i have a 1989 f-150 xlt lariat 4x4 with a 4.9 and 5 speed M5OD tranny. i just recently rebuilt the motor, and everything works awesome!... but my engine light is on. my check engine light is out but the red engine light is on. i researched it on ALL DATA and it said it was controlled throught the oil pressure sending unit BUT the sending unit is working because i have oil pressure.(aftermarket gauge and stock both say i do and theres no knocking or ticking). its also not a type of thing i can scan either because its not part of the obd 1 system. anyone have any suggestions? oh and the this was happening with the old sending unit too and nothing changed when i replaced it..

 

oh and the light was not on before the rebuild. i have double and triple checked my connections and they are all tight and secure

 

Well if you got two gauges and they say you got oil pressure and it don't throw no codes, live with it till the bulb burns out.That or check all the wireing to it and find the short and fix it. Sorry, I'm just in one of them moods tonight.

 

Usually that engine light comes on whenever the oil PSI gauge is on the "L" side or if the coolant is up into the Hot. How did you get the factory sending unit and your aftermarket gauge to work at the same time? T fitting?

 

well my temp gauge is maxed out when you start it. it just goes all the way up. but i didn't think that would through the engine light on. i thought it was just connected to the oil light? and the oil pressure sending unit is electrical. and i just tapped into the line

 

Hmmm... you have me baffled. The wire that leads to the stock sending unit is nothing more than a ground wire that runs to the instrument cluster. Whenever the factory sending unit recieves 6-7 PSI, it will will "switch" on and ground that wire out through the block.

As far as the "engine" lights go in these trucks, they've always struck me as similar to the "Check Gauges" light found in many Chevrolet vehicles in the early to mid 90's and mid 90's on some Ford cars.

As far as the coolant gauge, if the gauge stays on Hot, then somewhere between the instrument cluster and the end of the wire that was supposed to go to the temp sending unit has been grounded out, causing it to read fully hot. The temp sending unit is actually a resistor that fluctuates with coolant temperature, allowing for fluctuations in the coolant temp gauge. But remember, the Oil PSI gauge in reality is nothing more than a dummy light. Instead of having a resistor in the sending unit like the coolant temp gauge, it is mounted on the back of the cluster, which determines gauge position.

 

woops my bad i was thinking about my blazer on the oil gauge. the one on my ford is the stock on and it has been replaced. the light was on with the old one and the new one and the gauge itself works with both the old one and the new one. tomorrow il trace down the coolant wires and try and find a bad plug/connection

 

Actually in '89 it's not just a dummy gauge. It shows actual pressure. If the sending unit is a can type then it's a real gauge (as real as any electrical gauge). The temp is what's triggering the engine light. You need to verify coolant temp before going any farther. It sounds like you might have used a sealer on the threads of the sending unit.

 

Haha no problem. Find that wire and make sure it's not grounded out against something, or pinched. Personally once I found it, I would tape the end of it off if the elbow connector is still on the end of it, and get yourself a true mechanical temp gauge. I would also recommend an oil pressure gauge. The factory one, as I mentioned earlier, is a complete joke. I would highly recommend all of this especially after you just rebuilt it. This way you can moniter everything that is going on to make sure everything will be ok.

 

Ford actually had an oil PSI sending unit that actually read pressure?! I knew this came on select few Ford cars, but had no idea that the trucks ever had it.

Also, what would sealer on the threads do in terms of pegging the gauge? Wouldn't that just keep the temp sender from grounding out through the threads, thus not allowing the ground to be picked up from the gauge? If I'm wrong please let me know, I'm open to any and all opinions. I'm just stating what I've learned and or lead to believe throughout the years!

 

The sealer wouldn't allow the sender to ground properly. IIRC the temp gauge works the opposite of the oil pressure gauge. That is when grounded it goes to the L side not the H side. Ford didn't change to the dummy gauge until '91 or '92. I know my '87 had a gauge that read real pressure, as did my '82 and '83 E350s.

 

Hmm. Well in my 95 and in my friends 91 I have both the oil PSI and temp wires unhooked, and the factory gauges on both of our trucks sit on Cold and Low all the time. The red Engine light on the 91 mysteriously..... disappeared. When I grounded out the oil PSI wire, the gauge went up into normal and the Engine light went off. Who knows what's going on!

 

i did use sealer on the ECT sensor and on the oil pressure sending unit. but the oil gauge is working. ill take the ECT sensor out tomorrow and take the sealer off and put it back in and see what happens. worth a shot

 

Well, the easy way to tell whether or not the gauge works is to take the temp sending unit wire (single wire) and ground it out. If the gauge stays where it's at, then my theory of the "grounding temp gauge= rise in gauge position" was right, which means the wire is pinched, or grounded out, or the gauge is bad. If it goes back down to cold, then pfogle is right! I hope hes right! That's an easy fix!

 

The gauges read full hot or full pressure when grounded, not the other way around. The pipe sealer is probably not the issue, and for the computer temperature sender, the ground is irrelevant anyway since the wiring supplies the sensor ground. If the pipe sealant was a problem, the gauge would read colder than the engine was actually running.

As far as the oil sender being variable or being a switch, at least the 87 and 88 trucks used a variable sender, but I think the crossover year to a switch was 89 or 90. At any rate, the "Engine" light is on because the temp gauge is pegged at full hot, which is caused by a short to ground in the sender wire.