So, I went and invested in a new battery and the Blazer still won't start !!
At least I know the battery was not at fault.
Earlier in the thread you will see the Blazer did start, then after being driven would not start unless the battery was put on charge, now (with new and old battery) it will not start at all.
It still cranks over good and sounds like it wants to start, the 'Battery' and 'Check Engine' light still remain on when the ignition is on and the needle on the 'Volt Gage' dips into the red when the key is turned when trying to start the engine.
I bought a multimetre/volt metre and tested both batteries which both give positive readings though I can't test the alternator as the car won't start !!
Can anybody give a definitive answer regarding whether the dash warning lights should remain on or go off when the ignition is on (pre-start) ? I ask as one of the above posts state they should go off once the engine is running, another post states they should go off a few seconds after the ignition is turned on !!
Thankyou.

 

Go with the captains answer. He knows

 

The alternator will not directly cause a no-start condition. In post #9, low voltage to the fuel pump was mentioned. That will cause low fuel pressure and a no-start condition if pressure is too low. If the fuel pump is borderline bad, low voltage to it could easily cause the engine not to start. Check fuel pressure and leakdown per the sticky, and post all 4 readings.

 

So, I had an auto electrician look at the Blazer, he checked the wiring from the fuel pump to the fuel pump relay which gave unusual readings. He lifted the fuse box and tidied up the wires underneath. The wiring is a bit of a mess as the Blazer also runs on LPG (though it does not work) and whoever installed it has not done a very good job with the wiring !
The Blazer now starts and runs fine though the engine would not shut off when I switched off the ignition and removed the key !
Initially I had to stall the car to switch it off but after a bit of messing found if I remove the fuel pump relay it switched off, starts and runs how it should !
The auto electrician thinks it has something to do with the way the LPG system is wired, he doesn't know much about LPG so doesn't want to investigate any further !
So I now have a Blazer which runs great with the fuel pump relay removed, is this safe ?
The relay may have been bypassed to another switch/relay enabling the fuel pump to cut out when LPG is selected ?
The main thing at the moment is I have transport.

Thanks again for all your help. .

 

That definitely sounds like a fun one to troubleshoot!

I think you are correct that your fuel pump is now getting power through another switch/relay that was added with the LPG system. If this alternate power source is properly fused and uses a wire gage at least as thick as the factory wire, it should be safe.

Your "auto electrician" should be able to figure it out simply by inspecting all the modifications to the wiring. Not knowing much about LPG is probably an excuse...I'm guessing he doesn't want to mess with it because it's a liability. Cleaning up somebody else's botched wiring job isn't worth the risk for him. I've put myself in that situation before and it's a nightmare.