Constantly Blowing Fuel Pump Fuses

[Swedepilot]

So the ongoing saga of trying to get my 1978 99L back on the road continues. In the last chapter we fixed a rough idle / back-firing problem by replacing the warm-up regulator. Now that the car runs great, chapter 2 begins ...

Chapter 2 involves the 8A fuel pump fuse. So after 12 years of sitting in a garage the 99 L has running about 5 minutes today. During those 5 minutes we went through 3 fuses. So, here is a list of what I have checked so far ...

- resistance in fuel pump ground wire ... just fine
- resistance in power wire from fuel pump relay to fuel pump ... just peachy
- inspected ground connector in trunk (also feeds sending unit ground) for wear and corrosion

Oh yeah, the pump is brand new (o.k. 5 minutes old).

Looking at the wiring diagram that came with the car, I see that the power wire from the relay has two connection points. The first is a 12 terminal connector under the dash and the second is a 3 terminal connector. Both look fine.

On the ground side, there is one 1 terminal connector and it is located in the middle of the trunk. It looks good.

So going back to grade 9 electronics class, voltage = current x resistance. Too much current blows fuses. If we rearrange the equation we find that current = voltage / resistance. Voltage must be constant in a car to keep it running, so the only thing that can change is resistant. In order to increase the current, the denomenator must decrease ... in other words the resistance must decrease ... in other words a short.

Has anyone had this problem before, and if so could you point be in the direction of your short or recommend a good place to start?

Thanks!

[matt]

8a seems low to me for the fuel pump fuse. IIRC my 9000 had a 15 or 20 in that slot. Perhaps the new pump has a different resistance than the old one so it draws more current?

[rsp57]

The pump fuse should be 16 amps.

[Swedepilot]

So I put in a 16A fuse and ran the car for about 3 minutes. I then proceeded to touch the fuse and ended up branding myself on the finger tip because it was so hot. Any chance it could be a 25A fuse? The last thing I want to do is keep putting in larger fuses and force a fire somewhere else in the wiring system.

The new Bosch pump I put in the car is part #W0133-1817011. Does anyone know how much resistance this pump has or how many amps it draws. I looked on the Bosch site but couldn't seem to find any info.

[Jordan]

Agreed 8amps are too little...but I guess we figured that out already. Does the fuse go inline between the relay and the FP or between switched 12+ and the relay. Clean the fuse panel up really well. corrosion between the terminals can cause additional resistance and a really hot fuse. Usually with problems like this it is a corrosion issue around one of the terminal ends...or grounds

[Swedepilot]

The fuse is between 12+ and the relay.

I will try cleaning the terminals in the system today. I'll even use dielectric grease to reassemble them. I'll let you know how everything works tonight.

Thanks.

[matt]

Have you tried swapping the relay out? It may be corroded inside or outside, also causing trouble.

[rsp57]

You are on the right track cleaning the terminals. Also make sure you use a good quality ceramic fuse not the more common plastic ones which have a habit of melting.

[Swedepilot]

So I spent a couple of hours cleaning up the fuse box and all the connections in the fuel pump circuit. The good news is that I am no longer blowing the fuel pump fuse. The bad news is that the fuse is still hot ... hotter than I think it should be. I am still running an 8A fuse in the fuel pump circuit, just as the fuse panel label and wiring diagram ask for. Now, my dilemma is that I don't know the amp rating of the new pump I installed (Bosch #0 580 254 978). I have looked high and low for some information on this pump but have found nothing. I have posted a thread on Saabcentral and the closet they came was this information ...

"The correct c900 8v pump is the one with the number 0 580 254 953. This was subsequently removed from production and replaced with 0 580 254 909. This pump supplies 148lph, but at 5bar with a 10 Amp current draw.

Compatible or higher flowing model numbers which BOLT IN, are:
0 580 254 053 (175lph@5 bar) (11.5Amp current draw though, so this is why the fuse box melts!) (used on the Volvo 240, 260 and 740 with KJet pumps, also on Porsche's with KJets)
0 580 254 984 (165lph@5 bar) (wrong outlet type, need to install a female to male union) (10 Amp current draw)

Incompatible pumps:
0 580 250 910, as this does not flow sufficient fuel (only 130lph@5 bar), also has wrong fittings (used on some Mercedes)
0 580 254 069, as this does not flow sufficient fuel (only 98lph@4 bar) (used on the Volvo 740 EFI + 740 Turbo pump, note that in this case, the 740 KJet pump is actually higher flowing than the 740 Turbo pump!)
0 580 254 070, as this does not supply enough fuel, and is physically too small (only 130lph@3 bar) "

Great information, but none of these part numbers match my pump. Then I found this site ...

http://www.atpelectronics.co.uk/Catalogue/saab.htm

Another good resource. This site says that all 99 EMS/LE 2 litre (4 cyl) years 1974 to 1981 used a Bosch fuel pump with part number #0 580 254 978. Yeah! That matches my number. By to me that would imply that all 1974 to 1981 99's would have an 8A fuse. Can anyone else tell me what there fuse panel asks for? 8A? 16A?

[Swedepilot]

I found this on the Bosch Australia site ...

"Before fitting new pump:
Before fitting a replacement 12v fuel pump, please investigate the reason for the failure of the existing pump and rectify as necessary. Check the current draw of the fuel pump which should be less than 6.5 amps for EFI systems; for K and KE Jetronic systems 11 amps. Excessive current draw or noisy operation can indicate foreign material in the pump roller cell. Use only a fuse having the correct rating as specified by the vehicle manufacturer.

This would suggest that an 8A fuse will not work ... correct?

[TRAILINGTHROTTLE]

o.k., heres what i would do. take a car battery or other source of 12 volt D.C. and a real ampere meter, and wire the pump up, without a fuse straight to the power source...both + and - , and see how many amps it draws. screw the relay and the rest of the wiring. then compare that figure with the amp draw of the entire circuit and compare. my experience is mostly with VWs, but still its a bosch "roller" pump and all of the healthy ones that i have ever checked have drawn less than 10 amps. crap in the pump will make it draw excessive current, as will starving it for fuel, believe it or not, and, well its a bosch fuel pump, so it is prone to let you down totally randomly and at any point. i would also check the pump output whilst it is hard wired up. whatever you do, dont run the pump dry.

[Geoff]

I looked at my '80 99 last night. The fuse block label calls for an 8A fuse but there is a 16A fuse in there. I've had the car for 5 years and drove it about 20K miles 5 years ago and I've never replaced that fuse. The previous owner is on this board so maybe he'll chime in if he remembers if he ever had a problem with the 8A fuse. When he got the car there was a problem with a fuel smell and he went all through the car replacing lines, fittings, etc. I know he had the car for a few years and we estimated that he probably put 25 to 50K miles on it

So apparently the 16A fuse hasn't caused any other issues with the wiring and the wiring holds up to the 10 or 11A draw.

Your 8A fuse is probably getting hot because the draw is probably just around 8A. If you put a 16A fuse in there it shouldn't get hot and the wiring in your car should hold up to the draw of the new pump.

[Swedepilot]

So I finally got around to doing what TRAILINGTHROTTLE suggested. I took the battery out of the front and connected it directly in series with the fuel pump and ammeter. My ammeter has a 10A limit. If the current hits 10A it will act as a diode, preventing all current from moving in the circuit. Before taking an ammeter reading I disconnected the fuel pump wiring harness and connected its terminals directly to the battery. As one would expect, the fuel pump worked fine. Then I put the ammeter in series with the pump and tried again. Nothing. This means the fuel pump is drawing more than 10A, which is why I was blowing my 8A fuse. I guess I am stuck with a 16A fuse after all. This begs the question though ... what current is the fuel pump wiring rated for? My next step will be to borrow an ammeter that can actually read the proper current draw.

[Crazyswede]

So I finally got around to doing what TRAILINGTHROTTLE suggested. I took the battery out of the front and connected it directly in series with the fuel pump and ammeter. My ammeter has a 10A limit. If the current hits 10A it will act as a diode, preventing all current from moving in the circuit. Before taking an ammeter reading I disconnected the fuel pump wiring harness and connected its terminals directly to the battery. As one would expect, the fuel pump worked fine. Then I put the ammeter in series with the pump and tried again. Nothing. This means the fuel pump is drawing more than 10A, which is why I was blowing my 8A fuse. I guess I am stuck with a 16A fuse after all. This begs the question though ... what current is the fuel pump wiring rated for? My next step will be to borrow an ammeter that can actually read the proper current draw.

Old wiring has more resistance to it than when it was new. Rewire the pump with good quality newer wiring...perhaps something slightly heavier gauge and perhaps it will not need the bigger fuse.

[TRAILINGTHROTTLE]

crazyswede, i want some of that low resistance wire that will reduce the draw of the fuel pump. please send me a sample.