View Full Version : 737 NG both fuel fliter bypass lights

12th Aug 2002, 06:10
Can someof the experts please tell me how this can happen without fuel contamination. After landing no contamination in the fuel was found. Both fuel filters as well as the in line filters had to be changed along wiht the pressure switches. Thanx???:eek:

Golden Rivet
12th Aug 2002, 11:43
Was anything found in the filters ?

14th Aug 2002, 23:41
Cool Man,
Possible Ice !!! Regs require only 1 water drain a day(first flight)

Bobby Johnson
17th Aug 2002, 10:12
Did you get the maint fault code from the FMC? If so what was it.

Did you only get the lights come on after landing, you do not say when they came on.

For your info:

The Fuel Filter Bypass Light can only come on under the following conditions:

1. The fuel filter is about to be bypassed.
2. The EEC senses that the fuel filter signals disagree
3. The differential pressure swith has failed
4. The wires and connectors between the differential pressure switch and the DEU's failed.

The causes of these failures can be one of the following:

1. Filter contamination either fuel tank contamination, fuel pump contamination or HMU contamination.

2. Failure of the fuel filter differential swith.

3. A wire harness between the differential pressure switch and the EEC

4. The EEC

5. A DEU

6. The wiring (As in all things when you have isolated everything else) between the light and the DEU.

It would appear that as they changed the filters etc it was probably one of the first items.

I hope that helps - Or maybe not

18th Aug 2002, 01:15
I'm not familiar with this engine now, but, is it possible for the fuel heat system to fail and for wax to form in the fuel and block the filters? (it would be gone by the time the filters were checked?).

Appreciate that fuel heaters are often oil coolers so were the oil temps. high? Could the plumbing be incorrect?

18th Aug 2002, 03:25
Arn't the fuel pressure bypass lights (filter clogged)in the low pressure circuit (that is prior to fuel-oil heat exchangers)
Agree with the comment of the fault code of the FMC although have had a simmular prob due to water and when tested through to verify nil codes visible.
Good luck !!

Trash Hauler
18th Aug 2002, 12:16
Did both lights come on together or very close together?

Did the event occur after a rapid acceleration of the engines?

If so I have sen a similar event. In this case the switch is triggered during rapid acceleration of the engine. During the acceleration the fuel demand causes a pressure drop across the filter (outlet px drops below inlet px momentarily) and triggers the pressure switch.


Trash Hauler

27th Aug 2002, 07:07
Thanx for your reply bobby in this case the lights came on after take off in sequence the first one at 800' agl and the second one after about 4-5mins. Might i add they insisted that there was no contamination of the fuel bcos they did the regular checks etc.

29th Aug 2002, 09:24
Seagul 1,
Sorry to ask you these questions but am curious
1. Was the centre tank being used ??and if so was the order of light illumination No 1 engine and then No 2.

31st Aug 2002, 07:54
No problem at all the center tank was bieng used and the lights came on No2and then No1 please tell me what ure getting at fruit loop Thanx :rolleyes:

1st Sep 2002, 00:43
Thanks Seagul 1
I got the order of lights wrong (forgot about the scavange pump circuit) I've seen this 'occur'before,but it was a long,long time ago and on the 737-200(showing my age)and the culprit was (according to Boeing)Baffle plates and vent valves !!
I can't find my diary's so cannot give you the Advisory Notice but think it was 83/84.
Again thank you !!:D