Is the tacho pickup different in a P400?

[troopie]

G'gay folks.

I'm gunna sound like a dick cause I'm sure the answer will be 'NO', but in the interests of elimination I've gotta ask.

The now dead L65 (second donk) I had in my Toyota had the tacho sensor mounted behind the harmonic balancer. This fed to the only computer in my wagon; the Pollution Control Module, which then feeds to the factory tacho which worked fine.

With the removal of the Toyota engine (first donk) the PCM now only controlls the speedo and tacho and nothing else. The instrument cluster is the only other electronic component of the vehicle (besides the glow timer, which I don't need because it never gets cold enough (I should rip it out and throw it away to save weight! :))).

After installing the new donk (third) which has the tacho pickup in the same place, my tacho now 'responds' to engine rpm by jumping a few mill (um, poofteenth parts of an inch?) and then slowly climbs but gets never past 400 rpm.

I swapped the sender unit out with the one from the old donk (L65) which is visually identical and no change. I swapped my cluster out (just to make sure it wasn't the prob.) and no change. I looked at swapping the PCM but the cost of a S/H unit is prohibitive at this stage ('till I can find a willing temporary donor :D).

So I am wondering if there is a difference in the 'Pulses Per Revolution' in whichever part of the crank the sender picks up? If this was the case it would explain the behavior perfectly; thus why I'm asking. However, a couple of things don't add up.

For the numbers to add up the old donk would have to have four or more PPR. This is hard to know; as I don't have a functioning tacho I'm driving very conservatively but the screaming blower makes it sound like I'm going a lot faster than I am. I have read of 4 PPR units (from unknown engines) but this would mean I've never rev'd her more than 1600 rpm. Again, sounds like I have, but how can I now? Assuming I have rev'd her higher than this then the PPR on the old donk would have to be five or more and I haven't heard of that sort of figure before. :confused:

The other thing is why would AMG change it on a 'drop in' replacement engine?

Any thoughts would be appreciated.

Both senders are two-wire (not electro-magnetic)

I know I could F around running an after-market gauge off the alternator but when the provision is allready built into the donk I'd rather use that. Also, I also hate having electronics on board but really like the look of the factory cluster and so don't really want an after market tacho. I also like the idea of having a mechanical sender.

[DmaxMaverick]

The GM OEM tach drives off the alternator for mainline vehicles (pickups, etc.), or the oil pump drive in others. The sensor behind the balancer is the crank position sensor (unique to DS4/EFI applications, and never intended for tach use), but can be mod'd to be used as a tach driver with the correct buffer. Except for the oil pump drive, there is no other OEM engine speed sensor on the engine. If your engine has the OEM crank position sensor (and corresponding timing cover with the boss for it), and you just plug into it, it shouldn't work, unless previous mods allow for its use.

[troopie]

:eek::eek::eek:

Wow, this is why I love this site, thanks.

The P400 came with a DB2 so why the CAS? What's a 'buffer'?

By 'boss' you mean provision?

I don't know F all about CAS's but I'm guessing they'd have many PPR.

I had the new donk supplied with a new after-market alternator so maybe the old donk had the alternator wired up to supply tach info. If so my sparky missed this. But then, why am I getting any response at the tacho at all? And what was the old CAS plugged into (like I say, no computer)?

Unfortunately I've since sold the old donk so can't check it.


If your engine has the OEM crank position sensor (and corresponding timing cover with the boss for it), and you just plug into it, it shouldn't work, unless previous mods allow for its use.Thinking the CAS was a tacho pickup I was guessing that the Toyota engine and the old 6.5 used the same PPR and so it just worked regardless of the actual donk.

[troopie]

My oil pump drive has no sensor.

Actually the new donk came with no oil pump drive, the one I'm using is from my old donk. Maybe I should source a unit that has the sender incorporated?

[DmaxMaverick]

Simply described, a "buffer" is an electronic device that converts a raw input signal, to a signal usable for an instrumentation or control device. The crank position sensor on the 6.5L is used for fuel timing control through the PCM (Powertrain Control Module), on DS4 fuel systems. The tachometer on 6.5L trucks is independent of the PCM engine speed sensor, although the RPM can be read with a scanner (Tech II, etc.).

Also note, not all "pulses" are equal. The pulse/wave from the crank position sensor is different than that of the alternator or oil pump drive sensor. An electronic fuel system engine speed sensor system has a reluctor wheel on the crank sprocket. "Boss" refers to an accommodation in a housing or plumbing to accept another device, such as a sensor or outlet. There have been several detailed posts here, and some applications, using either of them successfully. The oil pump drive sensor is similar to your "plug" model, except it has a module with connector at the top. Peninsular Diesel is the only "new" supplier of these, that I know of (for marine and Hummer applications), but they can be found in wrecking yards in some van, RV and P-chassis (Workhorse) models.

Why your tach is responding to "something" is unknown to me. Perhaps some stray voltage or just field interference at the buffer/converter or gage. Your guess is as good as mine.

[Dvldog8793]

Howdy
Just a thought....
If the engine was supplied with the front cover that had the provision for the crank sensor "just because that's the cover we have" and not really intended to be used for anything. Is it possible that the engine has no reluctor ring behind the balancer? If that is the case and the prior engine WAS modified in some way to use the reluctor ring for a tach signal then it would not be there.
Like DMAX said, the small signal that you are currently get is most likely just spurious noise in system.

[DmaxMaverick]

Howdy
Just a thought....
If the engine was supplied with the front cover that had the provision for the crank sensor "just because that's the cover we have" and not really intended to be used for anything. Is it possible that the engine has no reluctor ring behind the balancer? If that is the case and the prior engine WAS modified in some way to use the reluctor ring for a tach signal then it would not be there.
Like DMAX said, the small signal that you are currently get is most likely just spurious noise in system.

I would agree with this, however, all GEP engines likely ship with the reluctor, and I've not seen one without. There is no designation difference between EFI and MFI long block models. It is doubtful that GEP, or anyone, is removing and replacing the sprockets, reluctors, and balancers, for any reason, at all. In all likelihood, the crank sensor timing cover IS an indication that the reluctor is present. From a cost effectiveness standpoint, it is much more efficient to produce the engines with a reluctor and sensor cover (the only cover in current authorized production), than inventory, track, and supply more than the necessary item, when the only difference would be a relatively inexpensive reluctor. The multiple application product also reduces inventory for original and replacement parts, such as the sprocket and/or balancer.

[disclaimer]
This makes perfect sense, practically, economically, and otherwise, so I'm probably completely wrong, because higher-educated bean-counters have entered the equation.

[troopie]

Thanks very much for the replies, that gives me some good info to work with.

[troopie]

G'day folks,

I'm going to endeavor to use the crank position sensor for tacho as it's already there and I hate having unnecessary stuff on my wagon.

My sparky would like to know; Is the sender unit (two wire) inductive (with the mounting face as an earth) or Hall effect?

Does anyone here know?