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745 #1 fuse getting very hot

alschnertz

alschnertz

Enough already!
Joined
Oct 4, 2009
Location
CT
1986 745T, 157,000 miles.

I found that the #1 fuse (fuel pump, Engine Management, ETC) is getting very hot. Almost to the point of not being able to touch it.

I've read that the main fuel pump should draw 6.5 amps.
Where would I check that? I connected my amp meter across the #1 fuse and found it was drawing between 10.5 and 12.1 amps. Seeing that high amperage, I elected to replace the main pump. So I put the new pump in and am still drawing 11.5 amps across the #1 fuse at idle.
Maybe that's a combination of the pre-pump + the main pump?

When I replaced the fuse panel a few years ago (due to meltage), I was very careful to clean and solder all the crimp connections. All were very clean to begin with. At that time, I replaced the fuel pump relay and drilled cooling holes in the relay body too. Additionally, I removed the #1 wires from the fuse panel and replaced them with a separate "stand alone" 30 amp fuse holder (though I only use a 20 amp fuse).

Just to be clear, after isolating the #1 wires from the fuse panel, the fuse panel and fuel pump relay do not get hot anymore.

Is it likely that the original pump was fine?
Is 11.5 amps across #1 normal?
Should I really be able to burn my fingers by touching the #1 fuse? That can't be right.
Could the Engine Management or ETC be causing this? The engine does have a slight intermittent hiccup at idle.
 
alschnertz said:
Don't take offense.
I just thought I'd post it here too.
Thanks.
Click to expand...

Not offended, Al, just feeling like a dope for getting the whole explanation of contact resistance typed into a post again before putting 2 and 2 together. Remember, engineers are often just examples of high-functioning Autism. :oops:

http://www.brickboard.com/RWD/volvo/1542476/740/760/780/check_fuel_pump_amperage.html

"Well ya see, Norm, it's like this... A herd of buffalo can only move as fast as the slowest buffalo. And when the herd is hunted, it is the slowest and weakest ones at the back that are killed first This natural selection is good for the herd as a whole, because the general speed and health of the whole group keeps improving by the regular killing of the weakest members. In much the same way, the human brain can only operate as fast as the slowest brain cells. Excessive intake of alcohol, as we know, kills brain cells. But naturally, it attacks the slowest and weakest brain cells first. In this way, regular consumption of beer eliminates the weaker brain cells, making the brain a faster and more efficient machine. That's why you always feel smarter after a few beers." -Cliff Clavin
Click to expand...
 
cleanflametrap said:
Not offended, Al, just feeling like a dope for getting the whole explanation of contact resistance typed into a post again before putting 2 and 2 together. Remember, engineers are often just examples of high-functioning Autism. :oops:
Click to expand...

OPTIMIST: "The glass is half full!"

PESSIMIST: "The glass is half empty!"

ENGINEER: "The glass is the wrong damned size!" :oogle:
 
cleanflametrap said:
dilbert_optimist-pessimist-engineer.gif
Click to expand...

I don't care WHO YA ARE>>>THAT'S FUNNY! :rofl:
 
It's been a few years, but this problem reappeared last fall. Left me stranded and I got it towed home and it hasn't moved since. Spring is here and my 17-year old wants to learn to drive a stick so it's time to fix it.

Cliff notes version of what happened last November...
After about 10 minutes of driving, the fuel pump wire went up in smoke. Strange that the fuse holder melted but the fuse didn't blow.
Fortunately I separated the fuel pump wire from the fuse box years ago as stated above.

In these intervening years, I've learned about the biodegradable wiring harness. I did see some missing insulation on the ground wires near the intake manifold but haven't investigated further yet.

Would it be likely that the deteriorating wire harness is the root of the problem? Again, the fuel pump wire and its independent fuse is isolated from the fuse box.
 
alschnertz said:
It's been a few years, but this problem reappeared last fall. Left me stranded and I got it towed home and it hasn't moved since. Spring is here and my 17-year old wants to learn to drive a stick so it's time to fix it.

Cliff notes version of what happened last November...
After about 10 minutes of driving, the fuel pump wire went up in smoke. Strange that the fuse holder melted but the fuse didn't blow.
Fortunately I separated the fuel pump wire from the fuse box years ago as stated above.

In these intervening years, I've learned about the biodegradable wiring harness. I did see some missing insulation on the ground wires near the intake manifold but haven't investigated further yet.

Would it be likely that the deteriorating wire harness is the root of the problem? Again, the fuel pump wire and its independent fuse is isolated from the fuse box.
Click to expand...

It's been only 5 years. Laws of physics haven't been repealed yet by the new administration - excessive heat (enough to melt things) in an electrical circuit is local in origin. Not saying you don't need harness help, but the melted fuse socket is a poor connection right there where it melted. Usually moisture caused oxidation, then heat takes the temper out of the female, and typical in an ashtray 740.

Given the wire is already separated from the fusebox, you should be able to replace just that wire, and splice in a new fuseholder.
 
Last edited:
"excessive heat (enough to melt things) in an electrical circuit is local in origin"
That is key.

You know, I never even thought that the fuse holder I installed could have been at fault.
Though it was a 30A holder, who knows what quality its connections really are/were.
First step, replace that fuse holder...again.

Thanks
 
alschnertz said:
Though it was a 30A holder, who knows what quality its connections really are/were.
Click to expand...

It isn't always the quality of the fuseholder. It certainly wasn't the first time it messed up, when that fuse was in the panel. It is the entry of moisture at the root of the problem.

Of course the circuits drawing the highest current continuously (with the motor running) are going to be the most vulnerable to contact resistance increase. And the fuse associated with those continuously run circuits is going to be hotter even when the connections are still good, because a fuse element needs to operate at elevated temps if it is going to melt (perform its function) when the current is actually exceeded.
 
All right then, here's this weekend's project...
 

Attachments

  • 745T Fuel pump fuse.jpg
    745T Fuel pump fuse.jpg
    59.3 KB · Views: 94
http://www.forums.turbobricks.com/picture.php?albumid=364&pictureid=2916


I am practicing current measurementsituation of the fuel pump using a scope and an amp clamp.


Hard to see in the pic, but according to my test I was only pulling about 2 amps, but I may have the settings wrong. I think it was gseelster that told us he measured closer to 6 amps on his tests.


Google search says average draw is about 1 amp for each 10 psi fuel pressure:

http://www.underhoodservice.com/fuel-pump-diagnostics-using-voltage-drops-and-current-draw/


12 amps seems too high.
 
ZVOLV said:
http://www.forums.turbobricks.com/picture.php?albumid=364&pictureid=2916


I am practicing current measurementsituation of the fuel pump using a scope and an amp clamp.


Hard to see in the pic, but according to my test I was only pulling about 2 amps, but I may have the settings wrong. I think it was gseelster that told us he measured closer to 6 amps on his tests.


Google search says average draw is about 1 amp for each 10 psi fuel pressure:

http://www.underhoodservice.com/fuel-pump-diagnostics-using-voltage-drops-and-current-draw/


12 amps seems too high.
Click to expand...

Zach, what pump were you checking, under what circumstances, and what voltage delivered? The current does look a little low to me for a main pump keeping a rail at 3 bars with battery charging (~13.8V).

Looks something like this tank pump drawing about 2A. (8 years ago on turbo brick 242Ti)

tankPump20.jpg
 
cleanflametrap said:
I think the current at fuse 1 in a 740 is a lot more than just the fuel pump's.
Click to expand...

Right, but it seemed like the fuel pump circuit was creating the high amperage.

It will be easy to check again with the fuel pump isolated from the other items powered by fuse #1. I'll check both the fuel pump and then across fuse #1.
 
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