Tune Up Info

[mickeyd]

I have a 1989 comanche with a 4.0. The motor started miss a little today. Doesn't do it all the time but when it does the tac drops to 0 then junp back up. Where does the tac pull the signal from?

Thanks for any help.

[luvmyglock]

First off, it is missing for a reason. Are you sure it is the speed sensor? Have you tried the other avenues (i.e. plugs, timing, distributor, etc.)? I would hate to see you go through all that when you just have a fouled up plug...

[Minuit]

Typical tune up includes stuff like plugs and wires and cap and rotor. If it hasn't been done recently it probably needs it anyway. If it's missing with any regularity, the first thing I would check would be the pre-cat oxygen sensor. You'll also notice a severe (like 25%) drop in fuel mileage with a bad O2. Not sure about your tach issue, but if it drops all the way to 0 I doubt it's completely related to the miss.

[cruiser54]

Renix CPS Testing and Adjusting

 

Renix CPSs have to put out a strong enough signal to the ECU so that it will provide spark.

Most tests for the CPS suggest checking it for an ohms value. This is unreliable and can cause some wasted time and aggravation in your diagnosis of a no-start issue as the CPS will test good when in fact it is bad.

The problem with the ohms test is you can have the correct amount of resistance through the CPS but it isn’t generating enough voltage to trigger the ECU to provide spark.

Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS, crank the engine over. It won’t start with the CPS disconnected.

You should get a reading of .5 AC volts.

If you are down in the .35 AC volts range or lower on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!! That’s a definite no-start condition. Best to buy your CPS from Napa or the dealer.

Sometimes on a manual transmission equipped Renix Jeep there is an accumulation of debris on the tip of the CPS. It’s worn off clutch material and since the CPS is a magnet, the metal sticks to the tip of the CPS causing a reduced voltage signal. You MAY get by with cleaning the tip of the CPS off.

A little trick for increasing the output of your CPS is to drill out the upper mounting hole, or slot it so the CPS bracket rests on the bell housing when pushed down. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

 

Revised 07-30-2012

Tach signal comes from ICU/coil. Loss of the signal is usually related to CPS failure. Here's 2 things you can do.

 

Renix Jeep ICU/Coil contact refreshing

 

The contacts between the coil and the ICU on your Renix Jeep can become corroded and loose causing a complete or intermittent no-start condition. I recommend the following procedure as a maintenance precaution to insure this is eliminated as a possible cause now and in the future.

The coil is attached to the ICU by two T20 Torx bolts. Remove these two bolts and lift the coil up off the ICU. You will see 2 pins and 2 sets of contacts. Clean both the pins and springy contact pieces with a good electronics cleaner.

Squeeze the springy contacts closer together with some needlenose pliers. Apply some dielectric grease to the contacts and bolt the coil back on to the ICU.

While you’re right there unplug the connectors from the ICU and inspect the pins in the harness connector. Make sure the pins are not retracted into the connector. Spray out the connector and the receptacle of the ICU with the same good electronics cleaner you used earlier. Apply dielectric grease to the connectors and plug them back in.

I feel this procedure should be performed at least once in the lifetime of a Renix Jeep.

 

Revised 11-29-2011

[mickeyd]

Thanks crusier 54. I will check this. Everbody likes to point to the basic ie plugs wires but they will not cause the tac to go dead like my. Minuit i have never seen a 02 sensor cause a miss, you are right about fuel milage. What ever this is it's causes the tac signal to drop to nothing

[Minuit]

Thanks crusier 54. I will check this. Everbody likes to point to the basic ie plugs wires but they will not cause the tac to go dead like my. Minuit i have never seen a 02 sensor cause a miss, you are right about fuel milage. What ever this is it's causes the tac signal to drop to nothing

 

O2 sensors can certainly cause a miss. A bad one sure did to me. Either way, hope you get it fixed. :thumbsup:

[HOrnbrod]

Typical tune up includes stuff like plugs and wires and cap and rotor. If it hasn't been done recently it probably needs it anyway. If it's missing with any regularity, the first thing I would check would be the pre-cat oxygen sensor. You'll also notice a severe (like 25%) drop in fuel mileage with a bad O2. Not sure about your tach issue, but if it drops all the way to 0 I doubt it's completely related to the miss.

 

Every O2 sensor in Renix and 91-92 OBDI MJs are "pre-cat". If it's bad, you stay in open loop. Bad mileage yes, but most always no miss. What the OP has is an intermittent ignition cut off problem with the tach shutdown. Most likely causes are a bad CPS or it's connector problem, a distributor pickup problem, a failing coil, or a grounding problem as Cruiser detailed.

[jeepmarine312]

how would you go about checking the O2 sensor? is it difficult? or expensive? or is it just a senses kinda thing (look, smell, etc.)

[dasbulliwagen]

When I got my truck, it would intermittantly act like it had a horrible miss, I tried almost everything to get it to stop, before replacing the O2 sensor. Hasnt done it since. This wouldnt cause the tach to drop out, but when it acts up, it can certainly be misconstued as a miss. If I were you, I would start with the Crank sensor. They are a common problem and fairly easy to fix. I'm not sure about the renix years, but in newer ones they come with a cardboard spacer on the tip. When you install the sensor, you set the spacer down against the flywheel and when you start it the spacer gets ground off leaving the proper gap for the sensor. If you pull yours out and reinstall it be careful about the spacing. If you put your old one in this way with no gap, youll chew up your sensor. Just FYI

[cruiser54]

Tach signal comes from the ICU to the dash. Another point to check. Also try unplugginf and plugging your CPS connector. a good ground refreshing and connector refresing may make this problem go away also. Cheap insurance. Try this:

 

Renix Jeep ICU/Coil contact refreshing

 

The contacts between the coil and the ICU on your Renix Jeep can become corroded and loose causing a complete or intermittent no-start condition. I recommend the following procedure as a maintenance precaution to insure this is eliminated as a possible cause now and in the future.

The coil is attached to the ICU by two T20 Torx bolts. Remove these two bolts and lift the coil up off the ICU. You will see 2 pins and 2 sets of contacts. Clean both the pins and springy contact pieces with a good electronics cleaner and some fine sandpaper.
Squeeze the springy contacts closer together with some needlenose pliers. Bolt the coil back on to the ICU.
While you’re right there, unplug the connectors from the ICU and inspect the pins in the harness connector. Make sure the pins are not retracted into the connector. Spray out the connector and the receptacle of the ICU with the same good electronics cleaner you used earlier.
I feel this procedure should be performed at least once in the lifetime of a Renix Jeep.

Revised 04-04-2013