Only 3v to eec

[beerhunter]

Hiya. So, after finally getting the dash lights to work, I was all set to actually drive the thing. Except it won't start.
It turns over, but won't fire. I couldn't hear the fuel pump, so hot wired the fp relay. The pump then ran, but the truck still wouldn't start.
I noticed the voltage to the fp relay from the eec relay was very low. The eec relay itself was only measuring 3v on the permanent live (2 yellow wires).
The relay and connector were both really gacky, so I cut it off in order to replace, and could then test the yellow wires separately. One reads at 9v, the other zero, and when squished together, gives a combined 3v.
It's an 86 2.9, and so (I believe) has only the dash fuse box, none of which are labelled as controlling the eec power.
I read about fusible links - how and where do I find these in order to test? It would appear one has failed hence reading zero. And maybe the other is on the way out hence reading 9v.
FYI, the switched feed (red with stripe) measured 12v, and the earth (black) measured fine.
Many thanks

 

[RonD]

Obviously test battery voltage first, so you have a reference voltage

Yes, they used fusible links, one end of each fusible link is connected to the battery Positive post on the starter relay(solenoid) on the inner fender

The starter relay has 2 larger posts, one has just one wire/cable to the starter motor
The other larger post has battery positive wire/cable attached AND several other wires, this post is the 12volt distribution point for the whole vehicle
Fusible links are connected here as is the alternator, which can also have fusible links

You will usually need to untape the harness a bit to get to the fusible links "other end" where it is spliced to the wire that runs to the device, i.e. fuel pump relay and EEC relay

Starter relay 12v post----Fusible link------X(splice)------(wire)--------EEC relay

Fusible links are just one or two gauge smaller wires connected to the "main" 12volt wire to the device
They are made to melt and separate without causing a fire if AMP load is too high, they act like a slow blow fuse but less expensive
These are usually color coded for AMP rating

 

[beerhunter]

Thanks for the reply. I threw on new relays and connectors to replace both the eec and fuel pump relays, and ran a temporary test 12v feed to the eec relay, and the truck fired right up. So the problem is definitely the power (or lack of) going in to the eec relay.
The fusible links sound like a likely culprit, but am I overlooking anything else that would cause a lack of voltage going in to the eec relay?
Many thanks

 

[RobbieD]

It's an 86 2.9, and so (I believe) has only the dash fuse box, none of which are labelled as controlling the eec power.
I read about fusible links - how and where do I find these in order to test? It would appear one has failed hence reading zero. And maybe the other is on the way out hence reading 9v.

Fusible links are on the "BAT" side of the starter relay. 1986 schematics follow. There are three layouts in the EVTM; this one, 2.0 with amp light, and diesel. This should be the one for your 2.9.

You can do a voltage drop test starting at the battery then moving to points closer to the EEC. First take a close look at your cable and fusibles connections at the starter relay; it's common for corroded ring terminals, corrosion or green wire rot inside of the the insulation, or even loose connections at this point.

 

[beerhunter]

Thanks all. I cleaned up the contacts and now get 12v to one wire, but still nothing on the other. I will hunt more tomorrow to find the break.
Would running it with only the one wire feeding in to the eec relay be OK? I will fix the other, but that would at least get me back on the road.

 

[RonD]

EEC relay 12v powers the EEC(computer) and also injectors and most solenoids, like the IAC Valve, it also powers the Fuel Pump relays COIL, not the fuel pump

Fuel pump relay has its own fusible link to power the fuel pump

You can't hurt anything by driving, systems either work or they don't

I would test battery voltage with engine running, should be 13.5 to 14.5v, if still 12volts then alternator is not working and that can leave you stranded when battery dies

 

[beerhunter]

Thanks all, all back together and running again with soldered contacts and new relays.
Weirdly, the live that was reading dead yesterday had 12v when I went out to it this morning, which made fault finding a bit difficult, but I soldered all the previous owner's crimped connections and sealed with heat shrink tube, so hopefully that will have done the trick.
I tested the voltage when running and it had jumped up to over 13v so the alternator seems OK.
If nothing else goes wrong in the meantime, I may get around to finding the parasitic drain.
Thanks again

 

[beerhunter]

Oh, one other thing... I pulled out the air box to give myself more room for soldering. In doing so, I found this plug lurking behind, not connected to anything.
It looks factory, and as far as I am aware everything works. Any idea what it is for?
Thanks

 

[RonD]

That's the OBD1 connector used to read codes

Read here: https://www.therangerstation.com/tech_library/OBD_I.shtml

Fuel Pump slot can be used to activate the fuel pump manually for testing, you GROUND that slot with a jumper wire, and fuel pump will activate full time with key on
Normally it only activates for 2 seconds with key on, then won't come on full time until engine RPMs are above 400, cranking speed is 200rpms

 

[beerhunter]

Many thanks. I'm glad I didn't need it, as it would have been a sod to find tucked away like that.
Thanks