First posting to this forum, thanks in advance. Retired EE, but more of a MatSci guy than a circuits guy, as you'll see by my question.

The application is a sensor in the Bosch D-Jetronic (circa. 1970) electronic fuel injection system. The sensor in question is the cylinder head temperature (CHT) sensor, which is a thermistor with a resistance of about 2K ohms with the engine cold, and about 50 ohms with the engine fully warmed up. Sensor is single wire back to ECU with the other side grounded. High resistance = richer mixture, low resistance = leaner mixture.

We would like to have a circuit that uses a single pot positioned at the middle which would have no effect on the resistance of the CHT sensor as seen by the ECU. Turning the pot to the right would richen the mixture (i.e. add series resistance to the CHT), turning the pot to the left would lean out the mixture (i.e. add parallel resistance to ground to the CHT).

I can do this with switches or adding active components, but can't figure a passive circuit topology to do this with a single pot, which would be more desirable. Is it possible? I'm sure if it is, when I see it, I'll say.... "duh". Thanks in advance.

 

First posting to this forum, thanks in advance. Retired EE, but more of a MatSci guy than a circuits guy, as you'll see by my question.

The application is a sensor in the Bosch D-Jetronic (circa. 1970) electronic fuel injection system. The sensor in question is the cylinder head temperature (CHT) sensor, which is a thermistor with a resistance of about 2K ohms with the engine cold, and about 50 ohms with the engine fully warmed up. Sensor is single wire back to ECU with the other side grounded. High resistance = richer mixture, low resistance = leaner mixture.

We would like to have a circuit that uses a single pot positioned at the middle which would have no effect on the resistance of the CHT sensor as seen by the ECU. Turning the pot to the right would richen the mixture (i.e. add series resistance to the CHT), turning the pot to the left would lean out the mixture (i.e. add parallel resistance to ground to the CHT).

I can do this with switches or adding active components, but can't figure a passive circuit topology to do this with a single pot, which would be more desirable. Is it possible? I'm sure if it is, when I see it, I'll say.... "duh". Thanks in advance.

The main control unit for the fuel injection system should have a small knob to do just what you are asking...

See this article
https://www.mbca.org/star-article/j...ths-and-magic-bosch-d-jetronic-fuel-injection

where it says,
These have a mixture adjustment knob on them. If the knob is turned clockwise, then the fuel mixture becomes richer (and a little smoother at idle).

 

Thanks for your entry, I'm aware of the ECU idle mixture adjustment knob. I have an extensive D-Jetronic page at the URL below, we reverse-engineered the system down to the ECU board component level, developed models for the manifold pressure sensor, modeled the operation of the CHT circuit, etc.

http://members.rennlist.com/pbanders

What I'm talking about is different from the function of the ECU idle mixture knob. When the throttle is closed, there's a throttle position sensor that closes a switch. The ECU senses this and enables the idle mixture control, otherwise, it's inactive at part-load, overrun, and full-load. The circuit I'm working on is intended to deal with a design problem in the system having to do with a hot start condition, where the CHT cools too quickly, due to it being mounted in the aluminum head, which doesn't retain heat as well as the iron cylinder sleeves. The result is that the CHT is telling the ECU that the engine has cooled and needs a richer mixture when it doesn't. By adding the circuit I'm working on to the CHT, the driver can lean out the mixture on hot start and avoid issues. By making it two way, that is a rich and a lean adjustment, other adjustments can be made that can help driveability under other conditions.

 

So, you have a thermistor that is grounded and connected to the ECU, but, there must be a current source for the thermistor. And, it cannot be a 12v feed from system Power or the ECU will always see 12v. Therefore, it will either have a simple resistor to make a voltage divider to get the various voltages back to ECU, or, a constant current source (a pair of transistor's or a transistor and a Zener diode with a few resistors. In any case, you'll have to dig a bit more to see how this is arranged. The, we can figure out where to wedge a pot in there or if an active circuit will be needed.

Almost finally, when would more than 2000 ohms be needed?

Finally, wouldn't it just be easier to move the thermistor off of the cylinder head and down to the oil pan area where heat lingers longer?

 

Thanks for all the input, re-engineering the system or going into the ECU are interesting ideas, we've considered them before. What I'm looking for is a solution to the question I posed, I think the answer is "no". Here's what we're going with, not as elegant as what I'd like but should work fine. Thanks for the inputs.

 

@Paul Anders

You could also change out the thermister for one slightly smaller in range,

You could change the thermister from one that is 50 to 2k ohm...
...to one that is 50 to 5k ohm.

Then, in parallel with the thermister, you can add a 10k linear pot (pot has wiper connected to ground to make rheostat). That can be set to 5k midpoint to achieve about 2.5k overall in parallel with the 5k thermister to mimic the current situation when the engine is cold.

You can turn to lower resistance to mimic the lower resistance of a warm engine. Turn to higher resistance to get higher resistance.