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'82 300GD TD (Sold), '02 G500, '09 B200
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485 Posts
Discussion Starter #1
Is there any way to figure out if my Tachometer is reading accurately? Either it reads WAY to high or there is something wrong with my driveline and I'd like to figure out what it is.

Steve
 

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1980 LWB 280GE
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3,504 Posts
Interesting comment.

BT's tach also reads WAY too high in his turbo conversion. I'm not sure if it did before or not.

One way is to know your tires, gears, and trans ratios and use a calculator to figure what engine RPMs should be in a given gear at a given comfortable cruising speed for that gear.

Even if you have an auto box, cruising on level ground, not trying to accel or decel, TC slip should be minimal.

Also know your ACTUAL speed via GPS, dont trust the speedo.

Other than that, the only thing I culd think of would be to try and sample the pulse off the crank sensor at the diagnostic socket.

I'm curious Steve, how is your tach wired? When I got my 617A motor I took the diagnostic plug, cap, and wiring from the car. In those cars the diagnostic plug socket is actually the tach signal conditioner. The crank sensor, +, and _ battery voltage go IN to the diagnostic plug cap, and a tach signal comes out. In the cars the tach is dead if the socket cap is off.

I've been hoping that I can just wire the tach output line from the diagnostic plug to my tach and have it work right....but so far it's only hoping. If you tell me yours is wired as I plan to do, and it doesn't work, then I'll be figuring on having the alternator wired for a tach that senses that way.

How isyour tach connected?

-Dave G.
 

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280 ges 1989
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321 Posts
If you had a petrol engine I could have just said invest in a snapon timing light with tacho readout – my tacho was way out causing stalling when cold.

There is a neat device that was used on my old Piper aircraft to check the tacho accuracy – a laser tachometer that measured prop rotation – there could be a way of using something like that – it worked pretty well.
 

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'82 300GD TD (Sold), '02 G500, '09 B200
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485 Posts
Discussion Starter #4
Thanks guys,

I used an rpm calculator and then did some math with tire diameter, etc, and this is what has scared the hell out of me. Anywhere from 500-900 rpm off at hwy speed (110kmh). Yesterday, my wife and I did an experiment where she drove behind me to tell me of my actual speed (close enough at least since I don't have a GPS yet). Also, while driving yesterday the tach suddenly jumped an extra 500rpm at one point though there was no noticable difference in engine noise and then it went back to "normal". However, from everything I can figure out, it still reads way high and I really don't think it could possibly be TC slip. The convertor is new and, from what I understand, is supposed to lock-up at high rpm. It seems to be doing this as there is no excess heat or anything else abnormal.

Dave, concerning the tach hook-up, I'm afraid you have become too technical for me. Email me some specific directions of what to look for and I will see what I can do, happy to help!!

I am going back to my mechanic on Wed for a quick check up on my engine leak problem(s) (now hopefully solved) and I will ask him if he can verify the tach reading or offer any insight into my (and possibly BT's) tach issues.

Steve
 

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1987 280 GD TURBO
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132 Posts
sedans up to 1983 take their signal from the front
of the crank shaft and use a signal converter
in the cover of the diagnostic cover. .
this part is often defective and symtom is intermitent sycnal. From 1984 sygnal is taken from
from flywheel teeth, therefore different and goes
to diagnostic socket connection only and on to tack.
This sygnal is converted in the tack itself and
tachometers are not interchangeable.
Early bellhousing adapters have a 110 engine part no.
From 84 they are a 617 part no. difference ie hole
on left side to receive sygnal pickup.
If using early engine use tach from 300GD, if using
84-85 engine use tach. from 250GD.
Steve---tachs are rarely inacurate but 460G
speedometers are often inacurate.
WARREN
 

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1980 LWB 280GE
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3,504 Posts
Your ear is your friend

Do your own experiment. Cruising along about 80kph, downshift from top to 3rd. That'll give a REAL rpm bump of about 1000 rpm. Hear the difference in the engine? Any time the tach takes a jump and you DON'T hear that correspoding tone shift from teh motor, it's the tach faking you out.

The one in my 280 gets a hokey signal and jumps around from time to time without any real change in engine RPM.

Now exactly what to DO about it.....that might require some diagnostics on-site.

Good luck

-Dave G.
 

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85 300GD 83 300TD
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978 Posts
RE: Tachy

The tachs in the diesels pre '84 that I've been around are not consistent. The one in my G comes on every once in a while to make me aware of it's existance. My wife's TD turbo does the same thing. I've spent money on the pickup wire that reads the front pully. 2 of them. I have tried several units that plug into the diagnostic electrical port. I think vibration kills them and you get faulty readings. I took one apart and it's pretty clever. The electronics are in the middle of a blob of silicone to resist failure but not for long. They are expensive little clever things. As are the pickup wires. I think the best solution is to '86 that stuff and use an aftermarket tach that reads a signal from the alternator. VDO sells them among many others. The stock setup is elegant, it just doesn't work very well in my experiance.

-Dai
 

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'82 300GD TD (Sold), '02 G500, '09 B200
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485 Posts
Discussion Starter #9
I talked to my mechanic again today and even got a second opinion and they both assured me everything is perfect with my tranny and the rest of my driveline. I suppose my question then is:

What do other owners find there rpm is at about 70-75mph or 110-120kmh in 4th (1:1) gear in any vehicle with a 4.9:1 ring and pinion? It shouldn't really matter what engine is in there because gearing is gearing right. It is a sickness I know, but I just have to get to the bottom of this tachometer readout.

Thanks,
Steve
 

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TJs, FJ40s, JGCs, G500
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2,343 Posts
Steve,

if you tell us your tire size and brand I can tell you the rpm at whatever speed or the speed at whatever rpm.

My software tells me 3658 rpm @ 70 mph with 235/85R16 BFG

Harald
 

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'82 300GD TD (Sold), '02 G500, '09 B200
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485 Posts
Discussion Starter #12
Harald,

I've done this and this is why I'm troubled[B)].

32/11.5R15 BFG MT/KM.

Is Dave's experience similar to others? Will an automatic add some revs onto this due to the torque convertor?

Steve
 

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1980 LWB 280GE
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A double check

Harald's right of course. The spreadsheets don't lie.[:)]

His post spurred me to verify my off-the cuff rememberance above with some real checks on the way home. With the speedo in the GPS showing 70.0, my tach on the dash was about 2/5 the way between 3500 and 4000 marks, so about 3700 RPM.

And as a confirmation, when the tack was pointing right on at 3500 RPM in top (1:1) gear, the GPS was saying 66mph. Doing the ratios on that one gives 3712 RPM at 70.

So it looks like something right around 3700, give or take, is the right RPM at 70 on 235/85s with 4.9 gears.

-Dave G.
 

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1981 300GD
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71 Posts
RE: 3500 @ 70 on 235/85s w. 4.9 gears

LOL - that probably means that I'm not really revving at 5k doing 60 on the same gears & tires.

Like Dave said - mine's just a touch off.

Dennis used the original tach and reconnected it to the new engine. It had not been working prior, so I've no idea what it would have read before the swap.

I've gone to the lower tech "butt tach". The vehicle complains loudly to the pants seat when it revs too high.
 

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'82 300GD TD (Sold), '02 G500, '09 B200
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485 Posts
Discussion Starter #15
Thanks all,

BT, I have to agree with you and your "seat of the pants" test. Last night I went out and found a loooonnnngggg stretch of road to do a bunch of tests and checked various speeds at various gears at various rpm and consistantly found it to be out by the same amount- about 20%. However, as BT says, the "seat of the pants" test really shows me what I am after I think. I have about 4400rpm at the same speed and similar tire size as Dave but there is no way it is revving that high. Trust your instincts I guess.

ST
 

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85 300GD 83 300TD
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It is unlikely that a 617 diesel motors can be overreved. The only way is to run down a very steep descent and have the weight of the truck spin up the motor. Other than that they are governed by the device the pump is connected to. They are also designed to be run hard and put away wet.

-Dai
 
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