MPS Tune-o-Matic - D-Jetronic

The Tune-o-Matic is a small, easy to use, hand-held device that can tune a de-tuned manifold pressure sensor (MPS) without any specialist knowledge or equipment.

Problem

Many MB V8 D-Jetronic engines have had their MPS messed with some time in the past. This involved removing the plastic cap (usually breaking the cap) and using an Allen key to adjust it, possibly to mask another d-jet problem in the vehicle. How does someone get their MPS back to where it should be?

1. Measuring inductance

This is the current method employed by Volker in Germany and @cushjbc and requires using an LCR meter to measure the inductance of a reference MPS and then tuning the problem MPS to match the same inductance. The more accuracy the better. Requires knowledge of using an LCR meter and requires a reference MPS to be available. However if a vehicle owner has a pristine un-altered MPS available to them they why just put that in the vehicle and job done? Therefore an MPS typically has to be sent off to the person doing the tuning.

2. Measuring air-fuel ratio (AFR)

Requires welding a bung onto the exhaust, purchase of an AFR gauge and O2 sensor and tuning to a pre-determined AFR setting. Doesn’t require a reference MPS and the MPS is tuned to a specific vehicle.

3. Send to Bosch

Probably slow and expensive but also the “gold standard”. Tuning method employed is unknown as far as I know.

Requirements

1. Easy to use and non-technical
2. Powered from USB
3. Doesn’t require additional equipment
4. Could be loaned out to vehicle owners
5. Uses one or more reference MPSs for initial calibration but doesn’t require the user to have a reference MPS
6. MPS can remain in the vehicle
7. Get the performance of a D-Jetronic system close enough that it operates in a reliable manner and without being too rich or too lean
8. For Mercedes V8 D-Jetronic only (type 3 MPS)

Note for item 7 we are not going for a system that can squeeze every last drop of performance or fuel economy out of a D-Jetronic system. The aim is to get the MPS back to close to where it should be.

Core Idea

@cushjbc hit on an idea last year. Instead of tuning an MPS to inductance why not tune to the width of the injection pulse from the ECU?

For a given set of engine conditions the pulse width determines how rich or lean the air fuel mixture will be. For the same conditions every MPS should cause the ECU to produce the same pulse width.

There are a lot of inputs that the ECU uses to create the final pulse width but the MPS is used quite early on in the process to generate a basic pulse. Looking at the ECU schematics (from @CaptainSLC and @cushjbc) the only inputs to the pressure loop are:

1. Air temperature
2. Speed control and idle mixture bias (I will refer to this as SCIMB)
3. Ambient pressure
4. Manifold pressure

For items one and two we can choose values and fix these so that every MPS connected to the Tune-o-Matic has the same inputs. We can also calibrate these values so multiple Tune-o-Matics produce the same results.

PCB