Brain Cramps - power in - nothing out

[tod914]

Well, car is dead in the water again. Was visiting a friend in NY state. Was getting some backfiring and had some idle issues when I started out from home. On my return trip, the car ran for about 10 minutes then shut off. It wasn't the relay board causing the problem. We tried swapping it with a good known unit. It was then sent to a local Porsche mechanic. The shop owner that diagnosised it, noted that power is going to the brain, but the brain is not sending a signal/power to the injectors. We tried swapping relay boards, trigger points, has a new distrib. from ottos, fuel system completely redone and cleaned. This weekend we are going to try to put my brain in another 914 to verify it's faulty.
A few people told me that brains very rarely fail. Any thoughts on what would cause failure? Power surge? Any ideas on what to check so I dont fry a new unit?

Thanks, car is a 75 2.0 w/pertronix

Tod

[Dave at Pelican Parts]

Two things, generally. Physical damage, like getting smashed or having water seep in through rust holes in the case; or an output transistor fails and the injectors come on but never shut off.

Other stuff can cause failures, but that is even less common.

Make sure that the grounds are good...

--DD

[pbanders]

I'd really doubt the brain is dead. I have some ECU's that show severe water damage on the boards (all the overlayer peeled away, hard deposits all over, etc.) that still function perfectly.

Did you check for voltage at the ECU plug on pins 16 and 24? And as DD says, did you check the grounds the junction in the back of the engine? Your problem could be a wiring harness fault. No voltage at the ECU plug on 16 and 24? Check back to the output pin on the relay plate (Pin I on this diagram: http://www.pelicanparts.com/914/parts/Electrical/914_electric_73E.jpg ). Voltage present? Bad wiring harness. No voltage? Pull out relay 74, check pin 30. No voltage? Bad relay board trace or bad wiring back from terminal 12.

If you verify that you have voltage at pins 16 and 24, and the grounds junction is in good shape, and you have continuity on the grounds in the harness, do the following. Turn the key to "on". Slowly open the throttle by hand. Do you hear the injectors clicking? If so, your ECU is getting power and is functioning properly. No sound? Pull the plug to the trigger contact points and short the center pin to either of the outside pins. Open the throttle again. Injectors clicking now? You have a bad set of trigger contact points. Still no clicking? Possible bad ECU. Swap with known good ECU to check.

[tod914]

wasnt able to swap comps yet. we did however go through all the wiring checks. it seems like the injectors are not getn enough voltage to spray. they click, but no fuel. they are getting max of 2 volts. what could be causing the drop in voltage?

[Dave at Pelican Parts]

Bad connections. Either on the power supply, or on the grounds, or on the injector wires. Check both ends of all of those. Verify a good solid +12V on the ECU connector end of the power supply wires. Check the resistance of the ground wires--should be as close to 0 as your meter will reasonably measure. Check from the injector ground connection (at the injector plugs) to the battery negative post, too.

--DD

[tod914]

well,

we tested the brain in a friends car and my unit was bad. water or corrosion are not the cause. what other possibilities are there for failure? anything i can check before i install a new unit to reduce the risk of frying it?

[pbanders]

If the ECU is dead, it's very unlikely that there's anything you can do to repair it, unless you have experience at electronic repairs and have access to the required tools, instruments, and spares. Common failures in ECU's are dead injector output driver transistors (which are germanium and not so easy to locate), shorted electrolytic caps, etc.

I'd just junk it.

[tod914]

Brad,

What I meant to ask was, is there something in the electrical system that could of caused the brain to go bad. Electric surge, short someplace that I could look for prior to putting in a replacement unit.


Thanks.


I hope it is just a plug a play fix.

[3D914]

Tod,

Thats the first ECU failure I've heard about.

If you need one, I have two working spares available - reasonable.

Good luck,

[pbanders]

Sorry, I misunderstood. I'd check the injectors to see if any of them are shorted (zero ohms). You should read about 3 ohms.

[froggy]

First thing I'd check is the auxiliary air regulator, there is one wire that comes out from the bottom. I have known it to come out totally. Also I have seen it short out from where the two wire connections come together when they make contact with sheet metal. This will cause a complet shut down of brain activity and engine function.

[tod914]

ok, the aar was replaced with a rebuild unit from AA about 2 years ago. The original 1 was bad and was shorting out the fuel pump.
It also melted the wire. That wire was replaced. We'll check the injectors again, but Im 99% sure they are ok. My brain caused the same problems in my friends as mine. So, hopefully the replacement will work and solve all the problems. I was in hurry to get a replacement and should have it by Wed. I did try PP but, it would of been about 2 weeks to get to the door . Kind of sucks when your in a rush to fix something. Anyway, thanks for the responces/help. I let you all know how it goes later this week.

[Dave at Pelican Parts]

Actually, Froggy, it will only cause the fuel pump to not function. The car still won't start, but the injectors will still click...

--DD

[pbanders]

A shorted AAR blows the 25A fuse on the relay board, disabling fuel pump and the heater blower (and the rear window heater, on those rare cars that have it). It does not disable relay 74, which powers the ECU.

[pbanders]

One thing I noted that's true on the early cars (may be true on later cars, too) is that the ECU is not on a fused circuit. Voltage supplied to the ECU is sourced from pin 12 at the back of the relay plate, and that pin is hot wired to the battery. Nice!

[froggy]

my bad when I had a similar problem it caused my brain to stop functioning porperly not my F\I brain. sorry.

[tod914]

Yup, the computer was at fault. Spent a ton of hours checking the electrical system and the brain was bad. New one is installed and the car seems to be running fine for the moment. Ain't I the optimist. Let's see what fails next. If any one is interested in a original paint concours car let me know. Im at the point where I want to sell it. Including my purchase price, I have well over 23K into it.

[jess p]

Chech the cylinder head temp sensor for proper resistance. This
senor will inhibit ecu function.

[Dave at Pelican Parts]

No, it will merely cause a rich mixture.

--DD