found the problem. The O2 sensor wires were all rubbed through and corroded. I had to trim them and splice it back together. Used tinfoil to patch up the shielded strand. Truck started and runs great, now check engine light. I still have a question: What would cause the truck to not idle up when the AC is turned on? I had to raise the idle to keep the AC compressor from cycling back and forth. The truck would idle so low before I changed the idle. I think it should, under normal circumstances, compensate when the AC is turned on.

Thanks

Any one know how to test the AC High idle? My AC turns on and functions just fine, but I had to turn up the idle so the compressor would stay on. I think it is suppose to idle up on its own, when the AC is turned on. I just don't know where to start.

Thanks