fuel pump voltage

[SwanJack]

Might be a stupid question, so aplogies !!

When I take a voltage reading across the fuel pump connectors when the pump is disconnected, I get 12v ish. When I take a reading across the connectors when the fuel pump is connected I only get 9v. If i rev the car, the voltage stay the same. Is this normal, again apoplgies if this is dumbass!

[SwanJack]

Great, thanks. A day with the multimeter it is then!

[SwanJack]

Before I started going through the maze of TVR electrics I made up a test lead from another 12v battery which was charged to 12.33 volts, spade connectors at the end all joints soldered. I had 12.33v at the spade connectors. I hooked the spades up to the pump and it ran, but showed 9v, would this indicate a pump fault?

[SwanJack]

Belle427 said:

Perhaps the pump is tired and working hard hence the excessive drop. I have a spare aftermarket pump in my boot if you wanted to try that, I'm in blackwood, south wales.
Pm me if you wanted to take a drive up one day.

Thanks for the offer. I bought a new fuel pump last week and put it on yesterday, but the car just wouldnt run over 3000rpm, serious fuel starvation, and the pump was making an awful racket. That's when I started with the voltmeter. My last pump was diagnosed by Avia Speed Shop as being on its way out, so I put that back on and the car ran better.

[SwanJack]

Penelope Stopit said:

You need to double check everything
If the wiring and battery are good there is no way that a faulty fuel pump can drop the voltage from 12 Volts to 9 Volts if the correct fuel pump fuse is fitted
My reasoning for the above is that it would take a high current draw to drop the voltage by 3 Volts, if a fuel pump draws too much current its fuse should blow
It is very important for you to know that the circuit is correctly fused, fires do happen

Edited by Penelope Stopit on Sunday 23 April 19:07

Why would it drop the same amount of volts when I 'bench' tested it by wiring it up to an another fully charged car battery, it wasnt connected to the car's loom at all at that point?

[SwanJack]

I read into this more and started from scratch. I did a voltage drop test and found 3.5 on the +tive side and 0.5 on the ground. I used the fuel pump circuit linked below and couldn't see anything unusual, I'll clean up all the connectors and test for voltage drops along the +ve circuit tomorrow. Am I right in assuming that the circuit is not fused as the linked diagram doesn't show one and as the fuse box schematic for my car, as per the bible, doesn't show one either. Is there anything else I need to do / look out for? Thanks.

[SwanJack]

Thanks for the input, i cleaned up all the connectors and got a drop of .5v at the wire leading from the relay (pin 87 in the middle) to the black bkock connector , all other relay voltages were fine. When i test at the black block connector with the loops and black wires, it increaes to 1.2v, and then on the fuel pump side of the inertia switch it goes up to 1.4v and then then 1.7 at the last block connector behind the seat, which is an improvement on the 3.5v I had yesterday. Didn't have a chance to see if thats enough to get the pump to push enough fuel through.

I did a drop test on the pump by running it on a direct feed from the battery and that alone gave a drop of .6v It's a new pump albeit it aftermarket from Powerspark. Im not really sure if it's any good as it sounds like a rattly bee hive.

I had to call it a day half way through tracking down the fuse, but so far it looks as if it shares the alarm fuse.

Edited by SwanJack on Tuesday 2nd May 00:04

[SwanJack]

Penelope Stopit said:

SwanJack said:

Before I started going through the maze of TVR electrics I made up a test lead from another 12v battery which was charged to 12.33 volts, spade connectors at the end all joints soldered. I had 12.33v at the spade connectors. I hooked the spades up to the pump and it ran, but showed 9v, would this indicate a pump fault?

Your most recent post doesn't tie up with this earlier post?

First off apologies for probably misleading you with my stupidity. In my earlier post I just ran wires from the battery terminals to the pump connections and took a reading on the voltmeter across the pump terminals with the pump running. After I'd read some more I realised that what I should be doing is measuring voltage drop on the pump by connecting my positive volmeter lead to the battery positive and the negitive volmeter lead to the positive pump terminal. When i did that i got .6v as opposed to the 9v before.

I did voltage drop tests all along the positive side of the fuel pump circuit and found ever decreasing voltage drops as I got closer to the relay. In the absence of any obvious faults in the wiring, would this drop simply be caused by electical degradation over time, coupled with TVRs inadequate fuel circuit wiring to start with (as mentioned in another post I've read on here)?

[SwanJack]

Thanks for the plan of attack, unfortunately won't be able to do anything anything until the weekend now.

[SwanJack]

Penelope Stopit said:

Belle427 said:

Probably easier to run some better quality cables with no connections in it, the route isn't that tricky.

Good post
As posted above
If by chance you are attempting to run a pump that consumes much more current than the original pump, you will need to check the cables max current capacity and rewire the complete fuel pump circuit if the cable isn't heavy enough
If the pump consumes a similar current to the original pump, the factory wiring will do the job
Bear in mind that should you rewire the pump circuit you must make sure that all the load carrying cables are uprated, it doesn't matter about the size of relay control cables as they can stay as is, also uprate the fuses accordingly

Would there be any advantage doing this http://www.gtir-motorsport-club.com/t1373-direct-p... or is it just overkill?

[SwanJack]

Thanks for all your help. All sorted, nice 12v at the pump. Also the new pump was rotten so chucked the old one back on and the car accelerates as it should (it used to die at 3,000 rpm on hard driving).

[SwanJack]

Ran some new wire 30 amp wire in the end.