I have a 1989 F-350 , 7.5L 460.....check engine light comes on from time to time...pulled code18...can anyone translate code into lamen's terms

thanks in advance

 

My book shows:
Loss of TACH signal to Electronic Control Assembly {ECA}

I think it means that the computer is not getting a RPM signal but I'm not 100% sure.

 

Also check the SPOUT connector to make sure it is in place and is making a good connection.

 

The computer gets two inputs from the TFI module. One is "PIP", the other is "IDM".

PIP is a low voltage signal coming off the Hall Effect pickup. This is the primary mechanism that tells the computer the engine is running, and how fast it is running, and triggers it to fire the fuel injectors. Internal to the TFI module, PIP also creates the signal that ungrounds the coil negative lead.

"IDM" is connected to the coil negative lead through a 20K ohm resistor buried in the wiring harness. It allows the computer to detect that the coil actually fired. The only use for the signal seems to be for the computer to flag or not flag code 18.

The resistor is rather fragile. If you've been rough on the harness, it is easy to break the connection. The engine will run fine, but you will get intermittant code 18's if the break is vibration sensitive, or hard code 18's if the break is complete.

The other way you get code 18 is if the TFI is on its way out. The computer may see PIP but not IDM because the TFI fails to trigger the coil. In this case, besides code 18, you would have symptoms of the engine missing, stalling out, or failing to start, especially when hot.

 

I am having this code 18 problem (which probably explains my pinging under load). I get it during KOER tests. The spout jumper is in place correctly. I think my situation sounds like the first problem...resistor in the harness. How do I change it out? Do I have to replace the harness? Part #s?

 

Follow the wires from the distributor back towards the rear of the engine. The harness gradually gathers more and more wires until it is about as thick as your thumb. As I remember it, the section from the last wire off the engine to the firewall connector is where the resistor is. You will have to unwrap the tape and separate the individual wires. You will find a place with two wires spliced together and soldered to an ordinary 1/4 watt 20K ohm resistor, with the other end of the resistor soldered to the single IDM wire running back to the computer.

You can buy a suitable replacement resistor at Radio Shack, actually a pack of 5 for 89 cents. You want 20K ohms, color code red-black-orange. The precision is unimportant. Either a 1/4 watt or 1/2 watt should be fine.

I suspect that all you will get out of the replacement is that code 18 will go away. My engine ran about the same before and after fixing this problem.

Your pinging under load is more likely to be caused by the static timing too far advanced, lean mixture due to low fuel pressure or vacuum leak, low octane gas, carboned up cylinder heads, or bad MAP sensor.

 

What is the IDM? Can you explain what it is and what it does?

 

IDM = Ignition Diagnostic Monitor

The computer can tell engine position and speed by monitoring the low-voltage Hall effect output on PIP (Profile ignition Pickup). It controls the spark timing by sending the SPOUT (Spark OUT) signal at the appropriate time. The responds to SPOUT by opening the coil primary negative lead. This creates a large voltage pulse in the coil primary (several 100 volts) which in turn is multiplied by the 200:1 winding ratio in the coil to get as much as 40000 volts to fire the spark plugs.

IDM is basically the coil primary negative lead, buffered by a resistor to help protect the computer from the several hundred volts which is sometime present. The resulting lower energy signal then tells the computer WHETHER the coil has fired and WHEN the coil has fired. If the computer is seeing PIP but not IDM, it means a bad coil, bad TFI, or bad IDM circuit. This state of affairs causes it to post code 18.

If the computer sees IDM, but does not see the correct time relationship with SPOUT, it basically assumes the TFI is in bypass mode, and posts code 213 (SPOUT circuit open). The usual reason for this is you left the SPOUT JUMPER out.

 

OK...code 213....is there a corresponding 2 digit code? I have 2 digit codes instead of 3 digit codes.

Basically, I think I have a problem somewhere in this area too and I'm dieing to get some real info on the 2nd 1/2 of the ignition (tfi, pip, idm, etc). I'm pretty good with EFI but not with some of these details.

Is the IDM just wiring or is there a module to it or what? I think I need to dive into this IDM circuit and check for problems and need info on how to do so.

My truck's got the TFI on the fender and not on the dizzy so my wiring is a little different than the older versions.

The pip sensor is in the dizzy. I've had code 18 (KOER) too, and my engine runs like crap. I checked my spout wire and took the plug out and it seemed to get rid of my KOER 18. But I havn't been able to run the truck and check codes since I played with the SPOUT wire. My tfi is fine as far as I can tell (swapped it w/ no difference), and i'm trying to look into IDM problems...I'm using an msd dizzy so my pip sensor is not original. I've had a flickering tach 4 times and each time it's been a little different (my tach needle flickered a little last night and for a few moments it ran like real poo poo).

 

I got it a bit wrong. In two-digit land, Code 18 means "loss of IDM input" (broken resistor) when it is posted as a continuous memory code. Code 18 means "SPOUT circuit open" (missing jumper) when it is posted after the KOER test.

A "flickering tach", "engine ran like real poo poo", and "code 18" posted in CM are all consistent with an ignition system that is not firing the coil properly, for whatever reason. Both the input to the tachometer and the IDM input come off the coil, and act as a basic diagnostic tool for whether the coil is firing or not.

 

I could very well have an improperly firing coil - I don't know if I still have a code 18 as I havn't been able to get into dianostic mode for some reason.

I had it under KOER and it went away when I fixed the spout wire, but it didn't improve the running condition of the engine and it's possible I have a CM code 18 now.

I've noticed that my timing drops out sometimes, as if there is an intermittent lost of SPOUT signal. I havn't been able to look at it much, but I did notice that with the timing light on it it would go from 30 to 10 and back if I played with the throttle.

The IDM is not part of the engine wiring harness right? It's part of the wiring on the chassis.

 

The harness for a fender mounted TFI is different than the harness for a dizzy mounted TFI. IDM works the same for both -- follow the wire off the coil primary negative side. It will dive into a bundle of wires and somewhere within the bundle, between the point of entry and the big connector at the firewall, will be a soldered junction, the resistor, and the IDM wire will start at the far end of the resistor.

 

"big connector at the firewall"....are you talking about the 60 pin computer connector or the connector on the fenderwell that allows you to disconnect the engine wiring from the rest of the main harness?

How can I tell which wire on the coil is the negative one? I've had that engine wiring harness apart before, and I don't remember there being an IDM resistor in it but I could of missed it.

I still have a dizzy TFI wiring harness off my 89 to look at too.

 

I looked up which wire is the negative...I looked up the wiring diagram which included the IDM circuit.

But still am wondering what connector you were talking about...

 

I'm talking about the first connector you get to after following the wires off the coil. It is not the 60 pin connector at the computer.

Rather, the coil wires join the big bundle coming off the left side of the engine. At the inner fender liner, the engine harness joins the chassis harness using 4 or 5 connectors. The IDM wire passes through one of these. The resistor is in between the point the harness leaves the engine and the 4 or 5 connectors on the fender liner.