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Fuel pump not operating


nrw34260

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Hi, I have an '84 3.2

 

Would not start yesterday and assumed it was the relay. Checked the relay in situ and there is power to Pin86 and when ignition is on there is power to Pin 87 but there is no power to the fuse.

 

Jumped the fuse and I can hear the pump

 

Any ideas welcome. Thank you.

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23 hours ago, GrahamTompkins said:

Have you checked the DME relay,..always the first thing to check . Also, alarm/immobiliser.?

 

Graham

Hi Graham, I can hear the DME relay clicking  I have run the tests and bridged 30 and 87 but the pump is not running.

The alarm/immobiliser would not let me turn the engine over if it was not deactivated.

The weird thing is I have tested continuity between the pump relay plug and I am getting continuity on 30 - 85 -87 to the fuse, But when the relay is in and I am showing 12v on 30 - 86 - 87 there is no voltage to the fuse.

STARTER RELAY CONT.jpeg

Edited by nrw34260
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IIRC, 87 is (output) feed to the injectors, 87b is feed to the fuel pump, via 25A Fuse 3 (fuse box 1).

Could still be the DME relay

Peter

Edited by PeterK
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29 minutes ago, PeterK said:

IIRC, 87 is (output) feed to the injectors, 87b is feed to the fuel pump, via 25A Fuse 3 (fuse box 1).

Could still be the DME relay

Peter

Must be  a relay. I have removed the relay and bridged the terminals and run the pump but it still will not start. 87b does not have earth when terminals bridged. Is it only when engine is running . I am now also considering crank position sensor.

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It’s worth swapping the DME relay . It’s a cheap easy fix and best to carry a spare anyway.

the dme has a habit of developing faults with the solder on the circuit board, can be fixed but easier to replace. Can’t remember exactly but think Matt at T911 do them for about £30 

 

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5 hours ago, nrw34260 said:

Hi Graham, I can hear the DME relay clicking  I have run the tests and bridged 30 and 87 but the pump is not running.

The alarm/immobiliser would not let me turn the engine over if it was not deactivated.

The weird thing is I have tested continuity between the pump relay plug and I am getting continuity on 30 - 85 -87 to the fuse, But when the relay is in and I am showing 12v on 30 - 86 - 87 there is no voltage to the fuse.

STARTER RELAY CONT.jpeg

The way the DME system works,to think the unpalatable,if your car was upside down, the relay would only power the fuel pump If the engine was turning.The relay knows this, as it receives signal from Speed and Reference senders.This will also prevent ignition spark.

When you bridge fuses 30 and 87 the pump should run. If it does not , leaving the fuse bridge wire  in, make up a T piece wire and remove the relay. Use the T piece to bridge the three relay connections.

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On 3/25/2019 at 6:29 PM, angusb said:

The way the DME system works,to think the unpalatable,if your car was upside down, the relay would only power the fuel pump If the engine was turning.The relay knows this, as it receives signal from Speed and Reference senders.This will also prevent ignition spark.

When you bridge fuses 30 and 87 the pump should run. If it does not , leaving the fuse bridge wire  in, make up a T piece wire and remove the relay. Use the T piece to bridge the three relay connections.

Hi, I have replaced the relay but no luck. I made a bridge from 30 to 87 but no pump, bridge 30 to 87b pump runs. Made T bridge 30 to 87 to 87b pump running but will not start :(

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My money would be a faulty dme relay. They are known to fail on the solder which causes either a total failure or an intermittent fault....I guess that what you’ve experienced . May be wrong but mine failed and I cracked it open and I could see the electrical burning on the pcgb. They control not only the fuel pump, but also a number of other functions incl spark.

Graham 

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22 hours ago, GrahamTompkins said:

but also a number of other functions incl spark.

 

You will still have spark with a defective DME relay, just no power to the fuel pump.

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7 minutes ago, angusb said:

You will still have spark with a defective DME relay, just no power to the fuel pump.

Not entirely correct. The DME relay is a dual relay, one half for the fuel pump and the other half for the DME itself (Motronic "brain"). So if that side fails you get no engine management control electrics, spark included.

(quote stolen from @SilverWT

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On 3/30/2019 at 4:57 PM, GrahamTompkins said:

My money would be a faulty dme relay. They are known to fail on the solder which causes either a total failure or an intermittent fault....I guess that what you’ve experienced . May be wrong but mine failed and I cracked it open and I could see the electrical burning on the pcgb. They control not only the fuel pump, but also a number of other functions incl spark.

Graham 

I replaced the DME relay and it still would not start. pulled the DME 36 pin connector to check it. When I put it back in, it fired straight up. Put the old relay back in and again fired up. Taken it on a 150mile run today, no problems. For some reason removing and refitting the DME plug sorted it, actually the same thing happened 3 years ago. All running fine now.

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2 minutes ago, mean in green said:

Do DME pins suffer corrosion perhaps?

Possibility. The last time this happened I was due to catch a ferry home to Spain. Packed the car the night before and was leaving early next day. No start! Had to find someone who understood old Porsches, eventually found someone 30 miles away, had to have it transported, luckily they said they would look at it in a couple of days, so the cost of the transport plus £250 from the garage to tell me the pins had corrosion. Must remember this next time it decides to play silly buggers.

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  • 2 weeks later...

Bloody thing has done it again! Been driving fine for a week then went into a "No Start" situation again. Replaced the DME Relay with a new spare and still no start. Had to spend 90€ to get a tow truck home. Went out this morning and it started straight away. I cannot trust this car any more! Think I am going to name her Christine, she has a mind of her own.

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