Renix startup time. Why?

I'm curious to know why the engineers decided to make the renix take a few seconds to start? Surely it wasn't accidental. Could it be that they wanted to cycle the engine a few times to get the oil everywhere, before firing up? Being an electronically controlled engine system, youd think it could start up almost instantly as the later years did.

I was also wondering what exactly it turns off. Fuel, or spark? I'm thinking fuel. I notice that if you crank for a few seconds, turn the key completley off, then restart it, it'll start up instantly. It got me thinking, is there some way to trick the system into starting up instantly? Some sort of fake signal. Would be nice.

mattbred said:

I'm curious to know why the engineers decided to make the renix take a few seconds to start? Surely it wasn't accidental. Could it be that they wanted to cycle the engine a few times to get the oil everywhere, before firing up? Being an electronically controlled engine system, youd think it could start up almost instantly as the later years did.

I was also wondering what exactly it turns off. Fuel, or spark? I'm thinking fuel. I notice that if you crank for a few seconds, turn the key completley off, then restart it, it'll start up instantly. It got me thinking, is there some way to trick the system into starting up instantly? Some sort of fake signal. Would be nice.

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I've heard (probably apocryphal - but you never know with AMC...) that the reason for RENIX to "spin up" before it starts is for oil circulation on startup.

I do know that the reason it does have to "spin up" has to do with it wanting two signals:
1) Valid 300rpm from the crankshaft position sensor (Mandatory)
2) Valid SYNC signal from the distributor (Optional)

It can and do start without the SYNC signal (I've tried,) but it will not start without the CPS signal. If you unplug the dizzy and try to start, you'll think that the timing chain has slipped a cog - that's because the ECU is "guessing" where the #1 cylinder comes up to fire. Once it has its guess, it can work without the SYNC signal - but performance does suffer somewhat.

However, until it sees 300rpm from the crankshaft position sensor, fuel AND ignition are both cut off. It will not fire fuel injectors or the ignition coil until it gets 300rpm.

To improve starting times, improve the mains (that seems to be the consensus.) I've put in 1AWG mains cables, and crank times are reduced (because the 1AWG will get a larger current supply to the starter motor.) I have heard reports from the field that indicate the same things.

Also, it can help to provide an additional ground from the battery to the chassis - the ECU grounds through the chassis, which grounds through the engine block, and then to the battery. Just something else to think about...

5-90 said:

I've heard (probably apocryphal - but you never know with AMC...) that the reason for RENIX to "spin up" before it starts is for oil circulation on startup.

I do know that the reason it does have to "spin up" has to do with it wanting two signals:
1) Valid 300rpm from the crankshaft position sensor (Mandatory)
2) Valid SYNC signal from the distributor (Optional)

It can and do start without the SYNC signal (I've tried,) but it will not start without the CPS signal. If you unplug the dizzy and try to start, you'll think that the timing chain has slipped a cog - that's because the ECU is "guessing" where the #1 cylinder comes up to fire. Once it has its guess, it can work without the SYNC signal - but performance does suffer somewhat.

However, until it sees 300rpm from the crankshaft position sensor, fuel AND ignition are both cut off. It will not fire fuel injectors or the ignition coil until it gets 300rpm.

To improve starting times, improve the mains (that seems to be the consensus.) I've put in 1AWG mains cables, and crank times are reduced (because the 1AWG will get a larger current supply to the starter motor.) I have heard reports from the field that indicate the same things.

Also, it can help to provide an additional ground from the battery to the chassis - the ECU grounds through the chassis, which grounds through the engine block, and then to the battery. Just something else to think about...

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Alright that somewhat makes sense, but when your Renix jeep is cold, and you try to start it, does it take 3 secondsish to fire up? Surely much before then the flywheel is at 300 RPM. Also, look at the statement about cranking it, turning key off, then cranking it again. It starts right up! Sure the 300 RPM thing could be valid, however maybe there's something else at play..

Also, I have run ground straps pretty much everywhere except to the starter, but it seems to crank fine. When it's hot it starts up under 2 seconds. It's just, compared to 91+ vehicles where you basically turn the key and let go, it'd be nice, heh.

Edit: about the cam sensor issue. Sometimes when I start it, it'll fire, delay, fire a couple more, delay, as in almost stalling, then it'll shoot right up to it's normal idle. You can pretty much hear each individual cylinder firing when it does that. It doesn't happen often, actaully it mostly happend in colder weather, but it did happen.

SYNC sensor is fairly easy to check - disconnect the three-wire connector from the distributor, and see if that changes anything when you go to start. If it doesn't, the SYNC sensor is shot. If it does, you're still good (it will get harder to start if the sensor is still generating a valid signal.)

For the other thing, the pressure in the fuel rail also bleeds off over time (like several hours to overnight.) Some have reported success in installing a BMW fuel check valve inline to maintain fuel pressure to the injectors when shut down (search here should give results,) but if it otherwise runs fine, what's the worry? Put the key in ON for about two seconds before cranking, and the pump will cycle up and pressurise the fuel rail anyhow.

Fuel pressure is definitely not the problem. Try it yourself. Crank it for a couple seconds, turn the key off for a second, then crank again and it'll start instantly. I mean pretty much as soon as you turn the key. I think my jeep starts like it did out of the factory, its just I would like to somehow fool the computer to making it start instantly. I doubt it's possible but was just wondering. Ah well, the joys of Renix.

mattbred said:

Crank it for a couple seconds, turn the key off for a second, then crank again and it'll start instantly.

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Exactly - while you're cranking, the fuel pump is running full tilt (ballast resistor bypassed) to prime the system. Try just turning the key to crank it - it will start, but it takes a couple of seconds. Part of that is getting the 300rpm and waking everything up - and part of it is getting the fuel system primed and ready.

It usually takes about a half-hour to an hour to "bleed down" fuel system pressure, from what I've noted. I just automatically turn the key ON and wait a beat before I crank - which seems to solve the problem. I've been doing it so long I don't even think about it anymore...

Damn, 3 seconds?? that must be nice.......I've been noticing more and more that my Renix is taking 10-15 sec. to finally fire up. She just sits there and cranks away.....

Yea my Renix is taking about 3 seconds to start no matter how long the key is "on" for. Try searching "crank crank vroom" there was a good thread about 6 months ago on this subject.

5-90 said:

SYNC sensor is fairly easy to check - disconnect the three-wire connector from the distributor, and see if that changes anything when you go to start. If it doesn't, the SYNC sensor is shot. If it does, you're still good (it will get harder to start if the sensor is still generating a valid signal.)

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Just to clarify, i disconnected this connection, and when i started the vehicle it seemed to make no difference whatsoever. I didnt leave it running long, but it seemed to start like it normally does. Does this definetly mean that the SYNC sensor is bad? Is there a voltage / resistence check i could do on it to make sure?

Thanks for your time.

EDIT::

Looks like i found some tests:

http://www.gps-spots.com/Tech_Articles/Engine/Cam_Position_Sensor_and_Sync_Pulse_Stator.htm

I just had to put a new battery in my heep, so i put new cables in too at same time.. It now starts faster than before.. battery was only 2 years old, post was loose but still worked

scottmcneal said:

I just had to put a new battery in my heep, so i put new cables in too at same time.. It now starts faster than before.. battery was only 2 years old, post was loose but still worked

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Makes sense - and matches up with reports from the field I've had on WiP cables going into RENIX rigs.

While the sensors themselves were still good, upgrading/replacing the mains cables will do two things for you:

1) The larger cables (if you use something larger than 6AWG) will allow more current to flow freely. This allows the starter to spin more rapidly, and will shorten the time it takes to reach 300 crankshaft RPM.
2) Improvement of the ground (by replacing the cables - especially if you follow my instructions, with or without my cables) give a cleaner "reference ground" for the sensors, which means less work for the ECU to figure out what's going on.

Both of those can only help.

5-90 any comments on my post above? Post #9 .. thanks.

Muad'Dib said:

5-90 any comments on my post above? Post #9 .. thanks.

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Go ahead and perform those tests, they sound about right (per FSM) and should help to verify things.

However, a failing SYNC sensor usually acts like the timing is off when it's trying to start, until it comes up with a working modus vivendi for running in that instance. Simple reason - it is. I've not known a SYNC sensor to fail personally yet (I'm just replacing mine at 19 years and 300K because I'm changing the rest of them anyhow,) but that doesn't make it impossible. I've only known of two RENIX ECU failures (I have both,) both due to blown injector drivers.

If you're just experiencing a "long start time," I'd start by checking and cleaning up the primary grounds - battery to engine block, and engine block to firewall - and consider adding a secondary chassis ground direct from the battery, using 8AWG wire at minimum. Painted contact patches for ground wires are stupid - clean a small spot down to bare metal, clean the lug, apply corrosion inhibitor (I like Gardner-Bender Ox-Gard) and reassemble.

That braided ground strap from the engine to the firewall is particularly suspect - the open braid tends to attract contaminants. If you replace it, do so with an actual insulated wire and seal both ends - this ground strap is known as "the RENIX Killer" for a reason. I would be only mildly surprised if you were to change this ground strap and have the problem resolve itself - use 8AWG as a minimum here as well.

Its not taking more then 3 seconds to fire up my Jeep, all my grounds are good etc. One question i do have about the ground it can you "add" a ground wire from the stud on the passengers side of the engine, to the bolt that holds the thin ground strap to the firewall? Would that be benificial?

My main reason for asking about the SYNC sensor is because im "anal" and i like to replace anything and everything that could be a problem or a potential problem. When you mentioned unplugging it and firing it up to see if there was a difference i did so. As i said i noticed NO difference whatsoever. I assumed then that it was bad, and since i have a digital fluke multimeter, doing the test's wouldnt be benificial unless i purchased an analog. I guess ill just replace it and see what happends from there

Muad'Dib said:

Its not taking more then 3 seconds to fire up my Jeep, all my grounds are good etc. One question i do have about the ground it can you "add" a ground wire from the stud on the passengers side of the engine, to the bolt that holds the thin ground strap to the firewall? Would that be benificial?

My main reason for asking about the SYNC sensor is because im "anal" and i like to replace anything and everything that could be a problem or a potential problem. When you mentioned unplugging it and firing it up to see if there was a difference i did so. As i said i noticed NO difference whatsoever. I assumed then that it was bad, and since i have a digital fluke multimeter, doing the test's wouldnt be benificial unless i purchased an analog. I guess ill just replace it and see what happends from there

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You can add all the grounds you like - just be aware that you will rapidly get to a point of diminishing returns, where each ground you add has negligible to no net effect.

I'd replace the ground from the engine to the firewall (you can attach that pretty much anywhere on the engine - use a fuel rail screw, for instance, and it will be easier for you to reach) and add the one from the battery to the chassis. Beyond that, you're spending money and effort for little to no gain.

While I'm rather fond of the RENIX control system and its open-ended programming, the RENIX XJ is probably about the worst-grounded vehicle I've ever seen. Fortunately, it doesn't take much effort to change that notion.

Similar to some of the folks in the "crank crank vroom" thread....I seem to have an odd situation with starting my Renix. I've noticed that it takes a few seconds of cranking before starting when cold. When hot (errands, etc), it will take 2-3x longer crank time to start, if it's been shut down for less than 10 minutes. If it's been shut down btwn 10-20 minutes, it will start on the 2nd-3rd crank, almost immediately.

Any opinions about what could be causing this?? Seems a bit strange for a heat-soak issue.....

Might be a loose ground that comes and goes with heat at the connector. Also might be the IAT temperature sensor not reporting the correct inlet air temperature to the ECU. Might even be a loose, dirty battery clamp.

Any other symptoms?

RaccoonJoe said:

Similar to some of the folks in the "crank crank vroom" thread....I seem to have an odd situation with starting my Renix. I've noticed that it takes a few seconds of cranking before starting when cold. When hot (errands, etc), it will take 2-3x longer crank time to start, if it's been shut down for less than 10 minutes. If it's been shut down btwn 10-20 minutes, it will start on the 2nd-3rd crank, almost immediately.

Any opinions about what could be causing this?? Seems a bit strange for a heat-soak issue.....

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Ecomike said:

Might be a loose ground that comes and goes with heat at the connector. Also might be the IAT temperature sensor not reporting the correct inlet air temperature to the ECU. Might even be a loose, dirty battery clamp.

Any other symptoms?

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I took the hood off over the weekend for some cutting, and didn't get it put back on yet. I noticed that while running errands yesterday, it started normally every time. You could def. see the hot air rising out of the engine bay.......

However, I noticed that my idle has gone from 250-300 to ~750, now that I took the hood off. That's really odd to me.....