1990 Ford F-250 5.8L manual transmission.
Ever since I replaced my motor, about 2 years ago now, I've been having ignition related and tach related problems. That is ever since I removed the engine harness I've been having these problems
Sometimes the tach needle would twitch, sometimes the tach had a mind of its own but the engine ran fine, sometimes it felt like the whole truck would turn off for a split second and come back to life, the latter sometimes happens when I'm making a turn or when I put my turn signal on switching lanes on the highway and the tach would blip down, the truck would literally jolt from the spark going off and coming back on again. I've even had it happen when its below about 45 degrees F outsides, doesn't matter if the truck is warmed up or not I can't go past about 2.5k rpm's with out spark shutting off and back on and causing the tach to jolt down and back up like a hiccup and truck lurching.

I've addressed this issue with this forum site before, with a mechanic, and others. I've replaced the ignition switch, had the distributor timed by a mechanic, replaced a faulty ignition coil at one point too. Well I still have this issue and to be honest I've been ignoring it. Until the other day, driving to meet up with some friends my truck was acting funny, tach needle twitched a bit, truck shut off on me once on an exit ramp for the highway, started it up fine, kept driving. (Truck seems to have good days and bad days, this was definitely a bad day lol). Driving down the road a while later and truck shuts off on me, thankfully I was able to roll into a parking spot, crank no start, it sputtered once like it wanted to start but no luck. Called a tow.

Went through some diagnostics, checked for spark at the ignition coil, nothing. checked primary and secondary at the IC, as well as voltage, all good. Ran thru the TFI/ICM tests (distributor mounted). 12volts power to TFI, check. 12volts ground to TFI, check. TFI pulse, no pulse. PiP pulse, good pulse. So I have a failed TFI.
Okay so I can replace this TFI and my truck will have spark again. Probably. But the underlying issue causing all this odd behavior; shutting off instantaneously in certain situations would probably still be present.
So I decided my best course of action; post installing the TFI would be to replace some of the crusty wiring and pigtails.
The 6pin connector for the TFI module has seen better days, the coolant temperature sensor pigtail (2-wire) has some missing wire shielding, and the ignition coil pigtail crusty as can be. Decided to purchase these 3 items on rock auto, and I'm noticing a difference between my ignition coil pigtail and the replacement pigtail.
I'll post pictures for comparison.



So really the only difference is the dark green/yellow wire coming from the ignition coil, I'll have two instead of one. This got me searching the electrical diagrams since I didn't know why all the aftermarket connectors for the IC had 2 instead of 1 wire.

I found the ignition suppression resistor, which I cannot find on my truck, I have heard it is tucked inside the wire loom and you can't see it without taking the wire loom apart but IDK if that's true.
Please don't mistake this resistor for the Radio Capacitor, they are two separate items on two separate wires.
I heard you can't find these Resistors anymore, Ford doesn't sell them. I haven't looked too hard to find a new one, and I haven't even checked to see if I have one, or if its any good still.
Would love any and all input from ya'll. Tomorrow I'm installing the new TFI which should get the truck running again. But I guarantee I'll still have this intermittent spark problem, so I'll post again when It's running and then I want to take the wiring harness apart and fix some wiring, OHM out the ignition related wires of the harness.. yadda yadda.
I feel like I'm on the right track here but please let me know what you think!

 

That is true. It (they on some model years) are in a wiring loom near the master cylinder. One is a 22K ohm resistor for the IDM (Ignition Diagnostic Monitor) signal going to the computer. The other, if you have one, is typically 1.8K ohms that goes to the tach.

If the tach is going wild, but the engine is running fine, it could be a bad solder joint or the resistor going to the tach is bad. A faulty IDM circuit will not cause the engine to miss/run rough. It will trigger an erratic PIP code.

A photo clipped from another thread regarding the ignition suppression resistor circuit:



Good reading here: Code 212 in memory



Probably true, but you can use a resistor(s) from any supply house. Nothing really special about them.

For reference here is the ignition diagram from my 1990 Ford EVTM manual:

 

Thanks for the response, For some reason I thought the resistor(s) were in between the ignition coil and the TFI, also I posted 3 pictures in my original post but they are not showing up for me, maybe you can't see them either? I'm gonna repost them here. Will install the new TFI/ICM today, someday in the future I want to relocate this module to the fender wall since heat kills these units. I've heard people mention that the gray ICM modules need these resistors but for some reason the black ones don't. I have the gray one.




Perhaps there's a short somewhere grounding the pulse signal, causing the truck to skip. Could i possibly unplug the spout connector and drive around? if I notice any skipping then I could narrow down the problem to a certain circuit? or would it be a bad idea to drive around town/highway with base timing only.
Also I am not getting any codes, thats partly why this is so tough for me to diagnose, I'm also not the best with wiring and diagrams but i'm getting better at it. I haven't seen the tach skip on its own without the engine skip as well in a long while, I think the last time that happened was around the same time my ignition coil was going out on me about 1.5 years ago. But its still very common for me to see the tach skip and engine skip in unison.

 

You can remove the SPOUT plug and drive the around. There will be no ignition advance since the computer is no longer controlling spark. The ICM/TFI will trigger off the raw PIP signal on it's own.

Please explain "Also I am not getting any codes". No codes at all for the KOEO tests and Stored Codes from memory?

 

what I mean by no codes is the check engine light is not illuminated.

I do have a obd1 code reader, so I’ll check for any codes that may not illuminate the CEL after I replace the TFI module. Been a long day I won’t get to it till late tonight or tomorrow and will update again

 

Installed the new TFI module: started right up.
going to let check for codes with my obd1 reader, might take me a minute to do cause I forget exactly how it works, it did come with a book so I’ll have to refresh my memory with that and I’ll post what I find.

 

@rla2005 I plugged in my obd1 reader and retrieved the codes. KOEO : 11 (pass) : 10 (separation code) : 11 (continuous memory pass)
KOER : 41 HEGO sensor low voltage / system lean

there is a test you need to perform prior to KOER which is the timing advance test, I did it but couldn’t confirm how the engine was running because my Dorman HB does not have accurate timing marks, which was confirmed by a mechanic, who also timed my distributor by ear for me. One of the things I’ve been meaning to get around to, getting a new HB and knowing if the timing really is at 10BTDC or at least close enough

 

I never use the KOER timing check. I remove the SPOUT plug then start the engine. Use a timing light to verify the timing is ~ 10* BTDC. I would ash can that Dorman balancer ASAP.

 

any recommendations for balancers? Ive heard of damper doctors been meaning to check out their work, also looked into Summits website and was thinking about pulling the trigger on this one

https://www.summitracing.com/parts/pbb-pb1214n

the KOER timing check runs the motor 20 degrees plus 10 degrees base timing for approx. 2 minutes (about 30 degrees BTDC) so that the operator can verify the electronic advance is working properly

 

I know what the timing check does. I never use it.

Damper Dr. is the only repair service I am aware of. No experience with ordering a new one personally.

 

I just used Damper Doctor. Ran around $230 with shipping both ways. There are no good aftermarket options for the 4.9 (all junk Chyna made rubber) and was unable to turn up a Motorcraft NOS unit. Turn around time was but a few days.

Lucky you have a 351W so lots of good aftermarket stuff is available even if you chose from Jegs or Summit.

Glad you sorted the issue for electrical.

 

thanks man, I decided to hold off on the wiring harness until I get my timing sorted out, got a new HB coming from Summit (powerbond brand) I’ll get my motor timed and check the timing advance, then start my electrical repairs

I have trust issues with HB’s having correct timing marks now, would really like to find TDC myself now just to confirm the new balancer is actually marked right