Hello,

I'm in the SLOW process of replacing the engine wire harness in my '85 EFI 5.0 with one from an '88. That harness is wired for a four wire (heated) O2 sensor. I was thinking about going ahead and using it, but I'm not 100% sure of the connections. There is some conflicting information out there. One source which is very highly regarded states that the 85-86 models already have a heated O2 sensor, as can be seen here: http://web.archive.org/web/201104301...ckpinouts.html

It states that the O2 sensor signal be connected to pin 29, and the sensor ground be connected to pin 49. I only have a single sensor wire connected to pin 29.

The diagram for the sensor itself states that the sensor signal ground go to 'SIG-RTN', which is pin 46 on the EEC computers. Does anyone have an opinion-or an EVTM for a later truck that they can check for me? The heater itself is a non-issue, with one side going to a switched 12V source, and the other to ground.

Thank you,

Rick

 

Are you using the ECU from the 1988 truck?

 

Yes, I'm using my original '85 computer. Most of the connections are the same. I just don't have some of the stuff an '88 does, like a speed sensor, and I have to add a lead for a fourth relay (EGR vent I think it was).

 

You need to use a later computer like an '88 or there is no point installing a 4 wire sensor as it's not compatible with the '85 computer.

 

You can hook up a 4 wire O2 sensor into a one wire O2 harness. Just use the O2 signal wire into the one wire harness and a ground from the 4 wire O2 signal ground to engine block.

I have done this before as for some reason the 4 wire O2 was half priced than the one wire O2 sensor! You just won't be using the 4 wire O2 heater.

 

Found out something on my 86 ranger that you may want to investigate; The early systems without a speed sensor use switches on the gear shift lever and the clutch pedal, or the neutral/park switch on the automatics. What little I know so far is the computer is programmed for different idle strategies depending on if you are sitting still out of gear, or coasting in gear. The later trucks with the speed sensor got rid of the switches and use the speed of the vehicle instead. Once the vehicle slows to a certain point they change the programming strategy of the computer and how it controls the idle of the engine. That's all I know so far.

 

I know what you're referring to, and my truck has no such functionality. It does have an A/C lead to the ECU, but nothing to the transmission...unless you count the neutral safety switch, but that is just tied into the ECU for a 'START' signal, not the reverse light circuit.

When I wrote this question originally I was at work, as I am now, and did not have my 'new' harness to look at. The next morning when I took a closer look at it I found that it never made use of the 'HEGO GROUND', anyway...and a schematic I eventually found for an early 90's truck shows HEGO GROUND to be just an ordinary ground. There is no particular ECU functionality for it, so I think I'm good at this point

Thank you very much to those that responded. Input from others is always appreciated in these projects!