How DME works when CHT inop?

[Flat Six]

Hi, all -- searched but can't seem to find the answer to this specific question:

What does the DME on a Motronic car do when the cylinder head temp (CHT) sensor doesn't work? For example, when you disconnect the O2 sensor from most ECU-managed vehicles, they go into a pre-programmed 'open loop' mode. Does the Motronic DME work the same way?

I have a couple of reference books (Probst's is invaluable, IMHO) but can't seem to find an answer. Any DME gurus out there who can shed some light on this for me?

FWIW, I own a 1985 Carrera 3.2, almost completely stock.

Thanks in advance,

Dale

[cgarr]

When I was trouble shooting my 86 when I unplugged the cht it ran like crap, up and down idle and sputtering.. It must screw up more than the o2 does because I run mine without the o2 just fine. I dont think with out the cht it goes into any type of pre-programmed setting.

[Flat Six]

Based on what I've read, the CHT provides temperature input which the DME uses to adjust fuel/air ratios; on a cold engine likely richens the mixture (kind of like a warm-up regulator [WUR] does on CIS cars), leans it out when at normal operating temp, and potentially enrichens some more if engine is overly hot to try to minimize preignition and detonation (whereas more modern engine management systems might rely on knock sensors to retard ignition timing to achieve the same objectives).

But what I'm curious about is what happens when the CHT is unplugged? I suspect that if it's not working properly it'll provide false readings to the DME which will cause wonky A/F ratios and poor drivability. But if it's disconnected altogether, does the DME revert to some kind of default air/fuel/timing map (kind of like 'limp home' mode)? Or does the DME assume just one temperature reading and -- if the engine is running at any other temperature besides the one default -- A/F ratios are off and the engine runs like cr*p?

Anyone know the answer?

[stlrj]

It defaults to full rich, as rich as the DME can take it, pure and simple.

[Flat Six]

Originally Posted by stlrj

It defaults to full rich, as rich as the DME can take it, pure and simple.

Huh. Makes sense; full rich is less likely to cause engine damage from lean running condition, right?

[stlrj]

Originally Posted by Flat Six

Huh. Makes sense; full rich is less likely to cause engine damage from lean running condition, right?

As rich as it gets can cause much more damage to any engine over time. Too lean on a lean burn DME will only cause the engine to start to run cooler to the point where the engine will no longer be able to ignite the mixture and result in no damage done.

Running too rich for extended periods damages valve guides due to dangerously elevated cylinder head temps and premature cylinder wear from excess fuel washing off the film of lubrication.

[Flat Six]

Thanks much for the information and education!

Dale

[ischmitz]

Just to clarify - it defaults to the coldest outside temperature that the DME is programmed to handle on the 3.2. (I think something like -20 C) While this is not full rich it is pretty rich. The O2 sensor regulation won't be able to adjust the mixture. As a result the engine will run like crap once it warms up and long-term use will cause engine damage (oil film washes off the cylinder walls, CAT gums up, etc).

On later DME versions (3.6 onwards) a missing or implausible it defaults to the engine at operating temperature setting. This will make starting difficult but the engine will run fine once it is at operating temperatures.

Ingo

[McLovin]

Interesting. So if you unplug the CHT, the car will start up and run?

Will it start up even if the car is already at full operating temp?

And how long is it safe to run it? I'd think a few miles wouldn't hurt.

This would be good info to have for an emergency. The only time I've ever been stranded in a 911 was when a CHT died while driving. Car wouldn't run at all.

[Jascha]

Basically, when the CHT sensor is disconnect from the DME it is as if you presented a very high (infinite) resistance (cold engine state) to the DME and as pointed out --> rich mixture.

In fact, if your CHT sensor fails and you can't get the engine running...depending on the engine temperature (cold/hot) a substitute resistor of approximate value (3K ohm or open-circuit or 300 ohm or short-circuit) should get you going again...

Jascha

[ischmitz]

If the engine is stone cold on a very cold day (below freezing) you will probably be able to start the car just fine. However, once the cylinder head temperature goes up the a working CHT sensor will adjust the mixture accordingly. And warming up will happens almost instantly (my guess is within the first 1 - 10 minutes). The range of the CHT is somewhere from 0C to +130C. And operating temperature is around the 130C mark. So you can imagine how quickly an open sensor (reporting 0C) will be wrong.

I would not run the engine for too long and not try to drive much but I don't know how quickly you damage the engine.

Ingo

[Jascha]

The truth be told...

http://forums.pelicanparts.com/uploads9/CHT1161879499.jpg


I travel with two cold/hot resistors (which speak for themselves)