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Discussion Starter #1
Still an ongoing problem. Been through multiple NEW distributors. Everything is wired right. Tested multiple known working ECU's, problem still persists.

Did an MPFI swap from DPFI on my 90 Civic DX hatch. Original dizzy fried itself, new ones won't start. Pump primes, CEL comes on and goes, it will crank, gets spark, fuel is getting to the rail. Proper time It will sound like it starts for a second then it dies out. Injectors aren't pulsing past the original key turn. You can prime it a number of times and it comes to life a little longer. But that's the extent of it.

D15b2 with MPFI swap using a6 intake and TD-02 obd0 dizzy and pm6 ECU.

No immediate engine lights, but upon cranking it for 20-30 seconds code 8 for TDC will pop up. It sure if that's related or not.

I'm able to unplug the CKP wires from the dizzy plug and it fires right up in limp mode with Code 4. Doing so, code 8 doesn't reappear. However, if you plug the CKP wires back into the dizzy plug, it's back to no start at all and TDC code ONLY comes on after a lonngg time cranking. The car has been down over a month now and I need to figure this out ASAP.

Everything was running perfectly fine before the new distributors. I've been through three brand new dizzys, and one used one from a CRX to try and figure this out. Have tested two known working PM6 ECU's with no luck.

Yes, I am running obd0 injectors with the resistor box. Like I said, everything ran GREAT for awhile till the dizzy fried.
 

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Discussion Starter #3
Would be easier to keep all of this in the same thread so people helping can remember what you've already tried. Did you check your mechanical timing?
Mechanical timing has been checked about a million times and confirmed. Even went so far as to advance/retard it a tooth or two in each direction just to see if that made any difference. It did not.

It runs flawlessly up to 3.2k RPM (fuel cut from limp mode) if you pull out the CKP wire from the plug to jump it in limp mode. Only spits out code 4 if you do that.

Otherwise the injectors don't seem to fire at all past initial key turn/prime with everything plugged back in.

All sensor wires for the dizzy read 350~ohms and have no continuity to body ground as stated should be the case by the FSM.

All my wires for the dizzy have continuity from dizzy plug to ECU plug, and I've looked it over about a million times over the last month that it's been sitting. Plus the added fact it worked fine for awhile prior to all this.

Is there a chance main relay controls injector pulse? It's the only thing I haven't touched yet aside from the injectors resistor box. My fuel pump primes, CEL comes on for a couple seconds at key on then goes away. I'm obviously getting fuel to the rail, just beyond that the injectors don't seem to fire/pulse.

I've been through spark plugs, wires, multiple dizzys, tried two known working PM6 ECU's, etc. I'm pretty sure all of that is ruled out at this point.
 

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93 Civic HB SI
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Depending on your specific schematics, the main relay can control 12V to the injectors, other times they could be fed 12V from a common power source from a fuse box or from a separate ECM commanded relay. Schematics will tell you how the injectors are fed 12V.

The ECU pulses ground to each injector individually. 12V at injectors should stay constant while the key is on or while cranking, again this 12V attribute depends on schematics.

The fact that it runs perfect, aside from limp mode with CKP unplugged, is proof enough that the ECU can control basic engine operation, even in a limited input state.

Everything should be fine, and it's even reporting a correct fault (code 4) so it is aware CKP isnt present when it does start.

If that's the only fault you are getting when it does run, and plugging CKP back in causes it not to start, it obviously isnt liking something it is seeing from CKP circuit.

About the only thing I can suggest to check now with the tools you have is to verify all of the distributor rotation sensor circuits with a voltage check.

With key on engine off, plug the dist back in with all rotation sensors connected. Backprobe those circuits at the dist connector, check the following:
  • One multimeter lead to each rotation sensor wire, one at a time, the other multimeter lead to ground
  • One multimeter lead to each rotation sensor wire, one at a time, the other multimeter lead to power
  • Both multimeter leads across each rotation sensor + and - wires.
Perform these checks in DC as well as AC. You should see ZERO voltage on any of these wires, in any of the above testing configurations, while the engine is off. Look for any issues on the CKP circuit wiring in regards to voltage.

Do the same checks while you attempt to crank the engine. This time you will see some small AC voltage generated from the rotation sensors. Again, your looking for anything out of the ordinary with CKP compared to the TDC and CYP circuits.

This is all I can suggest before moving to an oscilloscope to visually see what's going on.

The reason lack of CKP circuit causes limp mode is the CKP signal is a high resolution engine rotation feedback signal that the ECU uses to follow the ignition timing maps in regards to RPM. It wont allow the engine to rev beyond a safe range or advance timing beyond a safe range without this signal present.

All things I'm hearing is something funky is going on with this signal to the ECU. You've done fantastic ground work so far narrowing things down. Most people wouldn't have even tried to isolate the rotation sensors from the dist connector! Kudos on your efforts so far!
 
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