Protection Relay Fuse

10amp fuses keep blowing on my protection relay...as soon as I put one in it blows. Is this because the relay is bad or what might be another culprit? What should I check? No buzzing on frequency valve....
1984 308 GTB-QV.

Thanks

Brian
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It is hard to imagine what could be bad in a relay to cause the current draw to be more than the 10Amp fuse and blows it. It is possible that there is a short for the circuit following the relay that is eating up the current and blowing the fuse. That is a $110 relay so i would not go and change it just yet. Perhaps you need to find out what other problems you have down stream ...

 

Check the draw on the ECU, the relay panel and the frequency valve. May want to look at your O2 circuit also. Any porsche dealer should be able to get you that relay for less than 90 bucks.

 

$90 ... maybe if you have a 15% off coupon. My Porsche dealer nailed me for $110. The Ferrari dealer wanted $175.

 

That 10A fuse system is specifically designed to blow when/if the alternator output voltage goes crazy high (hence the name "protection relay" -- it "protects" the injection ecu from an overvoltage condition). Although it would also blow if the downstream current draw was excessive or if something went wrong inside the protection relay itself.

Measure the DC and AC voltage between the battery terminals (if you can get the engine running at 2~3K RPM even when the fuse is blown) -- if the voltage is some crazy high value like more than ~15VDC or more than ~0.3VAC = bad alternator (although it would be best to use a multimeter with an oscilloscope function to detect any sharp voltage spikes).

PS Your pictures show the 4-pin protection relay used on 328/TR; whereas, your model originally used the (no longer available) 5-pin protection relay. The 4-pin protection relay can be used on your model, but the wires need to be moved around in the harness connector per the following diagram. Are you replacing the 5-pin with a new 4-pin, or did someone else previously do that and your car was working fine with the 4-pin and the fuse just started to blow?:
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Protection Relay? I didn't know anything about this yet, where is it plugged in? dash or with the 'U Boat Electronics' in the trunl floor.

Is the fuse part of the relay or in the dash, and what are the symptoms when bad/blown?

 

This is found in the trunk partner, under the mat.

 

Only used on '84-'85 US version 308QV -- if your '85 308QV is a euro version, it doesn't have one.

Located on the RH side in the rear trunk (item C):
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On the older 5-pin style, the fuse is located inside the protection relay case so is not user replaceable if it blows. On the 4-pin style, the fuse plugs into the red receptacle on the top of the protection relay as shown in the post #1 photos.

The "with Lambda" part of the injection system will be dead = the frequency valve will not buzz = the frequency valve is always closed = the lower chamber pressure is higher than normal = the upper chamber pressure is higher than normal = less pressure drop across the slits = less fuel delivered for a given size slit opening = lean condition - although, sometimes, the engine can be kept running (but weakly) by "goosing" the throttle.

 

While being a simple relay at heart, a diode has been added to detect any transient voltage over 15v and deliberately blow the fuse. Steve's alternator test will look for this condition. If that proves negative, that leaves two things, either that internal diode has failed, or something in the ECU circuit is shorting and blowing the fuse.

To learn which, one needs to remove the relay and place a 10a fuse between contacts 30 and 30a of the socket, then start the car. If the cars runs ok, replace the faulty relay. However, if that fuse blows, you will need to research which item in the ECU is shorting but I doubt you get this far.

 

On the relays with internal fuses, why can't I just solder in a spade type fuse holder that uses replaceable fuses and tye wrap it to outside the case?

BTW the diode in question isn't a standard diode, it's called a 'Zener Diode' they conduct a preset voltage. Most commonly used as a reference voltage device in power supplies or in over voltage protection circuitry.

 

Yes, that would be fine, and would restore the "protection" function vs just replacing the (blown) internal fuse with a jumper wire inside your 5-pin protection relay as you mentioned previously doing in your PM.

You can also just leave the jumper wire that you have already installed inside the case of your 5-pin protection relay and add the 10A fuse between the large P (beige) wire in your harness and terminal 30 of the protection relay. Might be a cleaner installation/modification vs running wires into and out of the metal protection relay case.

 

No coupon... I just bought one last week he had 15 of them in stock. Porsche of Annapolis $88.25 plus tax. $175 from a ferrari dealer while still absurd is less than what I've seen it advertised for on other parts websites.

In any event my point was that it's available for a fairly reasonable amount if you shop it around providing that is actually the issue and not something else in the circuitry.

 

Larry's solution:
http://www.ferrarichat.com/forum/showpost.php?p=137337777&postcount=6

 

Thanks to all for the help...greatly appreciated. Connected a 10A fuse from terminal 30a to 30 and the fuse did not blow...so looks like a bad relay....?
As for the protection relay, I change mine over to the 328/TR setup a couple years ago.
Ordering in a new relay today...will let you know how it goes.

Thanks again

Brian

 

Steve,

Thank you for the posting with the wiring diagram for both old and new protection relays.

I went home and removed the jumper I had installed a few years back and wired in a fuse holder. I ignored the simple posting that showed how to wire a fuse holder, and "re-engineered" it by drilling a small hole next to the mounting post that protrudes thru the bottom........... Don't Do That.

I spent the next 3 hours trying to figure out why my engine hardly ran to find I drilled thru the tiniest wire you have ever seen. Then disassembling the relay, removing 1 turn off each end of the coil re-assembling, troubleshooting my miss wire, and using Steve’s wiring diagram.

I would still be stumbling around had you not posted.

 

Just called Porsche of Annapolis and they said they do not stock them and they did not sell any last week. Do you have a contact there that might know something?

Thanks

Brian

 

Let me know how you do, I should order one too.

 

Responded to your PM.

 

I normally replace this Stribel/Ferrari/Porsche OVP relay for a Mercedes relay. Has a different connection but works perfectly. Costs only about $45,- which is half the price of the Porsche relay and quarter the price of a Ferrari relay. They always have a couple of them in stock over here at my nearest Mercedes dealer (Holland) and it looks more solid and well built.
Partnumber: A2015400845
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