mordway

I've been searching the forums here to find out if anyone else has this issue of the Battery + latching relay not unlatching. I have refreshed all my grounds as best as I can, but I can't understand how this system actually is designed to work. Looking at the schematics it stands to reason that once latched, it can't be unlatched unless the ground side of the circuit is disrupted. Latching the relay sends 12 volts from pin 30 back around to the energized pin for the relay coil so will remain energized until the ground is removed from the other side of the coil. I'll add a picture later. Does anyone know how the circuit is designed to function? Is it unlatched within the ECU? Is the ground wire connected directly fly to the ECU or does it go somewhere else prior? Is the Battery Latch Ground at the ECU the same as vehicle ground? I can't imagine it is, but I'm not an electrical engineer. I can only Imagine the "timer" circuit that keeps this relay latched after key off, thus keeping some power to the IAC for a few seconds, is within the circuitry of the ECU. Any thoughts?

Jim Malcolm

If you ignore the orange highlights, you'll see the B+ Latch Relay on the LH side of https://www.cherokeeforum.com/attach...4d8680c186.jpg

You'll notice that the ECU controls both the power (pin B10) and the ground (pin A9), with no additional "loop around" wiring. Typically, the ECU has all of the control functionality built in and shuts off the relay when it's done "closing up shop" after you turn off the ignition. I can't say that some systems may not used a fixed timer, but it's generally not the case...

An electrical system won't function properly unless ground is the same throughout. Ground provides the "common reference point" from which voltage (potential) is "measured" against. In a vehicle, ground is the battery negative terminal. All other "grounds" are just additional "wiring" to get to that battery negative terminal; the "wiring" just happens to be the sheetmetal body instead of wire. The grounding system on an XJ Cherokee isn't the greatest and with a Renix being at least 30+ yrs old, it may be do for a refresh. Cruiser54 has a lot of good info on his website, but long story short, make sure you take the connection apart and clean it; a visual inspection does not work in this situation...

Two mods in particular that I've found worthwhile are an additional ground wire from the battery negative terminal to the inner fenderwell right beside it (has to pass through the engine block first otherwise) and a wire running from the dash metal structure over to the sheetmetal body (just the mounting bolts are used otherwise).

mordway

1990 xj battery+ latch relay

mordway

Thanks for the info. I posted a pic of what I see as a loop once the relay is energized. If the energizing 12 bolts and the output of the contacts are connected as shown here with pins 4&5, then I don’t see how it ever de-energizes.

im going to add a better ground to the ecu mounting surface and make double sure of the ground as pictured in the schematic and Yes, Cruiser's site and tips are brilliant. Thanks!!

lawsoncl

Looking at the diagram, I'd say the ECU applies power to B10 (pins 4/5 on the relay) and provides the ground on A9 (relay pin 2) at key-on. On key-off, it stays latched on until the ECU is done resetting the IAC position and drops the ground connection.

Is it possible you have the wrong relay type in there?

The Bosch A and B style relays have the same pin pattern and look otherwise identical, but pins 30 and 86 are swapped internally. That ends up swapping pin 1 (power) with either pin 2 or 5 (one side of the coil). I can't tell which side of the coil, since I don't know whether the picture is looking at the socket or the bottom of the relay. A quick sketch of either case, it looks like it would either cause +12 to stay applied to the ECU through the coil even if it lifts the ground connection, or it won't stay on when the ECU drops the power to pins 4/5.

Saudade

Here's a simplified pic of the B Latch.

mordway

Thanks!! That is quite interesting if this is drawn as if looking at the top of the relay socket. On mine I recall the solenoid coil connections, 2 and 5 here, are reversed. If this is looking at the bottom of the socket then it is what I have, other than one pink wire connected to pin 4, which I presume to go to the diagnostic connector, and also connects back to the ecu as shown in the Jeep schematic and the other schematic above. BTW, I am getting a "Battery + Open" error on my reader.

One other thought - It may very well be possible to have the wrong relay in here. I see on the schematic posted by Jim Malcom that there is a resistor within the relay? I just replaced the relay with the same type that the PO had in there and it does seem to be the same as the others. I couldn't locate a part number for the proper relay, it does seem possible that having a resistance within would be what causes the unlatching to occur. ??

Saudade

I always see it as looking down on the top of the socket. Sometimes the numbers are different depending on the FSM you're reading. The one above I posted comes from the Renix manual. The others are from the FSMs.

The relay part number should be 56002958. Looking at the one on Amazon, the pic indicates there's something connected between the 2 pins. I assume it's the resistor.

Saudade

If you haven't yet, you should look at Cruiser54's website. http://cruiser54.com. He has a lot of technical info on Renix there.

mordway

Many thanks for that part number. I've ordered two - to have one in the box. It certainly is pictured with the resistor or whatever it is across the coil too. The other relays I have do not show that so I am hopeful that this will fix the issue. The PO, we can always blame the PO, eh?? ;-), likely just put whatever seemed to fit into the slot.

I will try to connect my old 1987 Renix computer in the mean time just to see if there is any difference. I'm not expecting so, but that will give me reason to put eyes on all the pin connections at the ECU too - and ohm wires out without the ECU connected.

Cheers,
Mark
oh, and yes, I agree with your comments on Cruiser! Top notch site there. I've done most of the tips, check the grounds throughout the harness and relied on his advice for checking the function of the sensors. Invaluable advice!! I think he is just up the road from me (I'm in Phoenix) so maybe one day our paths will cross.

mordway

So, new relays arrived. Still the same error. Battery + latch open. I’ve checked continuity between the relay connectors and the ecu connectors and that’s fine. I’ve put three different ecus in with the same result, I’m really at my wits end. As drawn in the schematics above there is no way this relay should unlatch unless the ground is somehow interrupted. Or if the hot lead in from the fusible link is interrupted. So… heres My plan. I have purchased a timed relay which will supposedly unlatch after a specific set time. I’ll install that to interrupt the hot lead from the fusible link once the key is turned off. And, I’ll try to ignore the code reader saying “battery + latch open”.

lawsoncl

I'm wonder if someone got creative with the wiring. With the ECU unplugged, do you still see a ground to the coil?

mordway

I’ll check that tomorrow and report back. Should there be none with it disconnected?

lawsoncl

Since the ECM is what grounds the relay (and breaks the ground to unlatch), I'm thinking it should not show grounded if the ECM is unplugged if the ECM is disconnected.

Saudade

Looking in my '90 FSM, the BLK-T ground that runs to the ECU also runs to the diagnostic connector D2 Pin 3.

However the schematic in my 90 FSM show the PINK going to the diagnostics connector on pin 12, not pin 5 as above. Not sure that that's all about. It does agree with the diagnostic connector pinout.

Have you tested the runs to the diagnostic connector?