Brain dead?

tatanka · 10-07-1999, 08:39 AM

My '71 1.7L died at work - with key on, fuel pump does not cycle. Fuel
pump is ok (tested it isolated on batt). Relays are ok (however heater blower
relay blown (?)), and relay board ohms out per schematic. Haynes mentions
"12v on pin 86 but no ground on pin 85 - check main relay" in the FI
diagnosis section. Turns out that pin 85 is the ground from the fuel
pump relay to the brain. The wire from the relay board to the brain, pin 19,
ohms out ok - but no ground at pin 19.

If you can help, here's my questions:
a) have you seen this before?
b) what is the "main relay" ?
c) could a bad pressure sensor create this situation?

Thanks!

Paul

Ascaris · 10-07-1999, 10:13 AM

Just curious, how do you know that the relay is OK? I've never seen a diagnostic for them. Best I can do is plug in a relay I know works in that spot.

Double check the ground on the engine compartment wall beneath the relay panel as long as you are fishing around in there.

If the pump is ok, the relay is ok, you have a good ground, the only thing you really have left is the wiring harness. I'm not much help here since I have my pump up front.

tatanka · 10-07-1999, 10:52 AM

I knew the relay was ok by several tests. 1) when I turned the key, I heard the power supply
relay click. 2) I removed and replaced the heater blower relay while the key was turned on
and heard it click. 3) I bench-tested it by putting 12v across terminals 85 and I think 87,
then ohmed out the switched terminals (schematic not in front of me).

I just went out to the car and replaced the connection to the fuel pump, and lo
and behold it fired up. I'm still suspect of the control module; this is a regularly
intermittent problem. I'm also suspect of the pressure sensor. My only options
seem to be a) replace the control unit anyway, b) get a new control unit and wait
for the problem to resurface, c) wait for it to break for good.

In any event, my original question was asking if pin 19 on the control unit should
always be ground. I'm thinking it should only be ground when the fuel pump should
be running (during that 1-2 second prime, or when the engine is running). I will
work with it this weekend to determine the situation.

Paul

JP Noonan · 10-07-1999, 01:45 PM

I've done some REALLY stupid things to my "brains" and they still work. I've dropped one of them from the top shelf in my garage and I've dead shorted the battery to both of them by useing a wrench that was too long when un-hooking the battery (little "weld" marks on the case). In the Bosch fuel injection book the author talks about only seeing 3-4 dead ones in his career, one was due to the owner putting a screw through the case!

While I don't recommend treating the brain in this manner, I think they get a bad rap. It sounds like you have a fault in how something is grounded (the advice to look under the relay board is great), or the relay board is shot, and also check the four red wires going to the battery, make sure they are in good shape.

The relay board is a plastic frame with terminals on the top and runs of flat copper wire on the underside. (The Haynes manual has a diagram that lets you trace all the wires) Problem is that the two are then 'riveted' together, then the whole underside is covered in tar. Riveted electrical connections fail on their own, but add to this that the "tar" Porsche used tends to crack and let in water. Spray down the relay board with contact "TV Tuner" cleaner for now, but the only fix is to remove all the tar, solder the conections, then re-seal the bottom. This is all that those "re-built" relay boards are.

Only good thing about replacing your brain is that you would gain the idle mixture adjustment if you replace it with a 72+ brain. Downside is they are $$$$$

Ascaris · 10-07-1999, 01:47 PM

FWIW, I've got a 75 Ljet so my relay is bolted to the side of the battery tray support. Ever since I put things back together this summer, I've had problems with the relay being intermittant. It acts just like a loose connection - if things don't start wiggle the relay and whirrr goes the fuel pump. But I've cleaned the connection again and again and it still occasionally acts up.

I always assumed that relay failures are catastrophic, but maybe they can just "get tired" too. My relay is too expensive to just swap out to see if it fixes it, but you might consider trying it.

tatanka · 10-08-1999, 06:58 AM

Keep in mind that there are three spare relays in the car. Two are under the front
hood, and are used on the headlight motors. The other is on the relay board, and
is for the heater blower. Driving on one headlight or no heat will get you home if
your fuel pump relay blows.

I agree the brain gets a bad rap. Mine was bouncing around in the engine compartment
for years before I got the car and remounted it. I'm just a little bummed out that even
though I got it working again, I couldn't nail down the problem with this fuel pump.
I hate intermittent problems!

Paul
(thanks for your help, Dave....! )