Obd1 CEL code 12 help (1 Viewer)

[Pz10420]

So my 93 270k mile 1fz is giving me an issue I haven't had much luck identifying or finding much similar info on the forums. I have gone through the troubleshooting steps in the fsm to the best of my knowledge and still no resolve. so occasionally when I am cranking the engine it almost sounds like it skips a beat with a strange "weep, weep" sound. I can just keep cranking and it'll start after a couple more seconds. I know the code 12 is no rpm signal to the ecu in the first 2 seconds of cranking. Also of note, once she fires up she runs perfect without any issues. Most everything has been baselined on the truck over the last few years and here's what I've done so far for troubleshooting.
Ohmed out wires from distributor to the ecu: no resistance on any of the 4 wires.
Ohmed pickups on distributor and all were within specs and checked gapping also within spec.
Installed spare distributor and had the same issue
Tried a spare ecu and had the same issue.
Most other ignition components were replaced in the last 6 months(plugs, wires, cap and rotor, new fusible link, rebuilt the starter, all oem parts).

I only have 2 possible trains of thought left
Ignition switch(although rpm signal doesn't seem to run through that at all but may have to do with the starter losing voltage or something)
It has an aftermarket remote starter installed and the factory security system I removed years ago.
Thanks for any help

 

[CJF]

I know the code 12 is no rpm signal to the ecu in the first 2 seconds of cranking.

So what does the FSM say are the most likely culprits?

 

[Shoredreamer]

I had Code 12 on my 93 as well and the same symptoms of weird clunky start. It did it every 10 or so starts. I spent way too much time trying to diagnose it. In the end it was my start battery. Dropped in a new battery and all was good. Never had that weird starting again.

 

[Pz10420]

Hmm. I have a dual battery system that the 2nd battery is a dual purpose battery but I suppose that could be an issue. My start battery isn't that old either but it is a Walmart battery and I've found them to mostly be trash.

 

[Shoredreamer]

Does the weird start happen every time you crank to start or intermittently? I did not expect mine to be the battery either as it was not old and did not have signs of it being the problem.

 

[Pz10420]

It is intermittent. But has been slowly getting worse over the last 3 months or so. Mostly does it when the truck is cold, doesn't usually do it once the truck is warm but has started to occasionally also do it while warm.

 

[LandLocked93]

Any chance you can get the 'weep-weep' sound on video and post? I'm thinking it's like a horse whinny/dog wimper?

 

[Pz10420]

Any chance you can get the 'weep-weep' sound on video and post? I'm thinking it's like a horse whinny/dog wimper?

I’ll try to figure out to upload one tomorrow. I tried today but the file was too large.

 

[Shoredreamer]

It is intermittent. But has been slowly getting worse over the last 3 months or so. Mostly does it when the truck is cold, doesn't usually do it once the truck is warm but has started to occasionally also do it while warm.

yeah that sounds exactly like what I had.

 

[mudgudgeon]

If the battery is a suspect put a multi meter on the battery while you crank it. Watch the voltage. Should not drop below 12volts under load.

 

[Pz10420]

I'll try that this afternoon.

If the battery is a suspect put a multi meter on the battery while you crank it. Watch the voltage. Should not drop below 12volts under load.

I'll try that this afternoon and report back. Thanks

 

[Malleus]

I'm curious about your statement that there was no resistance in the wires between the distributor and the ECU. There should be. No resistance means you have an open circuit. Maybe I'm misreading your statement.

 

[Pz10420]

I'm curious about your statement that there was no resistance in the wires between the distributor and the ECU. There should be. No resistance means you have an open circuit. Maybe I'm misreading your statement.
View attachment 3501453
View attachment 3501451

I am speaking about the wires directly. I disconnected the plug from the distributor as well as the ecu and just measured the resistance of the wires to make sure the conductors from the ECM to the distributor were good. And they all ohmed out at less than 1ohm of resistance. I did not take the ohm readings as stated above. I believe the test you have posted is actually measuring the resistance of the sensors in the distributor. I did this directly at these pins right on the distributor plug and they all tested within the above specs. Thanks

 

[Malleus]

I am speaking about the wires directly. I disconnected the plug from the distributor as well as the ecu and just measured the resistance of the wires to make sure the conductors from the ECM to the distributor were good. And they all ohmed out at less than 1ohm of resistance. I did not take the ohm readings as stated above. I believe the test you have posted is actually measuring the resistance of the sensors in the distributor. I did this directly at these pins right on the distributor plug and they all tested within the above specs. Thanks

You also need to measure the resistance at the ECM connectors. This is part of the correct inspection procedure. Unless you do this, you won't know the signals are getting where they are supposed to go. You can do this with a multimeter. You need an oscilloscope to measure the signal directly, but that's unnecessary at this point. All the steps in the FSM are necessary; you can't skip and expect to solve the problem.

There are four likely root causes of the DTC 12 code. You need to go through the inspection procedures for each one in order to eliminate possible root causes.

 

[Pz10420]

You also need to measure the resistance at the ECM connectors. This is part of the correct inspection procedure. Unless you do this, you won't know the signals are getting where they are supposed to go. You can do this with a multimeter. You need an oscilloscope to measure the signal directly, but that's unnecessary at this point. All the steps in the FSM are necessary; you can't skip and expect to solve the problem.

There are four likely root causes of the DTC 12 code. You need to go through the inspection procedures for each one in order to eliminate possible root causes.

I understand what you are saying but what I did equates to the same test. By measuring the resistance of the wiring harness from ecm plug to the distributor plug and eliminating that as having any issues then testing those same pins at the distributor instead of the other end of the harness and getting good results. I could bring my oscope home from work to get a true test of the signals while it is running in case the resistance through the harness is greater once there is a load going through the conductor. But I am not sure what a good signal would look like on the oscope since there is no reference that I could find in the fsm. Of the 4 possible causes that is listed I have eliminated the:
distributor
distributor circuit
ECM
Starter circuit is the only one that I have not entirely confirmed is trouble free.

I appreciate the help and I will perform the test later exactly as it is laid out above but there is no reason that it should change the results that I already have gotten. I use a multimeter on the daily for troubleshooting, so that is how I have performed all my tests so far on my 80. I did not just do visual inspections of said components.

 

[Pz10420]

 

[Pz10420]

Here is the test I performed which as you can see has the same resistance values as what you have posted at the ecm connectors.

 

[largeone]

My bet is on the battery...and depending on your Dual Battery system, you may find it happens again. I ended up trashing my dual battery system as I had nothing but problems for the years I had it. Now that I'm using a completely separate lithium setup to run all of my accessories, not tied at all to my starter battery, I have not had a single problem. It would be worth a try to disconnect the dual batter system and confirm your starting battery is good, and see if you still have the problem? Your starting batter may already be affected, so I would start by making sure it is fully charged and can put out the amps required.

 

[Pz10420]

So it looks like my start battery may be junk. It was only at 12.2v before cranking and dropped to 10 volts before she started up. Like I said it is a crap Walmart battery that I think is only about 4 months old. I just picked up right before a trip because I needed something quick but figured it would last a little longer than that. I've had my dual battery setup for years and never had an issue with and my house battery is a blue top optima that ive been running for almost 7 years. And my previous start battery was an interstate that lasted almost 5 years. I will pick up a new battery soon and report back if this seems to cure my issues.
Much appreciation for all the suggestions!

 

[BakerFJ]

Did you end up finding the issue?