safulop

After several weeks with my shop messing around trying to get Ken's chips working, it now seems like in the process of installing the LH chip something broke down in the LH itself.

The shop says the LH (back to the old chips) now has no ground control over the Temp sender, although they traced the ground circuit into the LH as being good all the way. Their diagnosis is the LH needs rebuilding. Which is odd to me since it was rebuilt in 2007 or so.

Anyone else had to have LH rebuilt a second time?

Rich9928p

The LH fuel injection module pin 13 sources the voltage for the engine temperature measurement. The Temp II sender is connected to the ground (screwed into the engine). The different resistance of the sensor is what provides the different voltage for the LH module to measure. Are you sure the mechanics understand how it works?

If the engine temperature circuit is damaged in the LH it probably can be repaired. Pin 13 of the module is the circuit to the Temp II sensor. Internally it connects to a couple of resistors, a capacitor and the A/D converter (which reads the voltage). Worst cast the A/D chip was damaged and needs to be replaced. Another complete LH module rebuild should not be required as that circuit isn't involved with the other circuits.

Was another LH plugged in to verify that this really is the problem ... it isn't a common problem.

I could take a look at it for you. Send a PM.

Rich
'93 GTS Cover Girl
'87 S4
'79 5-speed rescue w/S4 Alternator
'79 5-liter track beast

safulop

I believe the shop has two people including the owner who were trained on 928s back in the day. They work on them fairly routinely, but unlike Greg Brown's shop or someplace they don't have the luxury of spare LH's lying around.

As for whether they "understand how it works" I'm not 100% sure. They do realize that the LH is no longer controlling the mixture using info from Temp, and they say the reason is that the ground circuit from Temp has broken internally to the LH. So the car runs but with random mixture, unrelated to engine temperature.

Roger's got rebuilt LH's in stock, I'm of a mind to just try one. They're not any more $$ than what the shop's going to charge me for tinkering around.

PorKen

I'm not sure I understand that diagnosis. The Temp II input goes directly to the A/D converter, not through the problematic 'tile'. I know little about electronics but I guess the pulldown(?) resistor or something could be burnt out.

I would lean more towards a bad engine ground (there's a bunch at the rear of the engine) or a bad/maladjusted idle switch, as below.

https://rennlist.com/forums/928-foru...m-install.html

safulop

Yeah, I'm going to have to see for myself how it runs, whether the problem is idle only or general running. Possibly a ground issue through TPS, but they said they traced the ground circuit and found no interruption except the LH itself. If there was bad engine ground I think they would have discovered it by tracing the ground.

safulop

Oy, this LH is definitely toast. They jerry-rigged a ground connection to bypass the broken temp circuit, car mixture is ?? and the idle is everywhere, rad fan polarity is reversed, the whole temp circuit is out of the loop from all the jumpering and fooling around, even the AC is dead. So we're going to ship the LH out for repair and see what comes back to life.

dr bob

Sean--

It almost sounds like the battery might have been connected backwards. Fan polarity reversed? How is that related to the LH?

Scratches head...

safulop

They did a bunch of jerry-rigged connections to bypass part of the LH control circuit. They'll figure it out, once we get the LH looked at.