Ecu issues

[Waine5550]

Had a shop drop a rebuild motor. Wont start.. They stated its an ecu issue. They didn't mark anything when the pulled the old motor. Here the biggest issue. Its an 85 that the dealership added the efi so the color scheme doesn't match. Does anyone have photos of ecu connections under the hood?

[Gary Lewis]

Wow! That sounds like a serious problem.

Our EFI documentation is pretty badly scattered on this website, but let me point out some of it:

• Documentation/Electrical/Electronic Engine Control (EEC): This covers EEC-III, IV, and V systems. Yours should be EEC-IV, assuming the dealership installed an '85 system in the truck. So go to the EEC-IV tab and then the Diagnostics tab to see how to troubleshoot as well as wire colors, connectors, etc.

• Documentation/Fuel & Air Systems/Carbs, Chokes, & EFI//EFI: This has illustrations from the Master Parts Catalog, pinout, acronyms, as well as the Factory Shop Manual section on EFI.

• EEC-IV Quick Test: This is part of the 1986 Engine Emissions Diagnosis manual and has some testing procedures.

• EEC-IV Light Trucks: This is also part of the 1986 Engine Emissions Diagnosis manual but has more detailed instructions than the Quick Test chapter.

• EEC-IV Pinpoint Tests: This is also part of the 1986 Engine Emissions Diagnosis manual but has more specific tests.

That's about as much as I can find, but I'm probably missing something. Anyway, as I said previously, I don't have one of the trucks with this system so cannot get pics. Perhaps others can. But we do have lots of documentation on this site so maybe some of that will help.

[85lebaront2]

If the truck is a 5.0L it was factory EFI. It needs Fuel, compression and spark to run and spark needs to be at the correct time. When the key is first turned on, the fuel pumps (2 or 3 depending on number of tanks) should come on, then go off, if this doesn't happen it will not run.

One of the biggest problems with the 1985/86 EFI 5.0 engines is the system ground, the entire EFI system has one ground, a 2 pin plug and socket pair that attaches to the negative battery cable at the clamp.

I do have the wiring diagrams and the 1986 system wiring is on the site, electrical/EVTMS/1986 EVTM

Gary just linked them above while I was replying.

[vjsimone]

Following

https://www.garysgaragemahal.com/1985-evtm.html

I think you can use the 85 EVTM manual for wire color code and use the 86 EVTM for component location.

[Waine5550]

Update on issues. Thank y'all for all the help. I've been reading a trying the stuff suggested. Was testing the brown relay.

The constant hot is good, the constant ground is good but the two alts are showing ground. I remove the relay and they are dead. Is that a bad relay or bad eec?

[mat in tn]

Update on issues. Thank y'all for all the help. I've been reading a trying the stuff suggested. Was testing the brown relay.

The constant hot is good, the constant ground is good but the two alts are showing ground. I remove the relay and they are dead. Is that a bad relay or bad eec?

you removed the relay. they are dead? what exactly did you test? the relay or the plug it goes into?

[mat in tn]

you removed the relay. they are dead? what exactly did you test? the relay or the plug it goes into?

there should be two relays. one for the ecu and one for the fuel pump. the ecu will not show any voltage on the control side till the ignition switch is activated. at that time, it will "prime cycle" the fuel pump relay for a second or two. it's the one you can hear time out. if the ecu relay is bad nothing happens from the ecu or fuel pump relay. another possible interference is the inertia switch. it is in line electrically between the ecu and fuel pump relay. physically it's above the carpet line on the passenger side of the transmission tunnel screwed to the firewall. it should have a little white rectangular button at the top and it pushes down with a click

[85lebaront2]

Update on issues. Thank y'all for all the help. I've been reading a trying the stuff suggested. Was testing the brown relay.

The constant hot is good, the constant ground is good but the two alts are showing ground. I remove the relay and they are dead. Is that a bad relay or bad eec?

Since many of us do not have a 5.0L EFI 1985/86 truck, relay location and wire colors would help immensely.

Of the two relays, the one under the dash is the EEC power relay.

On that one, large yellow wire is hot at all times, black with light green dot is always grounded (at the previously mentioned plug, C101), white with light blue hash is hot with the key on, dark blue will be hot with the key on and the relay energized.

The second relay is the fuel pump cutoff relay, it is underhood on the firewall.

On that one the yellow wire is hot at all times, pink with black hash is hot when the relay is closed, the red wire is powered by the EEC power relay through a blue fusible link near the power relay, this powers most of the EFI system, the tan with light green dot wire is grounded by the EEC, briefly when the key is turned on, and continuously once the EEC receives a signal from the distributor that lets it know the engine is turning.

I would start by checking, in the following order, power, key off, yellow wires on both relays should be hot.

Second, check the black with light green dot wire for a good ground, with a good meter it should have very low resistance to ground.

Third, key on, check for power at the white with light blue hash wire.

If these checks are good, plug in the relay with the key on, you should feel it click even if you don't hear it. If it doesn't click, check that the black with light green dot wire isn't showing battery voltage, if it is that C101 connector has way too much resistance.

With it closed, check for power at the dark blue wire, if you have power there, move to the red wire beyond the fusible link where you should still have power.

Once you have finished and hopefully found and fixed the problem, you should be ok. If you get through those tests, and still no start, it is time to check for fuel pump power, but if you can hear pump running sound and have fuel pressure you should be good there. The fuel pump relay can be activated with the key on, by grounding the tan with light green dot wire at the diagnostic test connector near the solenoids in the right front fender.

Good luck with it!

[Waine5550]

You guys are amazing!!!!

Wish I found this site 2 years ago when I started this sweet nightmare.

Trucks at the shop and I'll go up there tomorrow and apply all the new insite y'all gave me. Labor is scarce around here and the shop is taking forever. So I helping them along.

The fuel relay is yellow and eec relay is brown... for now. New relays are a different color. Fuel relay is located firewall driver side. The eec relay is located on the hump under the asstray.

[Waine5550]

Since many of us do not have a 5.0L EFI 1985/86 truck, relay location and wire colors would help immensely.

Of the two relays, the one under the dash is the EEC power relay.

On that one, large yellow wire is hot at all times, black with light green dot is always grounded (at the previously mentioned plug, C101), white with light blue hash is hot with the key on, dark blue will be hot with the key on and the relay energized.

The second relay is the fuel pump cutoff relay, it is underhood on the firewall.

On that one the yellow wire is hot at all times, pink with black hash is hot when the relay is closed, the red wire is powered by the EEC power relay through a blue fusible link near the power relay, this powers most of the EFI system, the tan with light green dot wire is grounded by the EEC, briefly when the key is turned on, and continuously once the EEC receives a signal from the distributor that lets it know the engine is turning.

I would start by checking, in the following order, power, key off, yellow wires on both relays should be hot.

Second, check the black with light green dot wire for a good ground, with a good meter it should have very low resistance to ground.

Third, key on, check for power at the white with light blue hash wire.

If these checks are good, plug in the relay with the key on, you should feel it click even if you don't hear it. If it doesn't click, check that the black with light green dot wire isn't showing battery voltage, if it is that C101 connector has way too much resistance.

With it closed, check for power at the dark blue wire, if you have power there, move to the red wire beyond the fusible link where you should still have power.

Once you have finished and hopefully found and fixed the problem, you should be ok. If you get through those tests, and still no start, it is time to check for fuel pump power, but if you can hear pump running sound and have fuel pressure you should be good there. The fuel pump relay can be activated with the key on, by grounding the tan with light green dot wire at the diagnostic test connector near the solenoids in the right front fender.

Good luck with it!

When out the truck today to used all the new info. The two of the wires into the fuel relay are are discolored to the point I had to guess. (Red and yellow are good colors) The eec relay tests good. My only question is the fuel pump doesn't shut off after primed. I have a fuel pressure gauge and it's reading 40 psi. I know before the new motor went in it would shut off in a few seconds. Will this be an issue?