# ECU logic



## bscloutier (Jun 24, 2004)

Does anyone have any background with the Nissan ECU (c1991 23710-88G11) to be able to offer any assistance in determining why there would be no injector signal (constant 0V) and therefore no start? Engine starts promptly with starter fluid. Fuel pressure is proper and fuel pump active. The crank angle sensor emits strong and clean signals for both the 1-degree and 120/180 degree outputs. Signals checked with oscilloscope at connectors at engine. ECU reports code 55 and thinks it is operating properly.

What else would the ECU be looking for before it would provide valid injector signal?

I am not convinced that this ECU is faulty and considering the cost of a replacement and the age/condition of the vehicle, I am not excited about merely trying a replacement.

Are schematics or connector pin outs available for this ECU? I would like to confirm signals at the box.

Any help for this challenged engineer would be appreciated. A running truck would help clear 500' of driveway of impending snows.

Thanks,

Bruce


----------



## bscloutier (Jun 24, 2004)

*Success*

[My final post copied from another thread]...

So since I believe that follow-up is important, I'm taking the time to post what I found and what was crippling my truck for all of these months.

Recall that my SEV6 began to not start reliably back in the spring. It would ignite and run for a second or two and then die and not restart again. Once and a while after persistent attempts it would take off and run just fine. Eventually it fell quiet.

Since a year or so ago I had substantial work done on the fuel system (new lines, sending unit, etc. ), I suspected a clogged fuel filter. A new filter and subsequent fuel pressure checks confirmed that it was not a fuel availability problem (electrical or otherwise).

At the plugs the spark was good and for good measure I replaced the distributor cap and rotor. We were still dead in the water.

I checked all of the fuses that I could find. I also visually inspected the fusible links at the battery. All appeared fine.

I turned to the E.C.C.S. (ECU), removed the passenger seat and read off the code. It reported 55 (normal operation). No help there.

I pulled an injector wire and threw the meter on it. There was no evidence of signal. Realizing that things happen pretty quickly at the injectors, I pulled out the oscilloscope and confirmed that there was absolutely no departure from ground (no signal) there. I noted that the lines floated (a couple of volts) when the key was off but snapped to ground once the ignition was turned on and stayed there.

I was not planning on blindly replacing a $600 computer only then to find out that it wasn't the problem.

A search of the Internet for ECU schematics turned up very little. In what I did find I noted that almost always there was a separate fuse in line with the injector circuit. I searched the truck again for a hiding fuse and there is none. In the Nissan wiring that I did find I noted that a few of the fusible links at the battery lead directly to the E.C.C.S..

So, I decided to remove the fusible links and electrically test them. They looked absolutely fine. There was no discoloration, swelling or anything apparent.

When I went to separate the connector for the grouping of three brown links from the terminal block connected to the battery the plastic (which appeared perfect) cracked and white powder from corrosion came out. In fact that connector was so corroded internally that I could not remove it.

Removing the terminal block instead. I found that two of the three brown fusible links provided resistance in excess of 15K Ohms. They both are wired in parallel to one connector clip.

I had to destroy the fusible link assembly to remove it from the terminal connector. One connection was extremely corroded. Electrically the fusible linkage itself was fine. I purchased a new fusible link assembly (<$20) however they had to backorder the terminal block. This morning I managed to disassemble the original terminal block, clean, and re-tin the corroded terminal.... almost as good as new.

Even though the battery was down, the engine fired up in a couple of revs... end of story.

Now after sitting for 6 months, I could barely get the gear shift to move and the brakes and steering are stiff (rust). Yep, it's an old girl/guy (1991) and clearly nearing the end... but it lives once more and I am relieved that I will be able to plow this driveway again this year without buying a new truck or getting the fancy John Deere utility tractor (that I'd love and almost had my wife convinced that we needed).

Thanks for all of the suggestions here. I do appreciate it greatly. I'm a happy camper this morning!

Bruce

:jump:


----------

