codes 18 and 28

[squishyss]

hi guys, I'm new to this site, so I'll give you a low down on my truck. I have a 91 2.3l 5spd truck. I bought it a couple of weeks ago and have changed all the fluids, plug wires, timing belt, water pump, and t-stat. I have been weeding out dtc's and only have two left that I am having some problems with. my tach only works when it wants to and I always have continuous memory codes 18 and 28. the 18 code is for the IDM/tach circuit. I tested the circuits as per alldata's testing schematic and can't find any wiring problems. I found some corrosion in the top connector of the dis module, and cleaned it up, but this didn't fix it. are these dis modules easily ruined by corrosion or other electrical problems? I have checked the rest of the engine, and computer harness and can't find any corroded terminals or bad splices. The 28 code is for dual plug inhibit, which according to the test is related to the 18 code. the truck runs pretty good, but seems a little low on power, and is only getting 18 mpg on highway. any help would be greatly appreciated. thanks

 

[Dave R]

The modules are suseptable to problems due to heat and age. Both your codes could be related to a failing module. If the IDM circuit is iffy then the computer will be quite confused by the info that it is receiving, it acts, in part, to confirm that the computer's ignition related instructions are being carried out correctly.

To test the dual plug inhibit circuit, check to see if the driver's side plugs are firing.

And, welcome aboard,

 

[squishyss]

thank you. I know the pass are working, I'll check the drivers side tomarrow when I change the plugs. you sound knowledgable about this system. did you work for ford? I am a chrysler tech, but I've never seen a system that works like this.

 

[Dave R]

did you work for ford?

No, I have just been diagnosing this system since the mid-'90 both on my own vehicles and on many others via this, and earlier, forums. The system is not that different than most any other dual plug, or even most waste spark systems. Ford just does things slightly differently.

After about 5-6 years the modules just get iffy. Other than age and heat, I've never found another commonality. Remote mounting them generally makes them last quite a bit longer. I just look at them as a wear item, every 5 years or 75,000 miles change the timing belt and ignition module. It's a bit wasteful but, cheap insurance for a motor that, with regular maintenance, can reach 300-400,000 miles.

 

[Joelzme]

Hey there,

Thanks for this thread. I have the same model year/engine and the same problem. Autozones website says to reinstall the IDM on the fender apron rather then back on the manifold, probably because of the heat issues you mentioned. Do you know if this will cause grounding issues? I am very new here and I am trying to learn a lot about trucks in general. I have this truck as a beater and enjoy learning this stuff.

Thanks for any help.
Joel