Fan control inhibits fuel pump

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Michael
Helpful MS/Extra'er
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Joined: Sun Apr 13, 2014 10:51 am
Location: Droitwich, UK

Fan control inhibits fuel pump

Post by Michael »

I post this for the benefit of those who may have made the same mistake, and for the amusement of others!
I’d had my megasquirt controlled Zetec running for two days and feeling flushed with success decided to disconnect the thermostatic fan control and wire the fan “porperly” back to the MS. I’d already done the necessary pcb mods so it should have been straightforward.
However, the engine would no longer start. After listening carefully, it was apparent that the fuel pump was not running at key on. I then found that if I disconnected the fan relay, the fuel pump ran OK and the car would start: connect the fan relay and no fuel pump. Took the MS out and connected it up to the simulator on the bench. It seemed OK until I connected a relay, any old relay or a bulb, between 12v and the fan output on spr1 (pin on db37 in my case). With the power turned off connecting the relay between spr1 and the 12v supply, the pump led came on and the MS came to life. This was the clue. I had connected the +ve side of the relay to a permanent 12v feed rather than a switched feed. What was happening was that 12V fed back through the relay coil windings to the 12v supply in the MS, MS thought it was wake up time and ran the fuel pump and then sat there waiting for ever, as it would never power off with the permanent feed, it never saw the key on and so never ran the fuel pump. ( I thought the pump would run during cranking to start the car, but perhaps it doesn’t run until it sees a certain rpm?) The solution run the +ve side of the relay coil to a switched supply – obvious when you know, a head-scratcher until you do!

Michael.
kjones6039
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Re: Fan control inhibits fuel pump

Post by kjones6039 »

Michael wrote:( I thought the pump would run during cranking to start the car, but perhaps it doesn’t run until it sees a certain rpm?)
The pump should start as soon MS sees any RPM. That is to say, there is no pre-set rpm at which the pump starts.

Ken
1979 Corvette - 383 CID SBC w/ Holley Pro-Jection 900 CFM TBI, 4-85 lb lo-z injectors & Walbro 255 pump
MS2 v3 w/extra 3.4.2 Release
36-1, Delphi LS2/7 coils in wasted spark, driven by v2.0 logic board from JBPerformance
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Michael
Helpful MS/Extra'er
Posts: 62
Joined: Sun Apr 13, 2014 10:51 am
Location: Droitwich, UK

Re: Fan control inhibits fuel pump

Post by Michael »

Thanks Ken - that's completely logical. I'm sure that is happening OK as it does start quite easily.

Michael.

Just checked on first cold start this morning: fuel pump is there as soon as cranking starts. As if your word should need confirmation!!
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