PFD / MFD loose crosslink during engine start

[hkavasch]

We operate a 2005 DA40 TDI with a CD-155 engine. Since one year we have issues with the G1000 during start-up of the engine. The screen shortly go dark, loose the cross link and then boot up with out the link. Only solution then is to pull both fuses and restart both MFD and PFD. This procedure is reastablishing the cross-link.

Diamond recommended to install: RSBD4-082/1 which is basically an additional Battery to buffer the time when the starter engine is engaged. total costs to install is around 4000€.

We are wondering why the plane did not have this issue for 18 years... nothing changed... we replaced the battery.. the issue is there also in sommer with worm temperatures.

Do you have any idea what the root cause could be ? Is there a chance to avoid intalling RSBD4-082/1 ?

thx

Hans

[Steve]

Since this issue is associated with low voltage on the Avionics Bus, I would actually measure Bus voltage during engine start. You could have a poor connection somewhere causing enough voltage drop to cause it. I suppose that a failing starter could also cause this, but that would be less likely.

[Rich]

As a background, it is normal for the system voltage to drop when the starter load is placed on it. If the starting point is low, this can be enough to cause problems. Also, of there are excessive impedances in the system, it can cause local voltage in parts of the system. Check these as you can:
1. System voltage when running. IIRC, the TDI is a 12 volt system, so it should be ~14,2 V. This is to conform the voltage regulator and alternator are working properly. An undercharged battery will cause this.
2. Battery resting voltage at rest before turning anything on: something like 12.5 V. Checking for parasitic drain.
3. System voltage just before start but everything else on and ready to go: 12-ish volts. Ideally measure the current drain from the battery in this state. This is to check for some excessive drain on the battery.
4. Try to get battery and system voltage during start. This might not be feasible for you. But if you can the difference should be no more than a couple of tenths of volts. This is an overall check of resistance in the battery/starter cables and starter relay.

If 1 and 2 check out, the next place I'd look into would be the condition of the battery cables, including terminations at both ends. Be sure to examine the ground cable termination to the airframe. Simple resistance checks with an ohmmeter may show something, but if these reading looks normal, it's not conclusive. Problems may not be apparent at low-current situations.

I won't detail it here, but check for the condition and operation of the master and starter relays. Excessive resistance across the output terminals (when closed, of course) of the master relay can be a player, as would cable termination issues, here. Note that failure of these types of relays is not uncommon.