AMC Eagle Den Forum
The Mighty 258 => Engine Electrical Systems. => Topic started by: Freeagle on November 25, 2022, 03:29:07 AM
-
1984 to 1991 (and possible some 1983) 258's have a relatively high 9.2:1 compression ratio and use a knock sensor screwed to the intake manifold. The knock sensor produces a small voltage (millivolts) when knock occurs. Every original knock sensor that I have tested produces zero voltage while knocking on the manifold or the sensor with a metal object and has allowed my engine to knock...sometimes rather hard.
The ignition system on these engines is really amazing. The ECU advances the timing up to a maximum limit and keeps the timing set at that limit unless it receives voltage from the knock sensor. The ECU then counts five more cylinder firings and retards the sixth, which is the cylinder than the knock signal was received for previously. It will keep retarding that cylinder until no knock signal is received. It does this for all six cylinders so that each cylinder runs at its own unique ignition timing setting that is on the ragged edge of knock for peak power output. The ECU doesn't know which cylinder is which, but it knows to wait for the next firing of that cylinder to retard the timing for that cylinder.
Guess what happens if the knock sensor goes bad and the ECU never receives a voltage to indicate knock? It stays at the maximum timing limit and lets the engine pound away until it melts pistons.
So a working knock sensor is critical unless you want to retard your timing way back and give up performance.
Standard Motor Products part number KS-1 Interchange it to other manufacturers as needed. It lists for 1980's Dodge four cylinders, as I recall. SMP doesn't publish what its resonant frequency is, but it cannot be too far off the 5500 Hz of the original AMC sensor. After all, engines tend to all knock at about the same frequency according to my ears.
The mounting stud is a smaller size than the original AMC sensor, but don't let that scare you. It will still thread into your intake manifold, but be careful! Do not tighten it down any more than snug. The threads will grab enough to snug the sensor down and hold it in place, but you will strip the threads in your manifold if you call upon your lube tech training and tighten the living :censored: out of it.
I haven't heard any knocking since I installed one in mine.
-
I've always been curious why AMC opted to mount the knock sensor way out on the edge of the #3 intake runner. Seems like a pretty distant place to be trying to listed to noises in the head.
I'm an '82 so my knock sensor is aftermarket from Carter, but from the late 70's. It sits directly on top of one of the head bolts.