Join Date: Aug 2010
Location: N-East Central USA
Blinking Econ Light = Speed Sensing Module failure
Thank you Van for this insightful investigation and a repair procedure.
Fri Mar 17, 2017 7:36 am (PDT) .
"Van Overhouse" van_overhouse
For 86-88 MY cars with the Bosch CIS system the green ECON light should come on steady while off throttle and in overrun condition. In two cases now I've seen that the light blinks rather than remain on. I traced this back to a bad "Speed Sensing Module" which I replaced the first time. Now that I've seen the problem a second time I decided to try and fix the Module rather than buying a new one. The first step was to do some reverse engineering and generate a schematic for the Module. I've posted the schematic at Turbo List files/tech/electric .
What I found was that either the Module design has an error or the Lotus application is incorrect. Within the module there is a 750 ohm 1/4W resistor connected between pin #15 (V+ from RPM Relay) and terminal #31b going to the Thermal Time Switch. During warm up, terminal #31b is grounded by the Thermal Time Switch until the water temperature reaches 35C. During this period the 750 resistor has 14.5V across it which generates (14.5*14.5/750= 0.3W) watts which is greater than the resistor specification allows. Once up to temperature the ground is removed and the power drops to about .13 watts. This extra stress on the resistor eventually causes it to fail.
In addition, the 750 ohms is too large a resistor and current flowing through it and out to the thermal time switch is causing too much voltage drop. The voltage at the node with the 750 ohm (?), 27K and ITT diode is too low to turn on the BC547 transistor. Making the resistor smaller will raise this voltage to the acceptable level.
?What would be the smallest 1/4W resistor you could use to replace the 750 ohm (R= E*E/P). At 14.7V that would be 865 ohms.
So if you put in a 1W resistor that would be 316 ohms. I'd replace it with a 330 ohm, 1W resistor.
After replacing the resistor, the should not fail again.
If you have a blinking Econ light and want to quickly test if you have the same problem just disconnect the Thermal Time Switch connector once the engine is up to temperature. If the Econ light no longer blinks you most likely have the same problem. If you do have this problem you should fix it as it is also pulsing the air bypass valve which will wear it out prematurely. Don't throw away your old Speed Sensing Modules they are repairable and we may need them some day.
Module: It's the one with the black base and you should see the Bosch part number 0 280 230 103 on the case [back wall, above the coil]. I didn't replace the Ecap on the PC board since I tested the module on the bench. Probably should have replaced it with 35V or 50V cap. Hopefully, it doesn't come back to bite me.
After thinking about the error I believe it must be the Lotus application of the module that is incorrect. Also, I believe that pretty much every Lotus module out there has a "750 resistor problem".
The original resistor looks to be a ceramic thin film resistor and the failure mechanism for this type of resistor is when it overheats is for the resistance to increase (if it doesn't fail outright like a fuse).
Once the resistance becomes high enough, it will no longer overheat. In some cases the higher resistance is not sufficient to cause the circuit not to work. In other cases it is too high and the module fails with the blinking ECON light syndrome.
Last edited by MRDANGERUS; 03-28-2017 at 01:59 PM.