Nukasu

I just recently got a 89 XJ laredo with a 4.0L Renix H.O, that will not start. It has has a weird electrical problem. There are three relays in the relay center and a spot where a fourth should be. This is going from the firewall to the battery. In the fourth location there is a red wire that was fitted with a blade terminal and plugged into the 87 location.
I figured out that that wire was on one of the fusable links and the person before had tried to give it power by plugging it in that location trying to fix it, but gave up on it. I hooked it back up to the starter relay with a 25 amp fused link and tried to start, no go. I tried putting a spare relay that I had, you can never have too many, in the fourth spot and still, no go.
But when I jumper from the 30 terminal to the 87 terminal with a jumper wire, It will start right up and run great.
Now the part that stumps me is this.... When I jumper those terminals I can hook a volt meter to the battery and it will instantly drop to 7.45v and as soon as the jumper is taken off it will go back to 12.14v????
Does anybody know what that fourth relay spot is supposed to control?

Thanks

Side note: The relay I put in it is good I tested it in my running vehicle on the fuel pump relay.

SNIDELYWHIPLASH

It's the O2 sensor heater relay. (diagram here)

I copied and pasted the following from an '88 service manual. It should be the same for yours:

"The oxygen sensor is mounted in the exhaust manifold to monitor oxygen content of exhaust gases. The oxygen content reacts with the oxygen sensor to produce a voltage output signal which is sent to the ECU.
The oxygen sensor is equipped with a heating element that keeps the sensor at a consistent temperature under warm-up and idle conditions. This allows the engine control system to enter "closed loop" mode of operation much earlier, and to remain in "closed loop" during extended idle periods.
The heating element of the sensor is controlled by the ECU through the O2 sensor heater relay. This is a normally closed relay that supplies voltage to the sensor under warm-up and idle conditions.
When the ECU receives information from the MAP and speed sensors indicating that the sensor will stay heated due to exhaust gas temperature, the ECU opens the relay to stop voltage to the heating element."

So, since the relay is supposed to be in an "on" state at start-up, it sounds like it's receiving an erroneous signal from the MAP sensor or speed sensor, which is turning the relay "off" prematurely.

But I could be wrong, lol

Just my $.02 - hope it helps some...

Oh, and BTW, the 4.0L Renix motors weren't H.O.

Nukasu

I knew that they weren't, had a dunce moment

The o2 heater relay is not the one I having trouble with it is working, from the diagram it the A/C Clutch Relay without the relay it won't start, with the relay it won't start, but with the jumper wire across the 30 and 87 blocks on the relay plug it will start right up. Relay is good I swapped it with fuel pump and latch relay

when looking at the bottom wires there is a red, two pink, black, and black w/ white stripe.

Nukasu

Started checking all the wiring and the main ground from the battery to the block was complete deteriorated and green from old age and the other grounds on that group were also the same on the first few inches of wire. So.... I had some spare wiring that I replaced the ground cable and new fittings on the others. and it is alive!!!!

Thanks for the help.

P.S. The diagram above is different than mine, I did a wire trace and found that where the A/C Clutch Relay is on that diagram, is the Power Latch Relay on mine.

SNIDELYWHIPLASH

Yeah, I replaced my grounds a few months ago. Bad grounds can cause all kinds of problems.

It's usually something simple, but not always simple to find.

Thanks for the update - glad everything worked out for you.

Nukasu

well I finally got a new battery and installed it, found out the ground cable I put on from the last one is too short now. Took that one off and made a longer one and hooked everything back up. Now I am right back to square one. The only way it will start is with the red and pink wires from the latch relay jumpered together. which I think are the 30 and 87 terminals.
Questions are:
Should the ECM be sending the latch relay a ground signal through the black-white wire or removing the signal from that wire?
right now I am getting a signal to: with the key off and no signal: with the key on.
Or am I looking at the wrong part of the system, and how does the latch relay tie into the rest of the system? Is it all ecm related cause every wiring diagram I have looked at all point that way.

I have the computer pulled down right now and was looking at the connections and noticed that it does not say renix on it, it says Bendix, is that the same?
I am fairly new to the non-HO engines, all I have had is the HO models and they are a completely different setup.

freegdr

a fsm manual might be more descriptive

Nukasu

I have looked at the one I have and I cannot find anything in it about the operating habits of the elusive latch relay. Unless I am missing something in there somewhere.

freegdr

latch relay functionhttp://www.jeepsunlimited.com/forums...d.php?t=179333

Nukasu

Ok so based off that info it is supposed to be able to run with out the latch relay at all? But if the ICS is sticking it can result in a no start or hard start problem?

freegdr

that makes sense

Nukasu

alright thanks for your help I am gonna try taking it out and cleaning it in the morning and see if it helps any

Nukasu

On the latch relay connector, i know that terminal 30 is supposed to have 12 volts, and it does, but the pink wire, which i think is terminal 86, does not have any volts cranking, on, or constant.

my question is: When does the latched command from the ECU pin B10 have a voltage on your renix, or is it a ground signal that is removed from it, because I can apply 12v to the B10 ECU terminal and it will start right up, but I don't know if this is good idea dont know how much voltage it should have and don't wanna fry ECU. SO...

Thanks for your help so far

Nukasu

Ok got to do some wire testing with the relay in. For some reason the power signal from the computer is not being sent to engage the B+ Latch Relay for the 3 seconds to prime the fuel. Found out that it get 12v for the entire time it is running, which is good
Fix: I ran a jumper wire from the B+ Latched side of the computer under the dash to the ignition wire that is hot only in the "RUN" position. Everything works now and there is a 3 sec pause after I shut it off, then everything electrical that is powered by that circuit shuts off. no big deal, it lives. But I am still searching to find out what is going on. I think I have a bad ECM? But I don't know.