ECU Voltage Drop

Ignition module is receiving a lowered voltage of 2.5 volts, instead of 5 volts, so it is not firing up the coil correctly...hit and miss. CPS is receiving a reduced voltage also. In short, ECU is working, but this voltage drop is the issue.

Question - Would an ECU's voltage regulator fail via reduced voltage output?

I plan to clean up connections Monday, and hot wire ECU's power lead.

thx

Your dvom will average out the voltage of a switching signal. Half of 5v at 2.5v sounds right to me.

What does your scope say?

ZVOLV said:

Your dvom will average out the voltage...

Click to expand...

I have an automotive DVOM, which will show min/avg/max.

CPS voltage is running about 3.5 volts

It's REX-I that I'm working with. ECU output to ignition module has right frequency during a start, but the voltage drop is too much to fire up transistor that supplies the ground to coil.

Injector computer is getting this ECU's timing (RPM) signal.

I'm taking the unit to an electronics shop Tuesday, and see what output voltage to CPS and ignition-module are when bench tested. On car, ignition-module's trigger wire is suppose to have 50mV with key on...its down to 30mV

Either defective voltage regulator in ECU, or its not getting a good hot or ground.

thx

You are in all likelihood getting a correct 0-5V swing to your coil, but your meter is averaging the reading. Your coil should not be getting a constant 5V feed.

Karl Buchka said:

...correct 0-5V swing to your coil...

Click to expand...

I hooked up battery 12 volts to ECU (REX-I) on my bench...2.5 volts is max voltage I found on various circuits.

Yes, ignition-module gets 5 volt on/off swing on pin 16, but my automotive DVOM only read 2.5 volts maximum via min/avg/max readings.

I'll see if electronic tech can examine voltage regulators Tuesday.

Rex-I's ECU circuit board used surface-mount technology (SMT) on bottom side, where small transistors are embedded. So when signal line (#16) to ignition module shoots craps, that's all. Voltage regs, caps, etc., are fine....I used the wrong pin to energize board, which is why I saw 2.5 volts.

Waiting on a new old stock ECU.

ZVOLV said:

Half of 5v at 2.5v sounds right to me.

Click to expand...

On a 740 with Rex-I/Regina setup, I'm reading about 2.5 volts at ECU-pin-16's wire on power-stage (item 420)
when engine is cranked.

On my 940, I'm only reading 1.6 volts when engine cranks, on two different ECUs. So, either both are good, or both are bad

I checked fuses, and I know memory and ECU voltages are on connector's plug...but Saturday, I'm going to send direct battery power to both of those fuses, and try to find the ground wire(s).

If no go, I'll try Sunday to check out those two ECUs on that 740. Either hot or ground sides are weak, or something else is pulling down ECU's voltage. I might disconnect load switch, MAP, etc and check their voltages.