I‘d like to introduce a simple self-built on-board test device that provides diagnostic information (mainly) about the KE-Jetronic fuel injection system anytime … while driving or parking. … Makes detecting or narrowing down problems a lot easier, especially if they occur intermittently. …
I built it into what‘s otherwise useless items in my cars … the ashtray. … On the first picture below you can see the device‘s interface in the opened “ashtray“. It has two push-button switches with integrated LED‘s, a digital panel meter (voltmeter), a rotary selector switch and three test sockets for external meters. The labels on it are not up to date … shortly after I installed it (about twenty years ago) I changed the circuitry, without changing the labels, to the following functions:
1) Orange and red push-button switches
When pressed in, its integrated LED, if illuminated, indicates that the FPR
(Fuel Pump Relay
) is providing voltage to the fuel pumps.
Depending on country version and production date cars with M103 engine have a MAS
) instead of a FPR. In that case the illuminated LED would indicate that the MAS is providing voltage to the fuel pumps.
When pressed in, its LED is illuminated if and for as long as the FPR
(respectively the MAS
) is providing voltage to the CSV
(Cold Start Valve
2) Digital panel meter (voltmeter)
Depending on the selection via rotary selector switch, it displays the following information:
(Idle Air Control Valve
) supply voltage … provided via OVP
(OverVoltage Protection relay
) in case of M103 engine, respectively via OVP and IACV-ECU
(N8) in case of M116 and M117 engine.
(engine speed signal) … generated by the ignition control module based on input from the CPS
(Crankshaft Position Sensor
(N3) supply voltage… provided via OVP
(Lambda control ‘off/on’-ratio) in volts.
Digital panel meter switched “off
3) Red, black and green test sockets
green & black:
Alternatively to the above mentioned option “Z”, the duty cycle
can be monitored in “%
” with a duty cycle featuring multimeter or, more comfortably, with an analog duty cycle meter plugged into the green & black sockets above the digital panel meter.
red & black:
With a (preferably analog) multimeter plugged into the red & black sockets the o2 sensor voltage
can be monitored.
Examples of use:
In case of a problem with idle speed
the selector switch can be kept in position “P“ in order to monitor the IACV
supply voltage on the digital panel meter. … Missing or wrong supply voltage would point to a problem with the OVP
or its fuse(s) or its wires, or (in case of M116 and M117 engine) with the IACV-ECU
or its wires.
With the selector switch set to “K1“ the supply voltage for the KE-Jetronic-ECU
can be monitored, which is always provided via OVP
, no matter which engine. … Very useful in case of various engine problems, like difficult cold starts
and bad idle quality
, acceleration problems
(especially if occuring simultaneously before the engine has warmed up) and higher fuel consumption
, etc.. … In case of M116 and M117 engine this voltage is also the supply voltage for the IACV-ECU
These two options “P“ and “K1“ (in case of M116, M117 just “K1“) make it very easy to check and monitor the OVP
(as for its engine related functions) … thus preventing unnecessary replacements of one of the most often unnecessarily replaced parts on cars equipped with KE-Jetronic. …
Talking about often unnecessarily replaced parts … with the selector switch set to “B” the voltage regulator
‘s function can be monitored too, of course.
In case of (sudden) engine stalling
and subsequent starting problems
the two most typical suspects can be checked in a matter of seconds immediately after the engine stalled. … You just press the orange switch “P” and set the selector switch to “TD”. Then during cranking the orange switch’s LED tells you if the fuel pumps are receiving voltage from the FPR
(respectively from the MAS
) or not, and the digital panel meter shows if there is a TD-signal and if its voltage is within plausible range, thus if the CPS
is in order.
Checking these two things (fuel pump voltage and TD-signal) is, of course, advisable in any case of starting problems
, not only after the engine stalled.
Additionally, a push on the red button shows if the system correctly orders the CSV
to spray during cranking when the engine is cold… or it shows if it‘s wrongly ordered to spray when the engine is warm.
With the duty cycle being displayed (selector switch set to ”Z”) Lambda control
can be monitored under all kinds of driving conditions. … Plus, keeping the switch set to ”Z” can help detect a range of (intermittent) problems. If, for instance, during driving the reading becomes static at ~8.4 volts, there is a problem with the AFM-POT
(Air Flow Meter Potentiometer
) ... static voltage of ~9.8 volts indicates a problem with the CTS
(Coolant Temperature Sensor
) ... ~11.2 volts a problem with the TPS
(Throttle Position Switch
) … zero volt usually indicates a problem with the OVP
(verifiable by above-described option “K1“) … etc.
For a little deeper view into Lambda control (i.e. the function chain “o2 sensor
”) the duty cycle
can be monitored either on the digital panel meter or on a duty cycle meter (plugged into the green & black sockets) while at the same time the o2 sensor voltage
can be monitored on a voltmeter (plugged into the red & black sockets).
Usually, however, I keep the device switched off and the “ashtray“ closed … and enjoy a pleasant ride. ...
Maybe some readers like the idea of such a device’s availabilty at a push on the ashtray panel and feel inspired to think about making one themselves. Works in any car equipped with CIS-E. Material costs are ~ €100 … the costs for unnecessary part replacements and troubleshooting time it can help to save I leave to the reader’s imagination. …
The only circuit diagram I have is the one I started to draw by hand when I had my first thoughts about the device. To post that one would be too confusing … there are too many subsequent changes in it. I’ll try to find some time to draw a new simple diagram soon.
P.S. to 124-series and 129-series owners among the readers:
Note that on the CIS-E engines M104 (124 and 129) and M119 (129) the CSV control and the IACV control work a little differently.