Reverse Polarity - What did it do to my car?

[jonl]

On the dumb actions list, I fear that this one is up there.

2000 V70XC, 185k mi.

I am getting ready to replace my fuel pump. This requires me to nearly empty the fuel tank. So I removed the Schrader valve from the fuel rail and clamped on a rubber fuel hose, in order to pump out fuel 2 gallons at a time (that's how big my container is). Things are great. It's working fine. Then I notice that the fuel isn't coming out quite as fast as when I started. I measure the voltage on the battery, while the pump is working, and it's down to 10.xx volts while under that load. That gives me the great idea of connecting my Dodge's battery to my Volvo's battery (as if I were doing a jump start).

Here's where I screwed up. I wasn't paying attention and put the jumper cables on the wrong terminals on the Dodge. The Volvo was pumping fuel out at the time and I heard the pump immediately shut off. This condition lasted around 30 seconds before I realized what I'd done and disconnected the jumper cables.

I'm a little freaked out now, because although the fuel pump starts humming when I jump the relay, no fuel is coming out and the starter just gives me a loud click without turning over the engine.

Any thoughts about what did I did to my car and how to fix it?

Thank you, thank you, thank you!

[erikv11]

30 seconds is kind of a long time for the cables to be on backwards but hopefully the damage is minor.

Start by locating the J relays in the fuse box under the hood. Oe or more of these will usually be cooked. Replace them (you can test them with a 12V source) and see how it is.

Could the fuel tank just happen to be empty or near so?

[jonl]

Thanks, erikv11.

I'll go take a look at the relays now.

If/when I pull a "cooked" relay, will it look black or burnt on the outside around the contacts?

To test one with a 12v source, where do I attach the pos and neg wires?

Still have about 8 gal in the tank per the fuel gauge.

[cn90]

I have myself done dumb stuff before lol... so don't feel bad.

Search for a thread recently (like last week), someone did the same thing connecting cables backward.
The bottom line is some relay diode "melted", I guess to protect the circuit.
Junk yards usually have tons of these relays that erik mentioned.

[jonl]

I'll try to find the thread you referred to.

I think my question is: How do you recognize a fried/melted relay since they are sealed up?

[jonl]

Checked all the fuses and relays (except the fan relay by the radiator) and everything looks clean. Just need to understand how I might go about testing the relays.

[j-dawg]

Have you checked out this thread?
https://www.matthewsvolvosite.com/forums ... hp?t=52252

Good tips on troubleshooting your fuel system. cn90 is an MVS god, dropping science since oh-ten.

[abscate]

The right search term to find this thread was

'reverse polarity'

and it lists the common suspects !!

https://www.matthewsvolvosite.com/forums ... e+polarity

[jonl]

I have read through that one and it does have good information in it but, so far, I haven't found any blown fuses and the relays look good. I've even been able to pop the plastic cover off of all the relays except one and they all look intact on the inside.

Is there a way to test the relays?

[abscate]

Yep - you look at them while you replace them...



Seriously, you want to pull them out, then apply 12 VDC to the switch side and listen for the click.

A more serious test is to look at continuity on the switched side as you apply 12 VDC to see if it goes from INF/open circuit to near 0 OHMS on applying 12VDC.

Usually, is they are fried, they don't click.

You can also jump them with wire in the car, but if you have had a reverse polarity incident, I would be careful doing that. Its possible you have more than one thing fried so it isnt that useful a negative test.