2.5V at Knock Sensor connector (should be 5V)


Active member
My 1990 4.9L throws a OBD1 KOER code 25 (knock sensor). It also exhibits other symptoms of a bad knock sensor (Symptom 2, and misfiring of the engine on starting as described here). I foolishly went through the trouble of acquiring an aftermarket KS (a scarce resource these days) before I checked the reference voltage at the connector. The Haynes manual states the voltage should be 5V, but I am consistently measuring half that @ 2.5V. I'm surprised how little information I was able to find online about my situation, but I am wondering if someone has a suggestion. My PCM (an EEC-IV) is a newly re-manufactured unit I installed about a month ago - I did not check OBD1 codes prior to replacing the EEC-IV so I don't know if the 25 code existed with the old PCM. The hard start existed even with the old PCM though.

I have not installed the new KS yet, I'd like to address the unexpected ref voltage first -- my old KS might not even need to be replaced.


midwestbird":3mhxrn9g said:
The Haynes manual states the voltage should be 5V, but I am consistently measuring half that @ 2.5V. I'm surprised how little information I was able to find online about my situation, but I am wondering if someone has a suggestion. My PCM (an EEC-IV) is a newly re-manufactured unit I installed about a month ago - I did not check OBD1 codes prior to replacing the EEC-IV so I don't know if the 25 code existed with the old PCM. The hard start existed even with the old PCM though.

I have not installed the new KS yet, I'd like to address the unexpected ref voltage first -- my old KS might not even need to be replaced.

Haynes is often wrong, as it is in this case.

:beer: Its running fine. (y)

Pin 23 is normally the EECIV Knock Sensor for Unleaded engine, so if its reading 2.5V, then its near enough to the required +3V for engine Engine Running


Depending on pinouts for your EECIV, it reads between 2.5 v to 3v normally if no knock is present.

They are super simple, as soon as there is knock in the fregency of 5600 or so cyles per second , it zeros voltage.

There is a conditioning for each sensor, but if its for a 4" bore 300 4.9 Ford or a 5.0 or 5.8 Ford Truck, it'll have its low and high pass set to between 4800 and 6400 Herts, to suit engine knock in the zone a 4 inch bore engine makes. Its not really something anyone needs to over think.

The frequency footprint just has to match, then, whammo, it cuts voltage, which cuts timming to the TFI module.

When the cycles per second match, it stops the base voltage to Pin 23. Computer cuts timming to the engine, normally right back to 10 degrees on the TFI, or about 20 total.

Here is a visual example. Knock is a high frequncy event which has fast cycles.


redxm":3mhxrn9g said:


bubba22349":3mhxrn9g said:
Here are a few part numbers for the Knock Sensors that might help you in your search. I don't know what thread size though. I don't think using a GM sensor is a good idea if you are using a stock Ford ECU, but it might work if you are using a Mega Squirt. Also some times you can get an error code because of dirty connections or a broken wire and the sensor would still be good. Good luck :nod:

1990-1987 4.9 (Y) 300 Bronco, E, F, Series Ford numbers E7TF-12A699A2A, E7TF-A, F0TZ-1 Tomco # 29047
1989-1987 4.9 (Y) 300 Bronco, E, F, Series Ford number EZZ-12A699A Tomco # 29047
NAPA Echlin listed for a 1988 F150 4.9 PN ECH DKS205
The aftermarket sensors listed are used on many other Ford Engines sizes too.

How it works? It counts frequency, and cuts voltage if the frequency is 5640 hertz. For a 4 " cylinder bore, that is incipient knock. On a 200 or 250, its about 6100 hertz.

Thats cycles per second.

Down here in the Antipods, we are stuck between a mixture of the old European ignition retard systems, and the later Link ECU system controls, which are for Japanese turbos. Ford Turbo Falcons and Supercharged or turbo Holdens with GM engines, there is normally a reflash, so they use the stock stuff.

Most guys are using a Jaycar KC5444 knock sensor by John Clarke.

It has a PSD= Power Spectural Density determination





Early knock sensors...well, all the same really, a knock sensor is a knock sensor, although the GM ones are more sensitive.

Later, the normal Ford 3 volt signal polling rate with a change to less than 3 volts being the descriminator has changed to the AFM type amp draw algorithim.

Just an Anolgue Digital "Do If True" ignition peal back.

Like the very bad 4.9 1-bbl feedback F150 truck system that screwed over so many shade trees who couldn't be frigged pullin the codes.

For the earlier resonant type, they are tuned to a particular engine’s knock frequency of interest.

Later KS's have a Power Spectural density determination, with a sampling cut off to sense the deap seated knock. Jaycar's does it that way, and so does GM now.

The combustion shock wave excites a characteristic frequency in the engine, which is typically in the 5000Hz–7000Hz range. Cylinder bore diameter and combustion chamber temperature are the main variables that affect this fundamental frequency. Variations in the fundamental frequency for a given engine configuration can be as much as ± 400 Hz in terms of combustion chamber design.

In terms of bore, the knock frequency can be greatly influenced.
Approximated using :

900/(Pi * r) where r = the radius of the bore in Sillymeters, um, millimeters.

Given a bore of 76mm the knock frequency will be 7540 Hz.
A bore of 3.7" or 93.98 mm, the knock frequency will be 6100 Hz
A fat bore Chevy or Ford with 4" bore, its down to 5600 Hz, with the very smooth combustion of a wedge head dragging its audiability down to a 5200 Hz threashold

Larger diameters and/or lower temperatures result in a lower fundamental frequency. Signals received by a remote sensor contain additional vibrational modes, which are structural resonances in the engine excited by the shock wave as it hits the cylinder wall. Typically, two to four additional frequency peaks are evident between the fundamental frequency and 20000 Hz. (PSD= Power Spectural Density determination)

Engine structure can have different higher vibrational modes. Sensor mounting location can affect which modes are detectableand the amplitude of each with respect to the background mechanical noise.

The later sensor’s all follow the Siemens or Bosch resonant frequency is above 20kHz with a wide pass-band to give a relatively flat response over a frequency range between 3kHz and 15kHz. This allows this sensor to monitor either the fundamental knock frequency or 1st harmonic for most engines.

The pinouts and use of MegaJolts EDIS6 means its a straight down the line FoMoCo.

On the stock Aussie EECIV for 1986-1992 Port EFi X flows, Ford Australia just copied the USA protocols. If knock occurs, the base 3 volt signal is pulled low.



"Do if true" ignition timing reduction if knock is present. How it does it is just switch related.

"Cut me a switch, bouy"


Active member
Wow, thanks for all the info. Very good news for me that Haynes is wrong in this case. I was hoping someone on this forum would be able to shed some light on my situation, and you certainly have. I will happily move forward with replacing my KS now that I know my ref voltage is OK. Hopefully this post will be discovered in a future search by someone if they too encounter the same question/problem.

It's been a long battle for me with this engine. I purchased the truck last year (I wanted a vehicle to tinker with in my spare time,and I've always liked the 8th generation F series, and the straight 6). But the engine ran rough, and I've slowly discovered with time that several causes attributed to how poorly it ran. So far, I discovered/resolved a few vacuum leaks, a leaky MAP sensor, a bad Fuel Pressure Regulator, missing SPOUT connector, and leaky capacitors in the EEC-IV. Today, it idles much better, but the rough cold start remains. I'll try to post back to this thread after I replace the KS as to whether the hard start is resolved.

Thanks again.


I love Fords, and FoMoCo thankfully opened its wallet right up at the worst times during recessions to employ Really Smart People to fix electrics, plant logistics and electronics.

1929 with the Model A Electrics,
1945 with the Whizz Kids,
The 1958 18 month crash course to get the Falcon compact into production after the Edsel disater and the late 50's recession

1968 with the open cheque book from the still smarting true costs of the LeMans win, the company then pored a chunk of bills into Safety and Electronics, like Anti Lock Brakes, ConLec EFi testing, Small Cars (Pinto and Mavericks) and a whole raft of Clean Air Parts Vehicle Emmission Components and the Information and education process that went with it, while still traying to win TransAm with factory made Tunnel Port engines while the plant was on strike, and Auotolite was being taken off Ford by Anti Trust laws from Congress.

and around about the Malaise Era 1978 to 1983 with Fords renewed on slaught into on board computer systems. IMHO, The true brilliance of EEC 1, 2 and 3 is that your EECIV is the culmination of about 12 years or earlier work, and its easy to fix if you know where to start.

EVTM = Electrical Trouble Shooting Manual

My shop formans was a Kiwi guy who was an ex Ford Service tech from Dunedin in New Zealand. He did Liquid Propane Gas instillations for my little village of Alexandrar, and he's still operating a gas station. He reaparied washing machines for a lot of years and was the Guru of everything electronic because he read stuff. For the seven years from 1985 to 1992, he ran the Service Deptaretment of our municipal Utilities compny, so he was head hunted for his ability to mix Propane systems into stock Gasoline Aussie and New Zealand market Fords and Mazdas (Ford Couriers down here). All our trucks had a goverment 1 k dollar kickback to go propane back then.

He said to me then that it was important to know that Ford had spent billions on making these on board systems work, and not to go beyond the Electrical Trouble Shooting Manuals.

He did say that when all the plastic clips and wires and capacitors started to break, then Service Techncians were gonna have a lot of extra work.

At that time in 1985, US company Impco sold a crap ton of Natural and Propane gas carbs, which would be bolted up to EFi 4.1 liter Falcon wagons and other linemen vehicles. At that time, Ak Miller was oe of the Impco Service drafter and technical Service Bulleten person, so the 1985 Impco LPG TSB book shows every Ford on board system for 1985, with refrence back to Feedback MCU's and EECIV. That taught good service technicians to Know the System 1st.

In very specfic langauage, TSB for October 1984 it lists the EECIV as being a very low reference voltage system, and that probing it using non Ford systems was not advisable as the base reference voltages had chaged from 9 to 12v of the MCU and EECI, II and III's to a working range of 5 volts or less with some of the parts the EECIV .

Haynes, AutoZone, Tomco et al just regurgitate non copy write FoMoCo stuff, and a Knock Sensor can take 5 volts, but its not supposed to poll more than abot 3 volts nominal. Its like taking a 1250 page book and trying to sumarise it in 1 page... just doesn't work that way. For example, the 1980 EVTM for the VV 7200 carb was just 245 pages

The Seven Woes main problems are

1. the ignition switch for partial separation (check it)
2. you MUST keep a spare TFI magnetic pickups (Everyone thinks it is the TFi igntion modlue that controls the timing, normally its not that. But keep a spare TFi module too.)
3. defective coils (much less common),
4. defective ECM control box (Very common now; non functional Capacitors,or a wrong earth on the O2 sensor causing a problem on the green PCB )
5. bad earths. Especailly to the Fuel Pump, Ignition, block to body, and ECU to its 60 pin wiring junction.....(wires....Ford had some major funding and backing up the training of the floor staff which stopped its wire supply and fuse boxes being 100% "Quality Is Job One"....it had less "at birth" goodness in its wire looms compared to GM and Mopar...espeacily the earth staps and over time, the exceptionally great Ford model Year EVTM info has been jumbeled up by AutoZone and Haynes, with out of date or wrong Electrical Diagams which on Fords, are always specific to the year and application),
6. Wrong postioning of the critical VECI details for your State, Application and Year. EGR especially, which always requires some periodic cleaning during a truck or cars life. The way it came from Ford was 99% correct, but often people replace parts (especially IAC partys) with non spec Resitor equiped parts which are subbtly different. That is more an issue with 1985 to 1990 Fox Ford Mstangs with any kind of Throttle Body or Port EFi... trucks are normally a little easier)
7. Not following the proper Star OBDI KOEO "Key on engine off", KOER "Key on engine running" process.

The fact that its linked, but so few people look, shows a lack of desire to respect Fords ordained 1980 to 1994 OBD 1 diagnostic protocol

http://vb.foureyedpride.com/showthread. ... ost1885657
Haystack said:
Ive linked this video probably 1000 times. It has 39 veiws.


xctasy said:

I love that clip.
"Key on engine off, 1987 bird codes"
Published on Feb 27, 2017


And "Koer test 1987 tbird"
158, ooops, 159 views
Published on Feb 24, 2013"


The 1990-ford-f-150-truck-electrical-vacuum-troubleshooting-wiring-manual is best.

Ben Wolf from FSP and the Four Eyed Pride has a list of functional EECIV outputs, that's probably what Haynes copied.

Older PC/ED manuals cover more of the EEC-IV era but sadly I don' have them on PDF. I can research if you need (...if I can find them in my big unsorted pile of books, which is in my big unsorted pile of automotive tools, which is holding up my big semi-sorted pile of building supplies...).

I go back to the late '80s in these manuals, with significant coverage back to the late 70s via other, related manuals

Search these file names with Ford EECIV in front

PC-ED S1 Sensor Descriptions.pdf
PC-ED S3 Diagnostic Charts.pdf
PC-ED S4 DTC Charts.pdf
PC-ED S6 Ref Values.pdf


Active member
xctasy":18br78r5 said:
The 1990-ford-f-150-truck-electrical-vacuum-troubleshooting-wiring-manual is best.
Thanks, I ordered a copy of that manual.

Apparently, some have had success with the TF-K141 knock sensor as a replacement. I ordered one, installed it, but my KOER code 25 remains, as well as my other symptoms.