1998 s70 bought yesterday. Test drove great. Trailered it home and had son hookup battery to unload. He reversed the battery and now the 15 amp ignition fuse blows when key is turned. Desperately seeking help

 

Ouch..... Not sure where to start, but does the fuse blow as soon as you hook the battery up, or only when you turn the ignition switch to start it? Does the dash board lights work, how about headlights and horn. I'm thinking something shorted out in the ignition circuit. If you can find a wiring diagram online we could find out what that specific fuse powers/protects.

 

You start with the wiring diagram. Short circuits are easy to find compared to the sort of intermittent electrical problems we normally deal with. The short will be there when you get to it (following the wiring diagram).


I have a 98 S70 so I can look at some things for you, but I notice at volvowiringharnesses.com they do not have the 98 S and V70 wiring harness. I think we can still figure it out. They do have the C70.

 

There are two fuse boxes, and unfortunately "ignition" is fuse #10 in both of them. Which one are you talking about? The owners manual is linked below.


Volvo Car USA Support | Contact us | Help

 

P.S. I found that I had earlier located the correct wiring diagram, and with some searching I found it here.
http://www.myvolvolibrary.info/Tech_...Y98_Wiring.pdf


Something I noted briefly there is that most of the simple little Volvo relays contain a diode which would create a dead short in a reverse polarity situation. I am not sure if these diodes burn up in the case of a backwards battery but that is certainly a suspicious point. if it is one of these relays then at least that would be easy to buy and replace.


To narrow it down, it would help if you could tell us whether the fuse blows in ignition switch position I or position II or position III.

 

volvowiringdiagrams.com to start. One thing I would do would be to pull all the fuses and major relays out then test resistance power to ground one by one (with the battery disconnected). if nothing, hook up the battery then turn the key... continue the check by adding the fuses and relays back in one at a time until the main fuse goes.

 

It blows when key is turned to the first position. Dash lights come as do head and tail lights.

 

When you turn to position 1 (accessory only) power from the ignition fuse goes to terminals X and S. After that I'll admit the wiring diagrams are hard to follow because each diagram focuses on just one purpose and it allows the branches off the circuit to get ignored.


Here is what I see:
Terminal X powers the SRS light and the SRS computer directly.


X goes to a relay coil that powers a bunch of systems:
Lights
Horn
Wipers
Radio
lighter
The 'accessory wiring" plug
Nav
power windows
power mirrors
power seat
heater
fuses 30, 31, 33, 34 and 35 at least (maybe more)


None of these accessories are powered by this fuse, just the relay that turns them on. These relay coils all have a diode bypassing them that is what I suspect has burned up. So you ought to at least pull this one relay, labeled "the system relay" in your owner's manual.


Terminal S goes to fuse 18 and you should pull that fuse to cut off everything downstream of that. S also powers a terminal on the central locking computer.


You can quickly pull the ECU and TCU just to see if that solves the problem.


If the lights are staying on, I don't see how that can be happening with the fuse blown. Could you clarify which fuse is blown. I may be on the wrong wire entirely.
Could you clarify what all works after it blows. That might help a lot.


Also if there was some sort of meltdown in the ignition switch, the problem might also be there, but I don't really suspect that.

 

I have removed all 50 and 60 amp fuses and inserted them one at a time with ignition to on position. The 60 amp fuse #6 labeled power to fuse box aliment boutique fusib blows the number 10 15amp ignition fuse when inserted.

 

Thanks. I'm on the right wire then. Power only goes to a small number of users (already listed above) when the ignition switch is in position 1 accessory.

That said, you may have a dozen problems or you may have just one. We don't know yet.

 

UPDATE the beast is alive once more!!! Thanks so much for all the input. Long story short I pulled all the fuses and tracked it down to main relay/engine system and relay with J on it inside the main fuse box. Only took about 4 boxes of 15 amp fuses to figure it out. Thanks again for the help

 

Good job!

P.S. check to see if your alternator is working. It has much bigger diodes and you might have blown something up in the charging path.