Main relay failures

[battlecry]

My main relay became flaky and stranded me a few weekends ago.  I found a replacement at NAPA but last week it began to act up and this morning I took another look and I think I have another winter project.

The M800 uses micro-ISO automotive relays for the main power switching and the fuel injection system.  These relays have fairly generous power ratings, say 20A, 30A, some even 35A.  What got me thinking is that these are current switching ratings, not current carrying ratings, which are typically around 10A less. 

The M800S wiring diagram shows all current for the fuel injection relay going through the main relay.  It also has directly connected headlamp and horn (no switching relay for these loads).

Doing a quick load calculation shows that a current carrying load of 20A on the main relay is not hard to exceed:

LOAD (W)         Max   Run   Start   Amps
low beam         70      70   5.8
high beam         80   80      
parking light      5   5   5   
sidelight         5   5      
tail lights plus brake
lights (5/21)      21   21      
turn signal lights      10   10      
number plate light      5   5   5   
instrument cluster bulb   2   2      
warning light bulb      1.2   1.2   1.2   
fuel pump (clean filter)   60   60   60   5.0
horn            60   60      
cooling fan         0   0      
electronic ignition      48   48   48   4.0
computer         24   24   24   2.0
starter motor      700      700   58.3
            
LOAD (W)         1091   321   913
AMPS          90.9   26.8   76.1   
            
Alternator Rating 520W

And that may explain why the relay has gotten flaky.  I think I want to modify the wiring so the main relay does not carry more than 20A. 

Some ways to remove current load from the main relay would be to:

1.  Wire the FI relay feed directly to the battery, and have the main relay just feed the  FI relay switching coil current (save ~10A).
2.  Add a headlamp relay and drop maybe 6A. 
2.  Add a relay for the horns(s).  (My two Fiamms draw around 5A, but rarely)

The  independent feed to the FI relay seems a must do mod to increase the reliability of the M800S.

The relays draw about ~2W and drop ~.5V, which is less that the headlamp harness drop, so if I can fit the extra wiring under the instruments, that may be a good mod also.

In the meantime I made a relay bypass pigtail and carry it with a spare relay under my seat.

[64duc]

  Your math is faulty.

LOAD (W)         1091   321   913
AMPS          90.9   26.8   76.1 


100 watts is 1 amp, not 10.

[battlecry]

   The loads are at 12VDC.

[ducpainter]

P=IE may come in handy on this one.

[Speeddog]

Starter is wired directly to the battery, so the 700W shown shouldn't be used in a relay power analysis.

[battlecry]

True Speeddog, the middle column, Run, shows a possible max load with hi beam, all lights, and horn on of 26A.  Goal would be to get it so it is under 20A continuous, and it is feasible with a fused jumper cable from the pos battery terminal to the source contact of the FI relay.  Unfortunately the relay holder on the M800S is one of those BMW parts that hold the female spade connector with two prongs from the front and two in the back. Don't know how to extract the connector without damage.  Don't want to cut and splice into the harness just yet.

[Speeddog]

The main relays do occasionally go bad, but it's not a real common thing from what I've seen and heard.

I thought the FI had it's own relay?

You're talking about the two small black relays next to the battery, yes?

[battlecry]

Yes.  The FI relay's +12V supply and the + trigger voltage both come from the main relay.  The two cables are spliced in the harness to the same source.  I'm just going to have the FI relay's + trigger come from the main relay, and the FI relay's +12V come from a fused jumper to the + battery terminal. 

I wouldn't think the relays would not be reliable, but I have had 3 units go funky on me, I think 2 mains and one FI.  I wonder if the PC III places an extra load on them?   

[Speeddog]

Odd that you've had so many go bad.

I suppose the PCIII would put a *tiny* bit more load on it, but IMO it'd be barely measurable.

Have you checked voltage on your system when the bike's running?
Occasionally the voltage regulators can go bad and be charging crazy high, but usually that borks the battery in short order....