Engine runs real rough when EZ display power disconnected

Webby · #1 (**permalink**) 02-26-2010, 03:06 PM

I think this subject has been mentioned before.
If I disconnect the power supply to the handheld unit the engine immediately runs very rough and soon stalls. As soon as I reconnect it is fine. I notice that the voltage reading on my dash is around 14 volts but the reading on the handheld unit is around 12 volts. Can anybody explain what is happening here?

bluzman2004 · #2 (**permalink**) 02-26-2010, 10:11 PM

The handheld unit does nothing but ask the ecu for info and then shows it to you. If you connect or disconnect it and the motor runs differently, you have a bad ground to the ecu most likely. The ecu must be trying to pull current through the communications cable's ground since the ecu doesn't have a good enough one. It's one of those paths of least resistance things. Make sure your ground wire is ran directly to the negative terimal of the battery and the positive, too.

Webby · #3 (**permalink**) 02-27-2010, 04:16 AM

pos and neg cables are run direct to battery. The pink wire which I thought was just a trigger to turn the unit off and on is connected to a shared ignition switched source. The voltage on the pink wire must do more than just switch the unit as it is the voltage that is reported by the handheld unit. Does it power the ecu?

bluzman2004 · #4 (**permalink**) 02-27-2010, 11:17 AM

The pink wire turns on a circuit that allows the battery voltage to flow to the unit. Kind of like a relay, only different. It also is where the unit captures battery voltage. Make sure that wire is going to a good source as well. Again, the only way the handheld can effect how the ecu runs is if the ecu is using the ground via the communication cable to pass current. Which should never happen as it can damage the handheld.

Webby · #5 (**permalink**) 02-28-2010, 07:36 PM

main neg wire runs direct to battery. I will change supply to Pink to make sure it is getting full system voltage. Then see how it goes.

Webby · #6 (**permalink**) 03-04-2010, 05:26 AM

After giving it a lot of thought I agree that the only common thing about the handheld supply and the ecu is the ground connection. So it seems I must have a fault in my main ground wire. I will get out my testers and check it from ecu plug to battery neg post. Maybe a dry joint or something.

Webby · #7 (**permalink**) 03-04-2010, 05:34 AM

Bluzman
Surely if the system was grounding through the PC RXD wire to the handheld it would either melt the wire or blow up the handheld?

Webby · #8 (**permalink**) 03-05-2010, 06:24 AM

OK so i have measured the resistance of the main neg cable back to the battery using a very accurate meter from pin 28 in the loom connector. 0.02 Ohms. I have also measured the resistance of the neg connection to the handheld back to the battery. 0.06 Ohms. So there is no problem there! The engine will still not run with the power lead to the handheld disconnected. Can a low voltage on the pink wire cause this problem?

Webby · #9 (**permalink**) 03-05-2010, 05:34 PM

Whilst trying to measure voltages on the pink wire I inadvertently tried to start the engine with the main relay removed. Somehow the ECU has become damaged. There is no fuel pump run at power on no injector pre squirt etc. So I have gone from a unit that worked fine as long as the handheld was plugged in to a unit that is damaged and not working at all Great!

Webby · #10 (**permalink**) 03-07-2010, 06:26 PM

Good news ECU not damaged! Faulty relay was to blame. Was testing green fuel pump run wire which is triggered to earth to run pump by ECU. I forgot that it only runs for a few seconds before engine startup. I was testing green wire for earth but by the time I had the probes connected it was too late. Making me think that ECU was not triggering pump. I have now given pink wire it own feed of battery voltage. When I get the new relay I will see if bad running with handheld disconnected has gone away. If so it was low voltage on pink wire not bad earth that caused problem.