Hi, I just recently purchased a 1991 ranger 2wd 2.3L with DIS. The CEL is always on and my tach does not work. Below are the steps I have followed.
1)pulled code "48"(no IDM to PCM)
2)did continuity test to tach and to PCM....checked OK
3)did "short to ground" test on IDM output wire(pin12 of ICM).....checked OK
4)disconnected tach from instrument cluster(to verify tach not faulty)...still
NO IDM to the computer.
5)checked voltage at pin 12 of ICM....steady 12.5V running engine
6)replaced ICM with used Jyard unit......still same problem
7)replaced ICM with ANOTHER Jyard unit.......still same problem!
So, I have 3 Ignition Control Units that will NOT produce an IDM output?!.......this could be true, but I doubt it.......is there anything else(like an ICM input) that could cause it to not produce an IDM signal?......I have tested both LH and RH coilpacks, they are good....TPS OK/ MAF OK/ ECT OK/ PFE OK/ IAT OK/ etc.......
Could it be the CMP(crankshaft/cam position sensor?)....is it true that sensor is split into two signals, 1 for the PCM and 1 for the ICM?.....what if one was missing....would the truck still run normally?....I'm out of ideas, and that is the last sensor I can think of that sends a CID signal to the ICM, but is the CID signal actually used for running operations OR is it simply for the ICM to output the IDM?...........
Any info would be great before I go and blow $240.00 for a NEW ICM at CarQuest, especially if this is not the problem.

 

The cam sensor generates the CID signal, and sends it to both the DIS module and the Control module. This is for cylinder identification.

The crank sensor generates the PIP (profile ignition pickup) signal, and also sends it to the two modules.

The DIS module generates the IDM signal and sends it to the control module. The signal is compared to the spout and determines which coil to fire.

This signal is used for the coil and tach.

Dr. Probst says if there is a problem with the IDM, it is either the DIS module, or the wiring to the control module.

Code 48: Engine running conditions = Measured air flow high at base idle

Continuous Memory = DIS coil pack 2 circuit failure or loss of secondary tach (IDM)-lft side;

Coil 3 primary circuit failure.

Depending on the fault, it will set the CEL and if the fault is severe enough, it will go to "limp home mode." jd

 

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See you on the boards. ….

 

Thanks for the reply.....the code 48 I got was from a KOEO test, so it is the loss of IDM LH side. I tested the LH coil primary windings and got 0.6ohms per coil, I would assume this is good. But I also disconnected the harness from the LH coilpack and the truck starts and runs exactly the same with same fault code 48.......If the secondary resistance of the coil is wrong, could this cause the no IDM?...........I'm basically stuck at a few options....1)replace entire computer (pcm) 2)replace LH coilpack.....3)replace CKP(crank) sensor...........it has to be 1 of the 3.........I also noticed that the PCM is a "remanufactured" unit....this cannot be promising. Also I get intermittant faults stored in memory usually about code "51" and "34"....these are "PFE sensor---EGR" and "ECT-coolant temp sens open circuit hot engine"..........recently I found the rear engine ground strap to body ground disconnected, I reconnected it and now my idle jumps to about 1500rpm at a stop and settles down to 700rpm after 5- 10 seconds.........could be a coincidence, but erroneous fault codes and screwy idle seem to be leading me to a defective computer?

 

Answer = LH coilpack secondary coil toast.....so if your tach dies + code 48....check your coilpack.......now onto the high idle problem and low speed stutter(engine under load) that don't throw any codes....

 

Good deal. A relief, huh? jd