I’ve been working on this 2.8 to 4.0 swap for some time now and it’s so close to complete! All relays click, I maintain 35-40psi of fuel pressure while it’s running, but the Bronco runs for about 20 seconds and then completely dies.

I’ve realized that after about 20 seconds with the key on and engine off, I hear a click and get an odd electrical high pitch whine coming from the fuel pump relay. This is why the 4.0 is shutting down, I’m losing power for some reason to the fuel pump, injectors, ecu, etc.

Anyone have any suggestions? I’m not sure why this is happening and I’m so close to getting this 4.0 up and running!

 

Welcome to the forum

There TWO relays involved

EEC(PCM) Relay, this is closed by Key on, it powers the EEC(computer), injectors, MAF heater, IAC Valve, ect.....AND it powers the fuel pump relay, NOT the fuel pump just the relay

Fuel pump(FP) relay is next to the EEC Relay, when key is on the EEC relay closes and powers the coil in the FP Relay, the computer then GROUNDS the coil in the FP relay to close it, but only for 2 seconds, after engine starts computer will ground it full time


From your description your EEC Relay is failing to stay closed, this could be because coil is losing 12v(key on) or losing Ground

Drawing of relay connections seen here: https://www.gtsparkplugs.com/images/...iring-840x.jpg

Pins/slots 85 and 86 are for the coil inside the relay, there is no polarity as long as one is 12v and the other a ground the relay will close

slots 30 and 87 are where power passes thru to activate devices connected to the relay, i.e. ECC, injectors, FP relay, ect.....
It doesn't matter which one has 12v or is connected to the devices, these two slots are connected when relay is closed

So pull out the EEC relay and test its slots with a volt meter, 85 or 86 should have 12volt key on, test battery voltage first, then test which has exactly the same volts key on, if its lower voltage then you have a corroded connector
If voltage matches then use the OTHER slot(85 or 86) as the Ground for the meter, if voltage drops then a bad ground connection

You can test slot 30 and 87 to see which one is full time 12volts, should also match battery voltage, key off or on doesn't matter

 

Ron, I see your name all over these sites helping everyone out with related problems and was hoping you’d respond - I really appreciate it!

I’ll test the EEC relay tonight and will report back with results.

 

Haven’t had much time to work on the bronco, but I did test the EEC relay slots a few days ago. With KOEO the battery voltage was 12.60v, 85 or 86 was 12.56v and using the other as ground was 12.52v. I measured until the whine started and the voltages and ground measurements stayed the same. Tested 30 or 87 (can’t remember) and the voltage stayed at 12.52v.

After checking various connectors, unplugging and replugging in to check for corrosion, I decided to test my fuses again in the 4.0 fuse box. I had already checked these many times, but of course this time I found that the 15a alt field fuse was blown. Replaced the fuse, turned the key and the fuel pump immediately turned on, no relays clicked. Cranked the engine and the bronco fired right up! I do have the alternator field connector plugged in, but I don't have the positive alternator wire attached to the starter solenoid yet so it died after about 3 minutes. Progress!

I’m going to hook up the alternator this week and hopefully it’ll keep running, but something is still up with the relays. I don’t hear the whine, but they’re not clicking like they should be. Would that alternator field blown fuse really cause the issues I was seeing?

 

Engine shouldn't have died after only 3min or even 15min with no alternator, with battery voltage at 12.5

Maybe fuel pump relay is not staying closed

 

Hm, yeah that’s interesting. I believe the voltage was down to 12.0v and wouldn’t turn over. I’ve recharged the battery and I’ll have some time to troubleshoot a bit more tomorrow evening. I’ll let you know how it goes!

 

Yes under 12.2v in cold weather would cause no crank, just too many AMPs(65-75) are needed to turn engine over
Running engine/vehicle usually uses about 20amps without lights and blower fan, so battery only can usually sustain that draw for an hour or so but very hard on a car battery to do that, to be drained that low, not what they are designed for