Electrical Gurus needed - DME RELAY
07-08-2010, 01:26 PM
#1
Instructor
Thread Starter
Electrical Gurus needed - DME RELAY
Hello all. Just did a little bit of investigative work, and while I fixed the problem, I don't know what actually fixed it, so I need some help.
You electrical gurus will be familiar with the electrical diagram of the relay:
So the pin layout is:
30 - 12V From Battery
87b - Fuel pump
85/86 - Switched 12V when key is in 'on' (2nd stop) position
87 - something - then ground
85b - ??? (something in the DME) 12V when 'on' but no ignition
So basically I believe what's supposed to happen is that when you turn the key to 'on' 85/86 are powered sending power through the first relay. When the key is put to 'start' or once the engine is running, 85b drops to about 1.5V allowing the switch to make and the fuel pump gets power.
The problem is that for 85b to make (and eventually send power to the fuel pump) there's some logic / sensors that have to be activated through the DME. The question I have is, other than the crank position sensor, what else drives whether 85b is active allowing the fuel pump to work?
I'm asking because I seem to have an intermittent fault here - sometimes 85b acts like it's supposed to, other times, not so much and the fuel pump doesn't run.
Tough question, but I'm sure someone knows the answer.
Thanks,
Flyguy
You electrical gurus will be familiar with the electrical diagram of the relay:
So the pin layout is:
30 - 12V From Battery
87b - Fuel pump
85/86 - Switched 12V when key is in 'on' (2nd stop) position
87 - something - then ground
85b - ??? (something in the DME) 12V when 'on' but no ignition
So basically I believe what's supposed to happen is that when you turn the key to 'on' 85/86 are powered sending power through the first relay. When the key is put to 'start' or once the engine is running, 85b drops to about 1.5V allowing the switch to make and the fuel pump gets power.
The problem is that for 85b to make (and eventually send power to the fuel pump) there's some logic / sensors that have to be activated through the DME. The question I have is, other than the crank position sensor, what else drives whether 85b is active allowing the fuel pump to work?
I'm asking because I seem to have an intermittent fault here - sometimes 85b acts like it's supposed to, other times, not so much and the fuel pump doesn't run.
Tough question, but I'm sure someone knows the answer.
Thanks,
Flyguy
07-08-2010, 02:40 PM
#3
Rennlist Member
I think you're reading the left-side relay diagram wrong. My take is that 85b is ground (and 85 is also ground).
86 is switched.
You'd have to look at the factory wiring diagram to see where 87 goes - but I think it goes to the DME. The first relay powers the DME and the relay for the pump simultaneously. The second relay power the pump.
30 is +12v (always hot).
87b is to pump.
86 is switched.
You'd have to look at the factory wiring diagram to see where 87 goes - but I think it goes to the DME. The first relay powers the DME and the relay for the pump simultaneously. The second relay power the pump.
30 is +12v (always hot).
87b is to pump.
07-08-2010, 04:10 PM
#5
Rennlist Member
When the car is cranking there is an overide supply (DME pin 4) that activates the DME relay to switch on the fuel pump. Once the car is running and you release the key to normal, the engine rpm then determine whether the fuel pump is running, via the DME, for safety cut off in an accident.