Hello its been awhile since I've posted on JF since I have been spending many months engineering a bellhousing for my engine swap. No one as far as I know have done this swap and I can't wait for it to be done.
But one hurdle I'm trying to over come during my 1998 XJ swap is I want to use the stock tach. After sending an email to Dakota Digital about there converter I wanted to use to convert a gear sensor on my engine to a signal for the tach. The guy at Dakota told me that...
" While we have the DSL-2 that can read the teeth, there is a problem with the way Jeeps reads the tach signal. Jeep used a staggered tooth crank sensor that indicated RPM and TDC for the ECM and in 1996 started using CAN bus to feed a signal to the dash cluster.
What info does the computer need to see to operate the factory tach?
I have heard that when they went to coil packs (1999 or 2000) that the computer needs to see a signal from the CPS and Cam Synchronizer. Also heard that if you don't get both not only will your tach not work but your A/C won't either.
If anyone has any flex plates or flywheels laying around and would like to donate them (I'll pay shipping) so I can have them in hand to measure to find out if the teeth are all the same degrees and widths so that I will have options of using either flywheel teeth or the flex plate band. If its old/broken flexplate, or worn out flywheel, with damaged starter teeth or needs to be resurfaced that will work just fine.
The tach signal input is provided by the ECU based on CPS readings. So the ECU connector is where you get the tach signal. The teeth setup is irrelevant to the tach; it only has to do with the CPS. Must the CPS be specific to the flywheel setup? that I don't know.
If your ECU and dash are originial to the vehicle, and ECU gets the proper input signal from CPS, then I don't see why the tach wouldn't work...
The crankshaft position sensor is mounted to the transmission bellhousing at the left/rear side of the engine block. Engine speed and crankshaft position are provided through the crankshaft position sensor. The sensor generates pulses that are the input sent to the powertrain control module (PCM). The PCM interprets the sensor input to determine the crankshaft position.
The flywheel/drive plate has groups of 4 notches at its outer edge. The notches cause a pulse to be generated when they pass under the sensor. The pulses are the input to the PCM. There are 3 groups of 4 pulses generated on 4.0L 6-cylinder engines, and 2 groups of 4 pulses for 2.5L 4-cylinder engines. The trailing edge of the fourth notch, which causes the pulse, is 4 degrees before top dead center (TDC) of the corresponding piston. The engine will not operate if the PCM does not receive a crankshaft position sensor input.
So the notches are in time with the pistons. 2 pistons are at TDC (top dead center) for each set of notches. Then they use the cam position sensor to time the injectors. The cam turns half the speed of the crankshaft.
Yeah that would have been helpful ohh well ha ha, I will just add that info in here also...
Note that the CKP (aka CPS) is the source for engine speed and the Cam Sensor is used to synchronize the fuel injectors.
CRANKSHAFT POSITION SENSOR
The Crankshaft Position (CKP) sensor is located near the outer edge of the flywheel (starter ringear).
Engine speed and crankshaft position are provided through the CKP sensor. The sensor generates pulses that are the input sent to the Powertrain Control Module (PCM). The PCM interprets the sensor input to determine the crankshaft position. The PCM then uses this position, along with other inputs, to determine injector sequence and ignition timing.
CAMSHAFT POSITION SENSOR
The camshaft position sensor is located in the distributor on all engines.
The sensor contains a hall effect device called a sync signal generator to generate a fuel sync signal. This sync signal generator detects a rotating pulse ring (shutter) on the distributor shaft. The pulse ring rotates 180 degrees through the sync signal generator. Its signal is used in conjunction with the crankshaft position sensor to differentiate between fuel injection and spark events. It is also used to synchronize the fuel injectors with their respective cylinders.
Came across this while back to figuring out tach options in my 4bt, been a few years and I want a tach!
I don't know about the 4.0 but using the 2.5 ecu originally in my TJ the crank signal alone is not enough for a tach signal, you will need to replicate both the crank and the cam position signals to get the factory tachometer to work. I have not yet made mine work with just the crank signal and the only company that I know of that makes a kit for the 4bt to use the stock tach replicates both the crank and cam position signals.
I did similar to you and replicated the factory crank notches by machining them into my crank pulley. Unfortunately I disposed of the stock distributor and removed all the wires for the cam position sensor so I have not yet wanted to spend the time repinning the ecu connector to get it to work that way. At this point he novak adapter and DSL-2 is going to be my best option as a thrown serpentine belt also just destroyed my crank position sensor and wiring but novak is on back order and next week I won't care about a tach.
The signal on obd2 jeeps from what I have read is all CAN based, there is no direct signal to the tach that could be used as an input from a coil or some sort of tach converter box. For me I have no coil it is diesel.