jwillman

I fried my fuel pump relay over the weekend after having pulled the fuse panel and cleaning ground points (all except the one behind the rear quarter panel).

As I am back tracking and double checking to determine if it was something I did I noticed that I likely had the original fuel pump relay, PN 928.618.113.00. The case shows it as 12V and 10A. PET shows 928.618.113.01 as the replacement and according to the pictures I have seen the 113.01 is a 12V 16A relay. The fuse for the fuel pumps is 16A.

Seems like a a 10A relay would be at risk in a 16A protected circuit.

Anyone know the history behind the change from 10A to 16A relays?

Am I understanding the markings and theory correctly?

Thanks

jpitman2

I think you are reading the relay correctly. When I had trouble here I found my single pump pulling ~7A. Its possible that startup inrush might exceed 10A, but its more likely a higher than expected load , or a fault in the wiring would blow a 10A fuse, and I would include a poor earth in that problem.
Run a direct 12V lead to the pump + side, and measure current, and then through the relay socket and see if there is a difference. Even measuring volts at pump +ve lead with the relay socket jumpered might tell you something useful, if there is a drop from volts at relay socket...
jp 83 Euro S AT 52k

John Speake

You need to use a 30A relay same as stock, readily available.

jwillman

So PEt lists the correct relay for the 1979 928 as PN 928 615 113 01. Looking online at pictures of this relay from two 928 suppliers the relay shows 12V-16A.

Is the 16A nominal and 30 A a max or is the 30 A for a later moderl car or am I just uniformed?

AO

If you haven't alreaqdy, I'd be looking at the fuel pump itself if you fried the relay. Auzivision had a fuel pump that satarted going out on him and it would really heat up the relay.

Mrmerlin

clean that last ground before you connect the battery the one behind the right quarter panel liner its also the ground for the fuel pump

John Speake

A 16 A rated relay is going to be loaded near its rated current if you're running external fuel pump, in tank pump and O2 sensor through it. A 30A relay's contacts are going to run cooler and have longer life.

Yu haven't blown the 16A fuse, so this appears to be a case of relay failure rather than overload of the relay.

dr bob

The factory CIS FP relay is not rated at 30A. Later ones that use a common 53B relay are 30A or 40A contacts.

In this case, the contacts weren't the weak point. The damage is in the timer circuit, not on the load side. There is literally no connection between 30 or 87 (supply or load terminals) and the damaged component(s) in the relay.

jwillman

So in some hind sight research I did today I read that soldering irons should have been 20 watts max to do the kind of work I was trying (Fixing the broken solder trace). My soldering Iron was a 40 watt. So based on Dr. Bobs analysis I am drawing the conclusion that my attempted repair likely subjected the relay board to excessive heat weakening the resistor that ultimately burned under a normal range load?

I still plan to yank out the right rear quarter to service the ground point back there, triple check the fuse panel and fuel pump connections before attempting any more starts.

Mrmerlin

I dont think that a 40 W soldering iron would have done much damage if you were quick on the soldering program especially if you were fixing the back board traces, now if you overheated a component than that might be another story.
Once you get all of the grounds cleaned and the power connections cleaned the elex system should work with less heat from oxidized elex connections.

To get to the rear ground first remove the cargo floor hold down ( 2 phillips screws, spray some PB blaster to the underside of the hatch floor ) to loosen the screws. Then remove the 3 screws attaching the rear of the panel to the floor and rear attach point under the tool tray, this will let you bend the panel out just enough without snapping it, a 10 mm wrench can be fit onto the bolt clean the wire and floor connections with an eraser use some dielex grease when you fit it back together ,
put some antiseize on the cargo holdown screws if you can get them loose.
( Note an impact driver may be handy to loosen these screws.) They will be rusted.

dr bob

Jim-

it wasn't the soldering iron. Contrary to what might it seem, a clean 40w iron will flow the solder faster, and lessen the risk of overheating components on the other side. Unless you were making a sandwich while left the iron on that trace, no risk. The damage to the cap on the component side is more typical to reversed polarity than soldering iron abuse.

jwillman

Well darn. Now that leaves allot of unpleasant possibilities open. reverse poilarity where?

Thanks

dr bob

Jeff--

I don't have the 555 circuits handbook in front of me, and don't know how they wired the FP relay smarts so I can't tell you exactly. That's why I suggested in the previous thread that you test the voltages at the socket, looking particularly at the 31b polarity and whether the diode on the relay board is doing it's job isolating the timer from that pin when/if the pin goes high.

I wouldn't be too tough to make a "new" FP relay with a few pieces on a perfboard, mounted in or on a standard 53B. But it wouldn't have the little 31b pin on it without more work.

John Speake

OK, my bad, sorry.

dr bob

More:

The max supply voltage is less than the 15 available from the alternator. A common way to limit the voltage to the 555 is with a resistor from B+ to a Zener diode to ground. If the Zener fails, the supply voltage to the timing cap will be too high and it will fail. Ultimately the voltage rating on the cap must be greater than the rating on the Zener. It's easy enough to get one with a 16V or 20V rating anyway, something to consider if you decide to repair the old one.