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Thread: Red engine light

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    Member 91merc's Avatar
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    Default Red engine light

    I got a red engine light on my 85 and it's acting sluggish, what's the difference between the yellow "check engine" and red "engine" lights on these cars?

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    The red "ENGINE" light comes on when oil pressure is too low (below either 6 or 10 psi, if memory serves) or coolant temperature is too high (250F or above).
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    Quote Originally Posted by Stocker View Post
    The red "ENGINE" light comes on when oil pressure is too low (below either 6 or 10 psi, if memory serves) or coolant temperature is too high (250F or above).
    Yellow "Check Engine" on '89+ is an actual check engine lamp. Like Stocker said, the red "ENGINE" light is for one of those two issues.

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    Well the coolant temp sensor has bare wire on the little snap plug. And I might've disturbed it when I pressure washed the engine, oil pressure wiring doesn't look the best either

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    That would be the "replace engine" light. Unless its a bad sensor, fair bet she's fucked if you've been driving with that on for any amount of time at all.

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    Quote Originally Posted by 91merc View Post
    Well the coolant temp sensor has bare wire on the little snap plug. And I might've disturbed it when I pressure washed the engine, oil pressure wiring doesn't look the best either
    With the engine cold and running, disconnect the wire from the coolant temperature switch (this is the one on the intake manifold - the one on the heater hose fitting is the temp sensor for the ECU). If the light goes out, the problem is the switch. If the light remains on, either the wiring is shorted or the problem is in the oil pressure side of things. Disconnect the wire from the oil pressure switch - if the light goes out, either the switch is bad or your oil pressure is next to nothing.
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    Doesn't an 85 have the "low oil" sensor in the oil pan? The sensor can be stuck or the wire going to it may be shorting out. WagonMan
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    Quote Originally Posted by WagonMan View Post
    Doesn't an 85 have the "low oil" sensor in the oil pan? The sensor can be stuck or the wire going to it may be shorting out. WagonMan
    That's a separate light - optional on Fords, and I think it says "Check Oil" (my '85 is a low-series car and doesn't have it).
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    does it go out when you rev it up?

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    The first time I drove it about 15 miles, parked it for about 10 minutes and it was on right from startup, then I went under the hood and wiggled some various wires and it stayed off for about 10 more miles, the last 5 or so miles there was no knocking or coolant smell, just a little sluggish

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    The light is staying off right now, but I haven't actually moved it at all yet

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    If the coolant temp sensor or the oil pressure sensor grounds due to faulty wiring and/or a faulty sensor, the light will turn on. By what you are describing, that sounds like your issue.

    A steady on red engine light for actual temp or oil pressure reasons basically means turn it off ASAP or you're gonna nuke it.
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    Quote Originally Posted by Tiggie View Post
    If the coolant temp sensor or the oil pressure sensor grounds due to faulty wiring and/or a faulty sensor, the light will turn on. By what you are describing, that sounds like your issue.

    A steady on red engine light for actual temp or oil pressure reasons basically means turn it off ASAP or you're gonna nuke it.


    I think you're right on the ground problem cuz it stayed off on my 20 mile commute to work and ran real good. If the ground problem is the case, would that basically fool the computer into thinking the engine was about to grenade and throw the car into limp mode and thats why it was sluggish?

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    Do these things have a limp mode?

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    Or is it rather to do with the fact that the coolant temp is one of the variables used for the air/fuel mixture?

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    91, I am guess from your answer to Lincolnmania that the light does not go off if the engine rpms are increased?
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    As mentioned above for the most part:

    The red engine light in a Ford (Mercury has two lights, Ford has one) represents either a low oil pressure, or high temperature, or both condition.
    The lights receive 12V at the gauge cluster, and the lights are turned on by the switches (like a "sender" but it is only on or off) grounding the wire.

    Oil pressure switch grounds the wire when oil pressure is below a certain pressure, from memory it's about 6psi. Therefore, the light is normally lit solid any time the key is on but engine is not running, and then turns off once the engine is running. As noted above, when an engine is worn, low oil pressure at idle that goes higher when you rev it is sort of normal (not good, but common as things get worn out).

    Temperature switch grounds the wire when temperature is above a certain amount, from memory it's about 260 degrees Fahrenheit.

    Reasons why the light can turn on:
    1. The oil pressure is low.
    2. The temperature is high.
    3. One of the switches is broken.
    4. One of the switches was mistakenly replaced with a sender. In parts store catalogs usually a switch is described as "with light" or "without gauge", and a sender is described as "with gauge".
    5. One of the switches is defective.
    6. A wiring fault is allowing the bare metal of one of the wires (oil or temp) to touch bare metal somewhere, which grounds the wire turning the light on.

    Sounds like #5 or #6 is your likely option.

    The computer has a temperature input, but it uses a completely separate sender. There is no interconnection between the red engine light in the cluster and the computer. They do not know that each other exist at all.

    A very hot engine will run sluggishly, but it is also possible you have an unrelated issue if wiggling the wire makes the light come on or off.
    Last edited by kishy; 08-12-2022 at 10:14 AM.

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    I think the bare spot in the wire to the coolant temp was touching the alternator bracket and grounding out

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    Quote Originally Posted by kishy View Post

    The red engine light in a Ford (Mercury has two lights, Ford has one) represents either a low oil pressure, or high temperature, or both condition.
    The lights receive 12V at the gauge cluster, and the lights are turned on by the switches (like a "sender" but it is only on or off) grounding the wire.
    I’d just like to point out the Mercury clusters when equipped with all the warning lights and the check level oil sensor will only feature the single red engine light. The base clusters without the extra lights or when not equipped with the check level lamp will feature the split temp/oil warning lights.

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    that's exactly what happened, the little clip on plug for the coolant temperature rotated and the bare wire was touching the coil bracket

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