DTAForum.com - a forum for the DTA ECU

The car won’t start and the S40 can’t be seen by my laptop (usb connection). I’ve checked that the power supply is ok, and the earth is ok. The main power relay and fuses are all ok. The fuel pump cuts in when the ignition Is switched on. Are there any other tests I could run to check the ecu itself?

Thanks, Richard

My limited knowledge says if the FP is priming / running when ECU is switched on - it is working as it grounds the pump so making it run!
Problem is likely the USB / Serial connection!

Thanks katana,

the s40 wiring diagram shows the fuel pump relay connecting to pin 1 so I guess (but don’t know) that if the ecu is dead no connection would be made.

Have taken apart and reconnected the serial and usb cables several times. Maybe I’ll get a new usb cable and try that.

Thanks, Richard

Looks like it was a combination of a dodgy USB extension cable and a dirty power supply. The S40 lives again.

Back again and mystified.

Having reworked the power supply such that I'm feeding off a battery live terminal on the fusebox (via an inline 20a fuse), I have been trying to start the car.

Voltage across battery terminals: 12.4v
Voltage on real time display with ignition on: 12.4v
The red band on the real time display voltmeter is set at around 12.3v. When I crank the engine, voltage drops to just below this figure and then the ECU disconnects from the laptop (message ECU not connected) and the car doesn't start. As I can't see any diagnostics, I don't know what's going on.

Surely the ECU should tolerate lower voltages than this? Checked batteries on other cars in the household and they are around 12.4v also, so this isn't an abnormal situation.

Am I missing anything obvious?

Thanks, Richard

ECU will work to 7 volts in my experiance. However try feeding the starter with it's own battery whilst you trouble shoot?
does the ecu main ground go to the battery or the same place as the battery to engine earth strap?

rjwooll wrote:The red band on the real time display voltmeter is set at around 12.3v. When I crank the engine, voltage drops to just below this figure and then the ECU disconnects from the laptop (message ECU not connected) and the car doesn't start. As I can't see any diagnostics, I don't know what's going on.

PC updates at about 10 Hz. So the voltage drops and comms stops, with the PC displaying the last known values while it waits for the ECU to start communicating again.
Presumably when you stop cranking, the PC starts communicating again?

Don't want to teach you to suck eggs, but are you confident the 12v supply you're using to feed the ECU remains "live" in the cranking position?

SMR wrote:Don't want to teach you to suck eggs, but are you confident the 12v supply you're using to feed the ECU remains "live" in the cranking position?

That is one thing I’m checking. When I got the engine started a few days ago, the real time display showed a sudden and very brief drop to 0v after which it started. I’m almost certain the permanent 12v feed is OK - but I will recheck it; it is possible that the 12v switched feed to the relay is playing up. Will connect my voltmeter to both the permanent and switched feeds to see what is going on and make sure the relay is working correctly.

Never be afraid to teach me to suck eggs; I clearly wasn’t given enough as a child!

Cheers, Richard

Good news. I had time to check the switched power to the relay today- 7.5v! It dropped to a lower figure while cranking such that it was not always enough to activate the relay. This circuit came off the vehicle wiring loom but I couldn’t easily trace it back so ran a new wire from a spare switched ignition terminal on the fuse box - battery voltage now getting through and the car started perfectly!

I’ve had cold start problems for years and posted about them on here. I think this dodgy switched feed to the relay may have been a big part of the problem all along.

Happy again!

Thanks again for all the suggestions

Richard