D-Jet vs. K-Jet

[Andrew280SEL]

Can anyone tell me any great advantages/disadvantages of the two injection systems?

I think K-Jet may be ever so slightly down on power, but i could be wrong. On the   other hand, i think i read that in 1978 even the k-jet models were pushed up to equivalent power figures by changing exhausts and compression etc.

So anyway, anything in particular? Mines k-jet, my dads blue 450 is d-jet, and his brown 450 is k-jet. Yet none seem much different in character at all, except for the D-Jet "Woorrrmmmm" when you turn the ignition key at first.

[Mercules]

Differences between the two
**********************
D-Jetronic pulses the injectors in reference to manifold vacuum to achieve A/F ratio
K-Jetronic sprays fuel from each injector constantly and uses manifold vacuum to reference fuel pressure (inturn fuel flow)


Advantages over each other
**********************
D-Jetronic gives more accurate A/F ratio as fuel is only injected when it's required... when the inlet valve is open
K-Jetronic... no real advantages I can think of - Bosch were just experimenting with different fuel delivery techniques maybe?


Disadvantages under each other
*************************
D-Jetronic... not sure really, maybe injectors more prone to blockage & failure as they contain moving parts?
K-Jetronic give poorer A/F ratio as they pump fuel when inlet valve is shut and are prone to dripping when not in use. (fuel sprayed when the valve is shut will pool together in droplets or wet the inlet port - this lowers fuel atomisiation, thus reducing burn efficiency)


MAYBE SOMEONE OUT THERE CAN CONFIRM / DISPROVE MY ASSESSMENT OF THESE SYSTEMS
Both systems are ancient and can be replaced with modern aftermarket systems easily & cheaply, resulting in far greater power & economy than Bosch D&K-Jetronic engineers every dreamed of.... kinda like comparing a IBM i286 to an Intel Core Duo 

[koan]

Didn't K-Jetronic replace D-Jetronic to enable the engines to meet the ever tightening US emission regulations?

Both systems measure air mass with a vane or plate.

Doesn't D-Jet fire the injectors twice, once when the valve is open and again when closed?

What happens in the inlet manifold is more complex than you might think, as valves open and close there are pressure waves bouncing back and forth, it isn't just a matter of injecting when the valve is open.

Personally I like the simplicity of K-Jet, and the way it uses fuel as the working medium to control how much fuel is injected.

koan

[Mercules]

Didn't K-Jetronic replace D-Jetronic to enable the engines to meet the ever tightening US emission regulations?

I think you are right I can't imagine they would have created K-Jet to make worse emmisions/power/economy

What happens in the inlet manifold is more complex than you might think, as valves open and close there are pressure waves bouncing back and forth, it isn't just a matter of injecting when the valve is open.

Not sure if engineers who decided to use a massively restrictive air-flow vane took the Helmholtz theory of resonance into account  Haha

[koan]

Not sure if engineers who decided to use a massively restrictive air-flow vane took the Helmholtz theory of resonance into account  Haha

It's not the ideal form for smooth turbulence free flow is it?

Modern systems use MAP sensors, Manifold Absolute Pressure sensors rather than flaps to determine how much loud pedal you're giving it.

koan

[oscar]

D-jet rocks

But I haven't touched k-jet which I will have to as I've just picked up a k-jet 280se.  D-jet is what I came across first, so I'm familiar with it.

In addition to d-jet disadvantages,
Any problems I've had with d-jet has been of a mechanical nature apart from the time I doused the engine in water and soaked the wiring loom and connections
My injectors have held up really well, but I would say that another is failure of the manifold pressure sensor.  At a time when electronics were expensive, this ingenious baseball sized electro-mechanical unit sufferes from metal fatigue in the diaphragm which leads to poor idle and part throttle transition and introduces a vac leak.  No replacement copper diaphragms are available and the whole unit costs $800US approx
The other part leading to poor running is the trigger points that signal the ecu to fire the injectors.  They wear out on the distributor cam and aren't adjustable unless you do some fine bending with pliers like I did.  Otherwise, new trigger points with loom costs around $400US.

These two parts last a loooong time, but our cars are at the age where these things are failing, so too the 2nd hand replacements.

Doesn't D-Jet fire the injectors twice, once when the valve is open and again when closed?

What happens in the inlet manifold is more complex than you might think, as valves open and close there are pressure waves bouncing back and forth, it isn't just a matter of injecting when the valve is open.

Koan, I haven't read that d-jet fires twice in a cycle HOWEVER, all d-jets, V8 or IL6 have half the number of trigger point contacts as there is cylinders.  So my V8 350 has four trigger contacts and each contact signals the ECU to fire a pair of injectors at the same time.  They'll be opposite cylinders too. One of my contacts was skipping last year, therefore the 4th and 8th injectors did not fire.  When I got it working again, by using test lights, both injectors fire at the same time despite the cylinders being in different positions.

Maybe the ECU detects the trigger pulse once but times two pulses to injectors, 180 degrees apart.  I don't know.

Also, re the "pressure waves", the MPS has a dampening feature/valve to counteract these fluctuations.  Can't go into detail without reading about it all again, but When I read it, i couldn't help but think at the time that these guys tried to cover everything.

[OldMercs3]

D-Jetronic does not have a MAF sensor or a so-called flapper door, it's what's known as a "speed density" system.  Bendix came up with it's "Electrojector" EFI system in the fifties, which was somewhat ahead of available electronics technology since there were gigantic injectors and vacuum tubes involved.  The Bendix system was available on some Chrysler products but nearly all were replaced with carbs.  Bosch, whose systems up to this time were strictly mechanical (300SL etc) bought Bendix's "Electrojector" patents (nice going Bendix) and eventually turned out D-Jetronic, the first widely available and reliable EFI system.  I can't imagine what the deal is with K-Jet or explain why it even exist except that this is perhaps what Bosch was developing before Bendix had it's garage sale.  What little I do know about K-Jet is that it does not meter fuel as efficiently or accurately as almost every other fuel injection system. It always seemed strange to me that Mercedes went with K-Jet considering that BMW and many others went with D-Jet derived L-Jet (now with MAF) or Bosch-licensed L-Jet like systems.  In an ironic twist Cadillac utilized a Bosch-licensed D-Jet system around the time Mercedes went to K-jet and before most of GM went to TBI systems.  Why was there not an L-Jet system created for the Mercs?  If change was motivated by emissions requirements it seems to me that an L-Jet/Lambda system would've been better at this than K-Jet.

[alabbasi]

D-Jetronic is very expensive, new injectors run for about $170 (Bosch). Also those computers and map sensors are prone to failure after 30+ years. K Jet has no computer or map sensor and injectors run for about $30. Not as sophisticated but quite reliable.

[Nutz]

D Jet explained

Trigger Points

The trigger points are located in the base of the distributor below the ignition point breaker plate.These points are used to produce the signal that synchronizes the injector to the crankshaft.They consist of two low voltage,low current contact points which are driven by a cam located on the distributor shaft.Each set of points initiates the opening of one or two groups of injectors.

Unlike ignition points,the trigger points can last 100,000 miles or more.As the distributor rotates,a pulse is created by the opening and closing of the trigger points.This pulse is sent to the ECU.The ECU uses this signal to open the injectors and will use the inputs from the other sensors to determine when to close them.

Temperature Sensor I

Temperature Sensor I is an ambient air temperature sensor.As the ambient air temperature decreases,the density of the air increases.As a result,the ECU must inject more fuel on a cold day than on a warm day.

Temperature Sensor I is a temperature sensitive resistor known as negative temperature coefficient thermistor (NTC). This sensor has a resistance of between 400 and 500 ohms at 50 degrees F. At 100 degrees F.,the resistance is between 150 to 200 ohms.

Temperature Sensor I actually has little effect on the operation on most D Jetronic equipped vehicles.This is because most of these cars have tens of thousands of miles on the engine and are running very rich due to this wear.The troubleshooting consequence of this is that disconnecting the air temperature sensor during the diagnostic procedure may have little effect on the way the engine runs,and in some cases it may actually improve the way it runs.

Temperature Sensor II

Temperature Sensor II is the coolant temperature sensor on water cooled engines.Like temperature sensor I, it is a NTC thermistor.The temperature of the engine is important because the intake manifold design of fuel injected engine does not permit the use of an air restictive choke.Additionally,restricting the air to enrich the engine during warm up would cause inaccurate readings from the manifold pressure sensor.The warm up choke function is therefore performed by Temperature Sensor II.

When the coolant temperature is about 50 degrees F.,the resistance of Temperature Sensor II is between 3000 and 4500 ohms.As the temperature increases to more that 120 degrees F.,the resistance drops to less than 1000 ohms.Thus the sensors work is done once the engine is warmed up.Also it must be continuously remind the ECU that the engine has warmed up.If damaged,Temperature Sensor II will not continue to partially function( such as the resistance values shifting). It will completely fail,creating an open short or ground.

An open circuit in Temperature Sensor II or the wiring leading to it will cause the engine to run extremely rich once warmed up.Symptoms would be dark smoke from the tailpipe (most noticable at idle),rough idle and poor power.Keep in mind that these same symptoms could also be caused by engine compression problems and ignition.

Should the sensor become shorted or the wiring harness grounded,the effects may not be noticable at all when the engine is warmed up.The symptoms would be more like a carburetor with the choke stuck open-rough or erratic idle,stalling or hesitation when the engine is cold and progressively running better as the engine warms up.


Throttle Switch

The throttle switch tells the ECU when the throttle is closed,when the throttle is wide open and when the throttle is moving toward the open position.The switch consist of twenty two contacts,with a set of wiping contacts that move across them as the throttle progresses from the closed position to the wide open position.

One wiping contact is used to inform the ECU that the throttle is closed.Another makes contact only when the throttle is wide open and a third makes and breaks contact twenty times as the throttle opens.The electrical pulses created by the making and breaking of the contacts signals the ECU to open the injectors more frequently,thereby enriching the mixture for acceleration.This feature behaves much like an accelerator pump on a carburetor.

Symptoms associated with a defective throttle switch include a rich(smoky)idle and hesitation.An intermittent condition at cruise-which feels like you shut off the engine and immediately turned the key back on,can also be caused by the throttle switch.

To test the throttle switch,open the throttle with the key on but the engine not running.The injectors should open exactly twenty times,evidenced by twenty evenly spaced clicks.


Pressure Sensor

The D Jetronic pressure sensor is known as a linear variable displacement transducer (LVDT).It consist of a pair of coils,one with about 150 ohms of resistance and the other with about 85 ohms of resistance.An iron core attached to a diaphragm runs through the center of these coils.As changes in manifold pressure moves the diaphragm,the iron core moves inside the coils,causing ripples in the current flowing through these coils.This signal is used by the ECU to monitor the relationship between barometric pressure and manifold pressure.

The most common symptom from a defective pressure sensor is a rich running condition.Of course rich running can be caused by several other defects as well.

Of all the sensors used on D Jetronic,this one both the easiest and the most difficult to test.Usually a simple resistance test of the coils is enough to determine if the unit is good or bad.On the other hand,the only way to be sure is to replace it with a known good unit.

Electronic Control Unit (ECU)

The ECU receives input signals from the pressure sensor,Temp Sensor I,Temp Sensor II and the throttle switch to determine how long to leave the injectors open.It is only able to respond to air-fuel ratio request from one sensor ata time.As a result,whenever a sensor fails,the tendency will be for the ECU to send the injectors a full rich supply of fuel.

The electronic unit has no servicable components.In the event of a failure,the ECU is replaced as a unit.Failures are extremely rare and usually result in a no start.

D Jetronic that have an adjustable air fuel ratio have a detent potentiometer on the side of the ECU.This potentiometer can be used to fine tune the air-fuel ratio during a tune up.

Pin # 19 and 25 of the ECU are connected to the fuel pump relay.When the ignition switch is turned to the on position,the ECU energizes the fuel pump to ensure that the fuel system is filled for ease of starting.If the engine is not cranked,the ECU will shut off the fuel pump after one or two seconds.If the engine is started,the fuel pump runs continuously until the engine is shut off.



D Jet Components

D Jetronic Fuel Components

In Tank Filter

Located inside the fuel tank is a screen or filter designed to protect the fuel pump from rust,dirt and debris.Although seldom the cause of a drivability problem,the in tank filter should be high on the list of items to check.In many cases these filters have been ignored,even on well maintained vehicles.


Fuel Pump

The fuel pump is a high speed roller vane pump.It is capable of pumping fuel at pressures and volumes much higher than the engine or injection system would ever require.This type of pump is very efficient at pushing fuel bur does not do a good job of pulling fuel.As a rsult,these pumps are located very close to the fuel tank or even inside the fuel tank to reduce the chance to vapor lock.

The fuel pump can react adversely and even fail as a result of using some fuel additives.Use extreme when selecting them to ensure that they do not contain methanol or other corrosive substances.

A defective fuel pump can cause low fuel pressure which would result in symptoms such as hesitation,stalling and poor power.Often a defective fuel pump bypasses the poor running stage and simply stops operating.This causes the engine to die or keeps the engine from starting.

Fuel Filter

The fuel filter is the only real protection the injection system has from internal contamination from dirty fuel.Therefore the filter should be changed every time the spark plugs are changed.

Fuel Pressure Regulator

The fuel pressure regulator used on D Jetronic systems consist of a valve connected to a spring loaded diaphragm.The regulator controls the fuel pressure at 28-32 psi and is adjustable so it can ensure the proper fuel pressure throughout the life of the vehicle.Incorrect fuel pressure can cause a lean running engine if the fuel pressure is too low and a rich running engine if the fuel pressure is too high.

A defective fuel pressure regulator can result in high fuel consumption,rough or erratic idle and poor power.

Injectors

The injectors are solenoid operated,normally closed valves controlled by the ECU.Grounded to the engine block or chassis,each injector is opened by a 3 volt pulse from the ECU.The lenghth of the pulse is only a few milliseconds(2 to 5),and it takes time for the injector to close from the spring tension.Thus the injector is open for a total of about 3 to 6 milliseconds.

Very little goes wrong with the D Jetronic injectors themselves.An occasional burned out solenoid winding or restriction from contamination is the most common problem.Another problem is leaking from the hoses that attach the injector to the fuel rail.

Typical symptoms of injector problems include rough idle and poor power.

Fuel Rail

The injectors and the fuel pressure regulator are attached to steel tubing known as the fuel rail.The inbound fuel lines from the fuel pump and filter feeds fuel to the injectors and the fuel pressure regulator through the fuel rail.

As a passive component of the system,very little can go wrong with it except for leaks and restrictions.

Cold Start Injector

Also attached to the fuel rail is a solenoid operated valve known as the cold start injector.Since the earliest applications of the D Jetronic system were four cylinders,the cold start valve picked up the moniker "fifth injector." It stuck in some circles,even for six and eight cylinder applications.

The cold start injector receives battery voltage whenever the starter is engaged and is grounded through a device known as a thermo-time switch.The thermo-time switch is a temperature sensitive bimetal switch designed to provide a ground for the cold start injector when the temperature of the engine is less than 95 degrees F. A second circuit in the switch is an electric heating element intended to heat the bimetal as the engine is being cranked.

Consequently,the cold start injector should operate only when the engine is being cranked,the temperature of the engine is less than 95 degrees F. and for a maximum of five to twelve seconds.

Two of the most common symptoms of a cold start injector problem are hard starting when cold becuse the cold start valve is not operating and a leaking cold start injector which can cause an extremely rich running condition.

[oscar]

Nutz, man u type quick

That's great info, have you got a similar rundown on the k-jet too?

I'd like a personal comment from you too.  In conclusion, since you've had a lot of expereince with both, which system would you prefer and why.

[alabbasi]

Wasn't a gentleman on this board converting his D-Jet to MegaSquirt?

[koan]

Excellent and most informative post Nutz.

koan

[oscar]

Wasn't a gentleman on this board converting his D-Jet to MegaSquirt?

That was Denis from Paris who sold his 350 to a German.  He's now enjoying that 280 w126.  So we never got to see a megasquirt conversion.

But following on from what styria asked- Mercules, if you know of aftermarket systems suitable for ugrading our cars, do tell.

[Andrew280SEL]

Wow, this is turning out great. 

very informative guys, thanks. 

and Nutz, anyone would swear you LIVE inside of a D-Jet system! 
very knowledgeable you are mate 

[Mercules]

Mercules, if you know of aftermarket systems suitable for ugrading our cars, do tell.

To convert to EFI, you will need a set of electronic fuel injectors, an 02 sensor, a MAP sensor & maybe an ignition pickup (if the standard one is not compatible with the ECU) then most of the other sensors like temp, throttle position etc will carry over. Whilst you are at it - you may as well piss off the old MFI ditribution block & depression plate - making the induction MUCH less cluttered & restrictive!

For intallation & tuning - http://www.pulseracing.com.au/Adaptronic.htm
(I know this guy personally and if there is anything about EFI he can't tell you, its not worth knowing!

For product info - http://www.adaptronic.com.au
This guy is an electronics engineer and has just designed & built his own ECU and it is simply awesome - priced well too!

All up I reckon you could get the 2nd hand parts you need for $150 - $200.... ECU with loom is $1100... all you need to do is wire it up and get it tuned! I've seen a few carby cars upgraded like this - the newfound economy, smoothness, power are excellent... and while the early jetronic systems are better than carbys - they still suck (my opinion - not actual fact) 

Hope this helps - Cam