MS relay board losing sensor readings and throttle position settings

[Dlander]

Hi some of you know I have been fitting a V8 to the Dlander following the demise of yet another oil burner. (I will post a build thread at some point with some pictures). Anyone who has seem me out in the truck will know i might need a bit of waterproofing, so the only way to go is megasquirt and EDIS. I have been having some trouble but have solved them now and I promiosed Nige that i would post some details here so others can make of it what they will.

I am using a relay board due to the lack of any wiring loom for a petrol engine fuel relays etc. and MS1 V3 ecu, I have also had addition work on the ecu to run a fan relay which switches on the return. Got the whole kit from Nige (www.megasquirt-v8.co.uk) who i have to say has been very helpful and a great support as has Fridge Freezer who modded the ecu for me.

Ok so I fitted it all, and apart from a couple of teething problems got the motor running, but thats when the problems started. Shortly after getting it going the engine started to run very lumpy and very rich the coolant temperature and the manifold temperature both started reading -40C, then the engine cut out and died no amount of turning over would get it to start again. The TPS had lost its settings and thought it was wide open on cranking so wouldnt fire the injectors.

Some phone calls and Nige sent me the readings for all the sensors. Out with the multimeter and started taking measurements at the CT and MAT sensors, and the TPS which was sending 5v back down the ground wire, FF tells me is what it should do, and that it should then go through an earth on pin 19 of the DB37 lead to the ecu. so on checking the leads all this was happening correctly. But if i removed all the ground wires for the CT MAT and TPS and sent them directly to ground they all worked fine. Great, but then it just moved the problem on, as then the fuel air ratio stopped working and the O2 reading went west as well.

All this was getting me a tad frustrated and i nearly pushed the truck outside and set light to it. Instead I drank a bottle of red and set about searching the net for some solutions.

It would appear that there has been some difficulties with wiring relay boards and grounds. The main earth from the board says it should go to engine ground, and all the others go back to the relay board and then on to the ecu. (with all the wires connected on the 37 pin lead except 25,27,29,31 as these are not needed). This was exactly what i had done and all was checking out. The lambda has the two ground wires, one is for the heater and the other the sensor. The heater was earthed in the engine bay the other i had taken back to the relay board and used the spare earth in the PWM relay slot. This I copied from Sam's instal post on here. For some reason this was what was causing the problems in my set up.

After reading some forums for Japanese cars I came across someone who was having similar symptoms to me, and had got so fed up with it he had dumped the relay board and connected everything back to the ecu which then cured his problems, but in his post he said he had earthed the lambda to the engine and the ECU to the battery. This got me thinking about the lambda wires. I swapped the engine earth to the lamda and the battery earth to the relay board and voila it all works as it should........ Dont ask me why I have no idea, all i know is i've had it running in the workshop all afternoon without a problem. Berore anyone asks there are three new braided earth cables from chassis to engine and the battery earths to chassis are all good.

Also to give everyone a laugh...... in my lack of confidence in myself i thought i must have the DB37 wired the wrong way round so swapped all the wires over and guess what - it wasnt so had to swap them back again.

Anyone who has wired up one of thses leads will know what a complete PITA they are -- to do it twice is just stupid!

Anyway thanks to Nige and FF for thier understanding and support, now i can fininsh putting the truck back together and get it down for an MOT after Easter so i can then get on with tuning it ready for Summer Sisters in Wales in June

[simonr]

I had similar issues with a relay board. The problem was just that some sensors were earthed to the engine and some to the relay board.

The reason this causes a problem is the voltages being measured are quite small and if you measure the voltage between the ECU and engine, though it should be zero, it can vary a bit just because of resistance in the wires etc. if all the sensors are connected to the same earth as the ECU and the ECU is measuring voltages with respect to the same earth, the problem doesn't arise.

The symptom I had was after starting, if you revved the engine, it would stall. If you waited 30 sec and rev the engine, it would be fine. The reason was, after starting g the battery is low. if you rev the engine, the alternator starts producing a lot of current and this was raising the engine earth a volt or two higher than the ECU earth and thus the sensors all read high. Once the battery had charged, the alternator was not spitting out so much current and the voltage stayed the same when you rev it. It took a bit of figuring out I can tell you!

Si

[Chazza]

Interesting story and thank you for telling us!

I have just been reading Phil Ringwood's opinion on the subject of earths, and he recommends putting all earths to the same point on the engine block, including the ECU - or at least all earths to where the ECU earth is. Dirty connections don't help either, as this increases resistance,

Cheers Charlie