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Thread: Function of Power Latch Relay in pre '91 XJ's?????

  1. #1
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    Icon314 Function of Power Latch Relay in pre '91 XJ's?????

    What's up guys?? I was just wondering if anyone was really 100% positive about what the heck that power latch relay does, and what is it's function? In the Renix manual it does'nt even say?? Just wanted to know since the Renix manual did'nt say! Thanks in advance, Bob.

  2. #2
    Registered Chris in VA's Avatar
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    'Power Latch relay'? I've never heard of it...
    94 XJ Country NP231 AW4, 145k miles
    Cooper Discoverer ST/AR767, front towhook, PIAA 40's and Hidden Hitch. 1.5" budget boost 10/08

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    Default B+ Latch

    The Renix ECU basically stops functioning when the Key is turned off... that's not a good thing because your XJ was just running with the idle stepper motor (ICS) set to idle a warm motor at 650rpm or whatever...

    The idle air requirements for a warmed motor are no where near the air requirements to start a cold motor... so the B+ Latch relay was added to the system (by Renix in the level I implementation of this ECU line - never released in the US, but found in Renaults) . When you shut off your key, the B+ Latch relay continues to power the ECU so that the ICS backs-out and is ready to admit the additional air-flow required for your next start (time of the latch is a second or so and is 'controlled' by the ECU).

    Additionally, the B+ line (in the ECU) provides the ECU with a clean and seperate power source (since it passes though the relay and not all the way though a noisy switch) -- This is useful for items like injector and/or ignition drivers where there are very-square output waveforms that would ordinarily drive noise back through power supplies and into other more delicate circuitry (ie: the knock sensor amplifier and/or the TPS and MAP voltage regulators).


    An unusual question - do you suspect a problem?

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    Icon314 high idle at start up

    Ya Satan, I've been trying to solve my high idle at start up problem for about 2 months now? For a while it would run right up to 2000 rpm and stay there until I shut her off. It would take a couple of trys with her doing the same thing, until she finally would start and let the idle come down. Then sometimes she would just run right up to 2000 for a second or two and then come right down into normal idle. Today I cleaned my C101 connector again and it got better! I still will need a couple of days to make sure, but she seems to be doing better. Thanks for the seminar on the B+ latch relay, I appreciated it! I have a '88 Cherokee all tuned up, all new sensors, and a funny start up quirk! ha ha. Thanks again, Bob. P.S. To top it all off, usually in the mornings during this problem she would start up perfectly normal?? But not every time. Hmmm?

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    Sounds like when my TPS went bad on my 89. I would suspect the tps the tps wire connectors , the tps ground wire and your c101 connector (not on my 89). If you replaced your tps then I would suspect the tps wire harness connector engine side, the ground or c101. Greg
    Black 97 Limited 5.2, ZJ Quadratrak II Tcase, Traclock rear,factory skids and tow hooks, factory tow package, Bridgestone Revos 235/70/16, Kolak'd suspension -- Edelbrock IAS Shocks and Addco Sway bars front and rear, Kolak Ignition (platinum plugs and MSD Coil & wires).
    Black 89 XJ Laredo 4.0, auto, Selectatrack (242) Tcase, Bilstein Shocks, Kolak ignition (20 yr old Daughter's car at Va Tech!)
    Both owned since new!

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    I posted the B+ latch relay a few times over the last few months. Since I pulled it, the idle doesn't immediately take off .
    Granted, I have to just "touch" the pedal on a cold start but I can let go and the rpm is normal.
    moderator of NAMDRA Forum

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    Default High initial idle not coming down... (or occaisionally not)

    The TPS and CPS are the only sensors involved in getting that morning idle to behave (a little flare is normal, but when you see a "flare" that runs more than a few seconds without any change, you've got other issues).

    Of course, the ECU output for this effort is the Idle Control Solenoid -- that little stepper motor has been known to stick and is well worth a periodic cleaning (below).

    And none of this is relevant at all if your throttle plate/linkage is sticking at all (ensure that it's free to move and closing as designed - those return springs are getting older and older...) - It may seem lame, but there are a few TSBs and many 'customer complaints' about the throttle not returning to closed position. This is complicated by the automatic & cruise control 3-cable system and Renix's silly bell-crank linkage...

    A few thoughts - the TPS (ECU side) ground MUST be damned good - you can add another one at the sensor. The "rest" of the TPS ground stuff needs to be top-flight as well (engine grounding, Battery to engine & battery to body grounds, C101 ,and the ECU's grounds) - Think about it this way - the ECU will try to control your idle when it's side of the TPS (only one-side for the manual folks) provides 0.9V or less seen at the ECU... If you have a crappy ground, it's not difficult to pick-up a tenth of a volt just from the "morning" charging going on from your alternator! -- If you are setting the TPS for an automatic, you owe it to yourself to check the quality (smoothness) of the ECU side of the unit and that the ECU side's output is no more than 0.9V (AFTER ensuring that the TPS's ECU-side Ground is damned good!)

    Of course, the CPS also has to have a good signal -- generally, if the rest of the ride runs fine, the CPS isn't the issue, but it's worth your time to see that the CPS connections DO NOT run through C101; that they instead run directly to the ECU (this was re-work for '87 and some of '88) - again, if you're running OK chances are that the CPS signal is adequate, but... it
    is a Jeep!

    I mentioned cleaning the ICS -- the ICS is a stepper motor that has a rotationally fixed pintle that's threaded onto the motor's shaft. I recommend cleaning the ICS with the pintle extended -- that is, pull the ICS connector while the engine is at 'normal idle', shut things off, then remove clean and reinstall the ICS. Don't forget to clean the ICS's seat in the TB while it's out - the pintle seat is not supposed to be black - it looks like soft copper. I use a sensor safe carb spray cleaner and a little shop-towel-action to get the ICS clean -- Short-period soaking (like pintle facing up and "filling" the body with a little cleaner) is OK, but don't let cleaner sit there for too long (5-minutes is fine, not a half-hour) -- blow dry and repeat -- once your happy enough - I like to soak the pintle for a long while (the 1/2-hour) in my favorite light lubricant (ENSURE that your lube has NO SILICONE, or you'll be shopping for an O2 sensor!) - non-silicone gun oils, WD40, Marvel, whatever -- just no silicones. Install the ICS without forcing anything plug it in and start the ride - let it idle (or try) for about 10-30 seconds then shut the key off (Oh, yeah -- WITH the B+ Latch relay in the cluster) - with the engine off, check to see that the ICS screws are in there tightly and things are fitting properly... ( Jeeze- it takes longer to write it down than it takes to do it !! )

    One more thing for Automatics <--- get your attention? The NSS in automatics provides a signal to the ECU indicating that your vehicle is "in gear" - this signal adjusts the limits and gain on the idle control routines... If your NSS is at all suspect - clean it or replace it, your ECU may be trying to control things when it shouldn't and may be using the standard idle data/curves when you're in gear...
    ... the NSS, it's not just for reverse lights...

    any help?

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    Thanks Satan. We appreciate the time, effort and knowledge you put in to your thorough responses. Greg
    Black 97 Limited 5.2, ZJ Quadratrak II Tcase, Traclock rear,factory skids and tow hooks, factory tow package, Bridgestone Revos 235/70/16, Kolak'd suspension -- Edelbrock IAS Shocks and Addco Sway bars front and rear, Kolak Ignition (platinum plugs and MSD Coil & wires).
    Black 89 XJ Laredo 4.0, auto, Selectatrack (242) Tcase, Bilstein Shocks, Kolak ignition (20 yr old Daughter's car at Va Tech!)
    Both owned since new!

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    White Knuckle Express BONEWAGON's Avatar
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    Satan does not tell you to convert to the darkside, nor does he tell you to kill people or anything stupid like that. He just tells people that doing evil would not be a good idea and if you do it don't let your friends catch you!

    I vote for a sticky!!!:rocks:
    1989 XJ Un-Limited
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