The fact that your fan comes on when the sensor is disconnected seems to mean that the pcm recognizes the temp sensor output. Do you have the code reader or are you going somewhere to test it. If its yours, connect the reader and run the engine until you read 225. If no fan then you have an issue . If its on by 225 then its all normal. In the real world...218 is 225. Also reading the sensor temp does not mean that the logic to engage fan relay based on that reading is functioning but you have to prove it.
There is a few things I would be doing before ordering a PCM.
First, to keep you on the road, runs some wires and a switch into the cabin..voila, fan switch on when stuck in traffic etc
Second. Using own OBD2 code reader I am almost sure that if a sensor is not properly detected by the PCM it will throw a code?? (check code list)
I personally would download the 98 FSM free from this site, study the circuit diagram, and identify the colored wires that stimulates fan from PCM and probe them out
imo, a bad output wire/connector is more likely than a bad PCM
There is a few things I would be doing before ordering a PCM.
First, to keep you on the road, runs some wires and a switch into the cabin..voila, fan switch on when stuck in traffic etc
Second. Using own OBD2 code reader I am almost sure that if a sensor is not properly detected by the PCM it will throw a code?? (check code list)
I personally would download the 98 FSM free from this site, study the circuit diagram, and identify the colored wires that stimulates fan from PCM and probe them out
imo, a bad output wire/connector is more likely than a bad PCM
It could be the PCM, just sayin
Thanks. That would make a lot of sense. The pcm must be receiving input from the sensor because my obd2 reader is giving me a temp. I've been just running the ac to keep the e-fan turned on. Thats gonna suck when it gets cold though.
There is a few things I would be doing before ordering a PCM.
First, to keep you on the road, runs some wires and a switch into the cabin..voila, fan switch on when stuck in traffic etc
Second. Using own OBD2 code reader I am almost sure that if a sensor is not properly detected by the PCM it will throw a code?? (check code list)
I personally would download the 98 FSM free from this site, study the circuit diagram, and identify the colored wires that stimulates fan from PCM and probe them out
imo, a bad output wire/connector is more likely than a bad PCM
It could be the PCM, just sayin
Hey, I can't seem to find where to download that 98 fsm here. Can you share a link or give me an idea where they're at? Thanks 🙏🏻
How did he test it? If all he did was check it at room temperature that's not checking it. You gotta get you a pot of water on the stove and monitor the range (or at least around the 210-200F area)
Also when a PCM gets fried it's usually a catastrophe, not a single signal.
That said, a solder connection can break at the cable connector, and that's only like 3 cents worth of solder.
How did he test it? If all he did was check it at room temperature that's not checking it. You gotta get you a pot of water on the stove and monitor the range (or at least around the 210-200F area)
Also when a PCM gets fried it's usually a catastrophe, not a single signal.
That said, a solder connection can break at the cable connector, and that's only like 3 cents worth of solder.
Thanks thats very helpful.✌️ I downloaded the 99 fsm manual but haven't found a diagram that shows what wires are the fan output wires from the pcm.
Both of the online sources for a pcm mentioned in this thread have lots of complaints with the better business bureau. 😔
Not surprising as many people mistakenly replace their PCM without correctly diagnosing the problem. And when they install the refurbished unit - it doesn't solve their problem, so they blame the unit and take their complaints to the web.
Did I not say all that testing in post 13. Lol...Secondly in Oregon in October he should not have to run the aux fan very often to not overheat. Does your primary fan have a shroud and have someone turn off the engine while you watch the mechanical fan. It should stop spinning almost instantly as the engine stops...if it still spins your fan clutch is shot
I have seen the coolant temp sensor wires break near the sensor as they are a bit short and pulled tight. I'm with Dave though. If the A/C turns the fan on, then the driver in the PCM, the relay, and wiring to the fan are good. If an OBD reader is showing the PCM reading higher than 217 degrees and it's not coming on, then something might not be right with the PCM.
So if the fan works with A/C it should work with a >218-degree temp sensor signal cause it's the same output circuit.
Measure the resistance of the sensor in boiling water and report back.
Finally got a minute to test today Any tips on getting a reading in a pot of water without burning my fingers? My meter doesnt have alligator clips. Readings with sensor installed at 47f outside were 20.8 kohms
How did he test it? If all he did was check it at room temperature that's not checking it. You gotta get you a pot of water on the stove and monitor the range (or at least around the 210-200F area)
Also when a PCM gets fried it's usually a catastrophe, not a single signal.
That said, a solder connection can break at the cable connector, and that's only like 3 cents worth of solder.
Readings were just a little high maybe. At boiling resistance was between 800 and 740 ohms between the two multimeters I used to test
Its mostly irrelevant, if its off a little. since its the only source for actual temp and gauge temp at the same time, if the actual coolant temp is 200 and it reads 210...so what. The bigger problem is if the actual coolant temp is radically higher than gauge temp...you will never know until it blows the head. The only way to test it is to remove it and drop in a pot of boiling water and read the resistance while its in the boiling water . should be about 6 hundred and something to 7 hundred and something. Did you let the temp go past 218 to see if and when the fan came on? Does the fan come on if you unplug the sensor connector with the engine running? This will set a check engine light and code. A decade box is a reverse multimeter...It has a bunch of resistors hooked up to a set of meter leads and you turn a bunch of selector dials that can change the resistance output from 1 ohm to several hundred thousand. My sensor reads 675 at boiling...so I set my decade box to 675 and connect the meter leads to the sensor connector car side...turn key and the gauge reads exactly 210...close enough to boiling for me. I did not have a code reader to see exactly what the number was but its close enough.
Could I use a decade box to try and trigger the fan with a higher temp reading(lower resistance)? My resistance tests being a little off seems close enough to just be a little lag where the metal takes longer to heat than the water? Thanks.
