FWIW, an aftermarket tach movement is just a volt meter with a capacitor/resistor network. The pulsing signal from the RPM sensor generates a voltage spike to charge the capacitor. The capacitor beins discharging through a resistor between pulses. Capacitors discharge at a (relatively) fixed rate, and how far the capacitor discharges (its minimum voltage) is determined by how much time elapses between the (spikes) pulses and the resistance of the resistor it is discharging through. The higher the RPMs, the less time between pulses, the less the capacitor discharges, and the higher its minimum voltage. Likewise, the larger the resistor, the slower the capacitor discharges, and the higher its minimum voltage.

The rest of the circuitry in the tach is just to average the voltage value (average of the highest voltage spikes and the minimum value the capacitor discharges to) and "smooth out" the level so the needle doesn't flutter.

The way the 4-6-8 selector switch works is it changes which resistor the capacitor is discharging through. A smaller resistor means it discharges more quickly - the smallest resistor would be the one for an 8 cylinder - because the pulses are closer together with 4 cylinders firing each revolution. The largest resistor would be selected for a 4 cylinder to keep the capacitor from discharging so quickly - because there are only 2 pulses every revolution (giving the capacitor more time to discharge, so you need to slow the discharge to get the same average voltage and the same RPMs reading on the needle).

Anyway, all that explanation of how it works is to basically say that when you are switching between 4-6-8 cylinders, you are only changing the resistance of the circuit to change the average voltage reading on the needle. It doesn't have to match the actual number of cylinders in order for you to get SOME kind of reading - only for you to get an accurate reading. If you had it connected to a 5 cylinder engine, and set it for 6 cylinders and everything else was working right you would get an RPM reading that was 5/6 of the actual RPMs (i.e. it would say 2500 when the actual RPMs are 3000). If you hook it up to a 5 cylinder and set it for a 4 cylinder, your RPM reading would be 5/4 of the actual RPMs (i.e. it would say 2500 when the actual RPMs are only 2000).

To get it to read accurately on a 5-cylinder you would need to determine the value of the resistor for the 6 cylinder setting and the value of the resistor for the 4 cylinder setting, and replace one of them with a resistor that is halfway in between. So if the 4 cylinder setting used a 330k resistor and the 6 cylinder setting used a 220k resistor, you would need to get a 275k resistor (a 270k would actually be the closest you could find), install it in place of one of them, and use that setting on the tach. I can tell you for certain that it works this way, because years ago I did exactly that to adapt a 4-6-8 tach to a two-cylinder Honda engine. I replaced the 4 cylinder resistor with one twice as big and used the 4 cylinder switch setting. So instead of reading only half of the actual RPMs it read correctly.

The problem with using this kind of tach for a diesel is that it is designed to connect to the ignition coil to get the pulses of the ignition system (firing of the plugs). On a 4 cylinder that is 2 pulses per rev, for a 6 cylinder it is 3 pulses per rev, and for an 8 cylinder it is 4 pulses per rev (4 stroke engines only fire once every other rev). The ignition coil is also a high-voltage signal.

You are trying to use a signal that only happens once per rev (the crank-pulley-mounted sensor only passes the pickup once per rev) and it is a very low voltage signal (a magnet in the crank pulley passing a pickup coil). Therefore the tach amp is the key piece. It takes that single weak pulse per rev and amplifies it (to a higher voltage signal) and it most likely also contains the capacitor/resistor discharge circuit, as well as the circuitry to average and smooth the voltage output to a level correctly calibrated for the tach to read. Of course the diesel tach is just the voltage meter part of the circuit - the amp is the key piece since it is what converts the pulses to an appropriate level of voltage to be read by the tach.

So, in order to make an aftermarket tach work, you have to have an amp that works correctly, then change the discharge resistors (4-6-8 resistors) to match that signal - or if the tach amp output is the correct voltage level you have to bypass the capacitor/resistor and voltage averaging circuitry of the aftermarket tach altogether.

What it all boils down to, is that in order to read accurately the tach has to be calibrated to the amp (either an internal amp like in the aftermarket gas tach, OR an external amp like the one from the Mercedes diesel), and the amp has to be designed to generate the right output signal from the timing and voltge level of the input sensor - which depends on the sensor type. If you are getting an incorrect RPM reading it is a calibration issue - which can be fixed. BUT if you are getting no reading it is either a failed tach amp (putting out no signal) or a tach amp that puts out a signal that your aftemarket tach can't decipher (probably too low of a voltage range for the tach). Try the Mercedes tach with the Mercedes amp. If you get no reading then it is either the tach amp or the sensor. Of the two, my money would be on the amp being the bad part. They are notorious for failing, but the sensor seldom fails - unless the wires to it get damaged.

If your tach amp and sensor are both OK and the Mercedes tach works, I wouldn't get my hopes too high that the output signal from the tach amp will be in the right range for the stock Cherokee tach to read it correctly. It would be worth a try, but I'm about 95% sure you're going to find that if it gives any reading at all, it is going to be WAY off. If you decide to give that a try, please post the results. You never know, if it is close enough, we may be able to figure out how to calibrate the Jeep tach to the Mercedes amp. Kind of a long shot, but worth a try.

I think you'll find that the easiest way to make it work is going to be to see if (once you have all the parts of it working) you can mount the tach from the Mercedes into the instrument cluster of the Cherokee. If that isn't workable, the next best option (IMO) will be to buy one of the signal converters that attaches to your alternator and generates an output signal that can be calibrated to be read by either an OEM or an aftermarket tach. That is the route I plan on trying to go for swapping my Olds diesel into a Cherokee - mainly because Olds made no provision at all for any kind of tach to be attached to their diesels. They never offered a tach option, so there is no sensor, no amp, no nuthin'...

 

Well, the good news is the OM617 runs like a dream. It's been getting smoother and smoother the more I drive it. The bad news..... Something is knocking in the flywheel/torque converter area. It only does it in gear when stopped, not park. Once you get moving, it goes away. I think maybe something inside the converter broke loose. I won't know till I get it all apart. If the trans is damaged, I will probably put the Benz trans back in and make it rear wheel drive. I could get a 700r4, but it won't bolt to my transfer case without an adpater and a new output shaft. I could get a 700r4 and the transfer case that goes with it, but would probably need custom driveshafts. I don't really want to put that much time and money into a $400 Jeep. I live in the city and don't really NEED 4X4 anyway. It would get better mileage being rear wheel anyway. I will let you all know what happened when I get it apart.

John

 

Hi John, sorry to hear of your trans problem. The 700r4 in the s10 with the 2.8 should bolt up to the adapter you have and the t-case most likely is a 207 or 231, like the cherokee so the drive lines may be the same. Tomorrow I am going to my dad's to use his lathe to fab my t-conv plate and bell adapter allignment tool. Hopefully I will have success with my design.

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Hey Cheeperjeeper, thanks for all the detailed info about tacs. That helps alot!

 

Good Luck!!
Let us know how it turns out.


John

 

Yeah, sorry to hear about the tranny trouble Benzer1. Keep working it, you'll work it out.

FWIW, the slip yokes of the drivelines are indeed the same for the NP231 and the NP207 for both Jeeps and S-10s - or at least the earlier ones.

No problem. I know it was a really l-o-n-g post, and probably contained more detailed info than many people would have any interest in. But I figured that if it even might be helpful to one person, then it was worth the effort. For anyone who wasn't interested in that level of detail, well, its easy to scroll on right on by!

 

Hi Yall! Interesting reading. Hay Cheeper Jeeper the 4.3 Olds which would be a better pick, cast iron heads or alum?

 

Good question. The cast iron heads are prone to cracking between the valves. They say that 95% of the time the cracks are pretty much only surface and cosmetic and don't cause any problems. SOme folks claim to have run them like that for tens of thousands of miles, but I wouldn't be all that comfortable knowing that the heads on my motor had cracks in them.

The aluminum heads don't have cracking problems, but from what I've read it only takes ONE serious overheat to warp them. Of course they can always be resurfaced but the more times they are the more warp-prone they become.

The cast iron heads came on some of the RWD motors. The aluminum heads came on some of the RWD and all of the FWD motors. Since the engines are pretty rare, you pretty much have to go with what you can get. I have 3 of the FWD motors, so they should all be aluminum.

 

I pulled the Jeep back in the garage. I hope to have the trans out tomorrow evening to see what happened. I found a local place that can make a custom driveshaft for a reasonable price. I am going to put the Benz trans back in and make it rear wheel drive for now. If I decide I need 4X4 in the future, I will find a divorced transfer case to put behind the Benz tranny. The only hard part will be figuring out the speedometer. The Benz speedometer is electric, driven off a signal from the trans.The jeep speedometer is cable driven off the transfer case. Maybe I can take the motor out of the Benz speedometer and transplant it into the jeep gauge cluster?

John

 

FWIW, I think I'd have a go at transplanting the whole Mercedes cluster in that case...

 

That's a good idea,
Or maybe put the speedometer pod in. I know the tach in the Jeep is the same diameter as the Benz, I thought about mounting it in the Jeep dash for a more factory look.

John