Kevin in Atlanta

Bought a pair of computers and used a 87S4 to test them before sending off a check. One computer let the car start and idle (note this car has not been started in over a year.) The other would let the car start but not hold an idle.

I have never seen this before and wonder what is failing in the computer that cannot hold an idle. I assume it needs to be repaired.

Mongo

I remember LH expert John Speake a while back talked about a hybrid type composition making up some of the circuitry on the board. One section in particular was the portion that controls the idle. This circuit has delaninated and failed on occasions, so I wouldn't bw surprised if the idle circuit is toast.

Mongo

Hope this link helps clarify the problems between the LH ECUs you have.

http://www.jdsporsche.com/23fiecu.html

worf928

The hybrid circuits in the LH2.3 boxes *ALL* fail. It's just a matter of time. Every '87 I take care of has had it fail. Several of the '88s too.

Near-100% probably that at least on the OP's spares has a hybrid circuit failure. There's no 'one' failure characteristic; they are many.

Mongo

Worf mine died too a while back and had it replaced. You're right. It is just a matter of time.

jcorenman

Kevin, Louie Ott is the US agent for John Speake / JDSPorsche, his web page for LH and MAF rebuilds is https://www.performance928.com/products.html

John Speake

The hybrid circuit "tile" on the LH ECU controls quite a few functions:
1) closed and WOT throttle positions
2) Idle control loop
3) O2 loop.
4) Battery voltage monitor.
5) Kicks the ECU microcontroller into life and monitors it continually.

It's the hybrid that is the pathway between many signals to the 5v electronics in the LH. So it is subject to any spikes or excess voltages on its inputs. Unfortunately it's not well protected and Bosch never learnt the lesson - the design is the same from an 87 S$ to the last GTS.

The replacement hybrid I fit is a completely new design with much more protection against unwanted impulses etc.

worf928

... grounds the fuel pump relay.
... injectors.

What signal's aren't in it's path?

John Speake

Hi Dave
Well, those two output signals don't go through the tile. Nor do the temp 2 values, MAF burn off, MAF input signal, tank venting drive or fuel pump relay drive signal.