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Renix tps voltage source

Greybullit

NAXJA Forum User
88 Renix, started experiencing hard starting when dead cold. Starts right up when its warm. I've done most of Cruisers tips already, ground refreshing, C101 connector elimination, etc, etc. I've got fuel pressure and spark. I replaced the TPS a while ago and adjusted it to spec.

I started there first with this new problem and have Zero input voltage at the TPS. I believe the TPS gets it voltage from the ECU but I'm not 100%. Can anyone confirm this?

I'm thinking one of the contributors to this new problem is my son took the jeep out(its his ride) and ran it through a huge water hole 5-6 times and got water everywhere in the engine compartment and some came in through some leaks in the dash/firewall...hoping the ecu didn't get fried...
 
Ok, I found a wiring diagram and the input source does come from the ecu, pin C7. I've check for voltage at C7 with key on and have .84 volts there. Obviously not enough. Does this mean its time to order a new ECU??

Seems like one thing after another...
 
C-7 is the TPS "output" voltage, ECU TPS input voltage. 0.8 is what it is supposed to be, .87 is close enough.

C-16 is the TPS input voltage, ECU 5 volt output supply voltage. Typically around 4.8 volts or so.

No start temperature related and I automatically start thinking CPS, but it is usually the other way around good cold start and iffy hot start. If you have spark it is unlikely to be the CPS.

Water and my first thought is most always something with the ignition. Moisture in the distributor. Moisture in the coil to cap cable boots etc. Weak spark can make for hard starts and a terrible cold motor idle. It can also affect near top end RPM or higher RPM.

The ECU on the Renix is pretty robust and not a part that often fails. I've had a Renix for 28 years and never needed an ECU. I do have a spare just in case. My issues always turned out to be something else. Half the time a component or sensor, half the time a connector or the wiring.

Just tossing this out there, but make sure to look in the air filter box. I've drowned my Renix numerous times. The next most common is water in the bottom of the distributor. A high volt cable boot, coil to distributor or spark plug is also pretty common. The cable boots can get stiff with age and often don't seal well. Having spark doesn't necessarily mean having good spark.
 
Ok, thanks 8Mud, I'll check those spots.

Does anyone know which color wires correspond with the A, B and C on the TPS connector.

This is how its set up right now, looks as if the connector was changed at some point.

A - Yellow/green
B - Black
C - Blue

Again, I have a manual transmission.
 
I set mine up like this for testing once, about ten years ago and just left it that way. It hasn't caused me any issues yet.

 
Thanks mud, I've put the wire back to the way it's pictured.

Cruiser, that's probably a good idea.

I've got to be honest, the tps did have voltage, I just wasn't getting the probes far enough in the back of the connector to do the testing. I've since set it up to spec.

However...the hard start when dead cold problem still persists. I had changed the coolant temp sensor a while back and decided to test it the other morning. I got 4 ohms resistance on a morning that was around 45 degrees. According to the chart, that seems off to me. If that is the problem it seems to me that it might contribute to a cold start. But the thing starts and runs just fine when warm. Another thing I've read is the IAC valve being an issue on cold starts. I have had the throttle body off for cleaning and got everything really nice and clean in there. I've read that if you push the pintle/valve in, it can damage it? I might have done that when I was cleaning it...not totally possition though. I've unplugged it while running and the idle shot up, plugged it back in and it came back down to normal, seems like it's working. Once warm, the truck idles around 800-900.

I need to recheck my fuel pressure, any other ideas on a hard cold start? By that I mean I have to crank it 4 or 5 times for at least 10 seconds each time. I've tried priming the rail with key on/off a couple times and doesn't seem to have much help.
 
My hard starts have been;

Stuck IAC or lazy IAC. It is either gummed up or it was low voltage and often a battery or charging issue. Try starting it with around 1/8th throttle and see what happens. A normal start is, no throttle and the IAC supplies the air.

Weak spark, warm air and fuel ignites easier than cool or cold does. The best way I've found to check it is to use and old plug, open the gap up to around 0.60, ground the plug on the AC or another good ground and look at the spark while cranking. You should actually be able to hear that spark crack if your ears are OK. I usually check the number 2 plug it is easy to get to. A solid crisp (mostly) blue spark. A spark you can barley see or a yellow spark that can splash on the electrode is weak. Be careful. Weak spark usually also causes a really crappy cold idle.

Have you checked your CPS? When mine screws up, it is usually heat that does it. Maybe yours is odd and screws up cold? Cruiser has a write up on doing a voltage check on the CPS. Or you can do a basic OHM test or both.

A long shot, but water in your fuel can cause this also. Not common but it does happen. Especially in alcohol blend fuel. Really old crappy fuel can also cause this.
 
I've load tested my battery and its got plenty of CCA's. Charging system is working properly. I've tried to start with the throttle slightly open before and it doesn't make a difference. I checked the cps per cruisers test and the ohm test and its also fine. I don't think water is an issue, I run gas stabilizer and seafoam every 6 months or so. Plus I've gone through 2 tanks since this all started happening because it is a daily driver.

I pulled the IAC to recheck it and its totally clean as is the throttle body. One thing I did recently do is install a jumper wire in place of the ballast resistor. It is starting a little easier, even on the cold mornings. Still takes longer than I would like but for the time being its livable. In doing that, it seems to me that maybe the fuel pump/filter could be and issue. Again, I really need to double check my fuel pressure. I hope to do that this week as soon as I can borrow one from my shop.
 
The only time I ever had a cold start problem and a no start warm or hot problem also had a high idle issue when hot after it got good and hot, and the problem was the intake manifold air temp sensor. I suspect a bad coolant temp sensor might also be a bit of a problem the same way.
 
The only time I ever had a cold start problem and a no start warm or hot problem also had a high idle issue when hot after it got good and hot, and the problem was the intake manifold air temp sensor. I suspect a bad coolant temp sensor might also be a bit of a problem the same way.

Clean it. The IAT.

Or test them both......
 
Problem resolved, it was the Coolant temp sensor. I had installed a new one about a couple months back and apparently it was no good. Got another one and tested it plugged in and sitting in a glass of ice water and the thing fired right up. Thanks for all the input.
 
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