Author Topic: 75 TS fuel pump wiring weak point  (Read 600 times)

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GTVeloce

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75 TS fuel pump wiring weak point
« on: August 10, 2019, 09:50:20 PM »
I have always believed Alfa made a mistake with the wiring for the 75 TS to the fuel pump. With my 75 I have had to change (and upgrade) all the wires related to it because they eventually cooked themselves. Now my GTV TS has just had the same problem although I rectified this one in a better way I believe. I also have a 164 3L engine and the wiring loom appears to have suffered a similar problem where you can see the fuel pump wires have all drawn too much current and have become hard and brittle.

The problem is that the relay for the fuel pump is fed via the ignition switch, not a clean power source. This means juice needs to run from the battery to the ign switch (red cable) and then back to the relays (green/black cable) where it jumps between the motronic relay and the fuel pump relay and then back to the fuse box (pink/white cable) before winding its way to the fuel pump underneath the rear seat. All the way it is 1.5mm cable. This means even a good system in working order rarely sees over 10V at the pump and once they get older that drops even further. The other major power users (ignition coils and injectors) receive clean power from a decent sized cable straight off the battery so it is confusing the fuel pump didn't.

My fix this time was just to install another relay (with built-in 15A fuse) underneath the rear seat. The battery in the GTV is boot mounted so it was easy to run a decent power wire to the rear seats. I used the existing earth point but made a thicker cable going to the pump. The existing earth cable became the earth for the relay and the existing fuel pump cable becomes the switching wire. The car started straight away (I couldn't get it to start before the mod) and runs beautifully.

I would suggest the mod to any other TS owners before they have to deal with this problem and possibly other engines as well.

bazzbazz

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Re: 75 TS fuel pump wiring weak point
« Reply #1 on: August 10, 2019, 11:21:47 PM »
Just checking, are you referring to a 916 series GTV Twinspark here?

If so, everyone please read -

GTV 916 Series

One thing to be aware of when doing this is that the earth wire from the fuel pump does not go directly to earth, but runs through the Fuel Cut Off Inertia Switch, then to earth.

Just thought I would mention this in case anyone was tempted just to run a new beefier earth direct from the pump to ground somewhere, as I have seen this done, and while it works, bypasses the Inertia Switch, which is not a good idea in case of collision or rollover.

However the method that GTVeloce has used is the way to go, as the relay uses the original pump earth wire running through the Inertia Switch, which in a collision would still render the relay, and thus the pump, inactive.
« Last Edit: August 10, 2019, 11:29:51 PM by bazzbazz »
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GTVeloce

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Re: 75 TS fuel pump wiring weak point
« Reply #2 on: August 12, 2019, 03:31:47 PM »
Good point bazzbazz. I was referring to a 160 series TS (8V engine). It doesn't have an inertia switch but the ECU kills power to the fuel pump if the engine isn't turning over (i.e. a signal from the CAS). This will still work with my mod as I am using the original fuel pump feed to switch the relay on therefore as soon as the signal is lost the relay will disengage.

Colin Edwards

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Re: 75 TS fuel pump wiring weak point
« Reply #3 on: August 12, 2019, 03:53:18 PM »
Although the resistance of notorious pink / white wire and resultant voltage drop can contribute fuel pump performance issues in the 75, we need to remember these cars are 30 years old.  The condition of the beloved pink / white wire and its connections is as likely a result of the condition of the fuel pump / fuel system as anything else. 
Before replacing the fuel pump wiring on my 75, I decided to ascertain the condition and performance of the fuel pump.  The specifications for the Bosch pumps are readily available.  Depending on the model 75, the FULL LOAD current draw of the pump will be around either 5A, 10A or 13A.  Its critical your car has the correct pump to begin with!!
Once you are certain of the correct pump specification the current draw and pressure delivery needs to be measured.  A decent fuel pressure gauge and multimeter will provide this info.  Also an idea to measure voltage drop across the pump terminals AT THE PUMP, supply voltage between pump and ground and voltage between pump ground AT THE PUMP and nearby chassis. 
I found the pump current draw was around 25% higher than specified, suggesting a failing pump or fuel restriction somewhere or both.
Replaced the fuel filter - some reduction in current draw.  Replaced the pressure regulator - more reduction in current draw!  Replaced ALL rubber fuel hoses - minor reduction in current draw.  Current was still out of spec so time for a new pump.  Fixed!

As the pump ages it will become electrically less efficient so current draw will increase.  An iffy non-return valve can hurt the pump, however a blocked fuel filter will be the pumps worst enemy. 
« Last Edit: August 12, 2019, 03:56:57 PM by Colin Edwards »
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2018 Abarth 124 Spider
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1987 75 3.0

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2012 Giulietta TCT Veloce
2006 147 Ti 2 door Selespeed
1979 Alfasud Ti 1.5

ALF750

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Re: 75 TS fuel pump wiring weak point
« Reply #4 on: December 11, 2019, 03:04:51 PM »
I haven't visited for a while, sorry for posting late.   Elsewehere on the forum I went through some of this problem on my 75TS.   Basically I recommend a dedicated wire and fuse feeding the fuel pump directly, not via the circuitous white/pink-numerous-relay route.   More concerning when I pulled the in-tank lift pump out of the tank I found the wires to it disconnected!    They were too short to have ever been connected, I suspect they had been left disconnected since the factory!   I cleaned the filter and lengthened the wires, replaced it all and have had no problem since (should never boast that, I know...).   Pump runs at Warp 9 now, you can hear it from 20ft away!