fuel pump voltage

fuel pump voltage

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Discussion

SwanJack

Original Poster:

1,911 posts

271 months

Saturday 22nd April 2017
quotequote all
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!

Penelope Stopit

11,209 posts

108 months

Saturday 22nd April 2017
quotequote all
There is a problem, you should be reading battery voltage at the pump, you need to check for where the volt drop is

Paulprior

864 posts

104 months

Sunday 23rd April 2017
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Basically you have a poor connection or contact somewhere, that resistance will create a voltage drop across it, but only when the circuit is complete and you have amps flowing,
So you can try and measure resistance in the line when not running, but to drop 2 or 3 volts in line with a motor it won't be a high resistance you are looking for.

Belle427

8,857 posts

232 months

Sunday 23rd April 2017
quotequote all
This is a helpful diagram, its for a griff but I'm guessing its much the same. Some people have found corrosion at the connector which lives near the passenger side central locking solenoid.

http://www.bertram-hill.com/fuel-pump-schematic.ht...

SwanJack

Original Poster:

1,911 posts

271 months

Sunday 23rd April 2017
quotequote all
Great, thanks. A day with the multimeter it is then!

SwanJack

Original Poster:

1,911 posts

271 months

Sunday 23rd April 2017
quotequote all
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?

Belle427

8,857 posts

232 months

Sunday 23rd April 2017
quotequote all
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.

SILICONEKID345HP

14,997 posts

230 months

Sunday 23rd April 2017
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I pulled the pump live through in to the boot abd picked a batterly live which is located in the boot . Add a relay job done .

SwanJack

Original Poster:

1,911 posts

271 months

Sunday 23rd April 2017
quotequote all
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.

ClassicChimaera

12,424 posts

148 months

Sunday 23rd April 2017
quotequote all
Sounds like a faulty new pump. frown

How old is the fuel filter.

Edited by ClassicChimaera on Sunday 23 April 18:51

Penelope Stopit

11,209 posts

108 months

Sunday 23rd April 2017
quotequote all
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

Belle427

8,857 posts

232 months

Sunday 23rd April 2017
quotequote all
Worth checking the filter as suggested or a pinched fuel line.

SwanJack

Original Poster:

1,911 posts

271 months

Sunday 23rd April 2017
quotequote all
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?

Penelope Stopit

11,209 posts

108 months

Monday 24th April 2017
quotequote all
SwanJack said:
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?
There are a number of reasons why
1) The battery used for testing was not a good battery
2) The cable used for testing was not heavy enough
3) The pump is taking far too much current

Did you fit the correct rated fuse in series with one of the test cables. If a correct size fuse was fitted in series with one of the test leads, the battery used was a known good battery and the test cables were the correct size the fuse would have blown if the pump was drawing too much current

Penelope Stopit

11,209 posts

108 months

Monday 24th April 2017
quotequote all
If you are still not sure about what's going wrong with your test the following should help

12 Volt 70 Amp/Hour battery discharge test
To test the above battery a load of 3 x its Amp/Hour capacity should be applied for 15 seconds and the battery voltage should not fall below 9.6 volts

3 x 70 Amps = 210 Amps discharge for 15 seconds : Voltage must stay at 9.6 volts or more to pass the the test

There is no way that a fuel pump can apply a massive load similar to the above and drop the battery voltage to 9 volts

SwanJack

Original Poster:

1,911 posts

271 months

Sunday 30th April 2017
quotequote all
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.

Belle427

8,857 posts

232 months

Monday 1st May 2017
quotequote all
The fuse will probably be doubled up on another and not marked, you will have to pull them one by one to check.
I'm not sure what else you can test if your getting the same results on a good battery and connections etc.

QBee

20,903 posts

143 months

Monday 1st May 2017
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I know nothing about electrics, but do have an original TVR fuel pump available to borrow if it would be of any help with your testing. It always worked perfectly on my car up to 315 bhp, so should be fine on yours. For BHP reasons (my car is now more then 315 bhp) I have changed mine for a Bosch 044 pump, so the TVR one is available to buy if it solves your problem. I visit Cwmbran every three weeks or can post it to you if needed sooner, as the next Cwmbran trip is not for a couple of weeks.

Aussie John

1,014 posts

230 months

Monday 1st May 2017
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On my 94 Griff there is a pump fuse in the loose wiring near the fuel pump relay.

QBee

20,903 posts

143 months

Monday 1st May 2017
quotequote all
I had a discussion recently about fuel pump current draw when installing my Bosch pump. I was told fairly firmly that the pump draws 12 amps, so the cables need to be 17-20 amps and the fuse 15 amps........ those numbers for the Bosch pump, but the point is there is a fuse.