acceptable tachometer accuracy

stlthe · 10-11-2021, 01:52 PM

My machine came sans tach. I installed a new cluster and figured id try a tach while i was at it. I bought a used tach (from an intl) to install in my bus. I stuck it in the dash and wired it into the harness but it would not work, i assumed no signal. i ran a wire from the R terminal of my SI40 alt to the tach and turned the key. initially the idle showed to be 1000 (per cat ET my idle is 700), which thrilled me, accurate or not. I was able to calibrate it down to like 800rpm at low limit of potentiometer.

Heres my question. How important is a 100 or 150rpm discrepancy? Id like for it to be accurate, but im not sure its worth the ~300 bucks for a new one.

Im also torn deciding whether to identify and intercept the proper wiring harness or just leave the direct connection as is and zip tie it.

any opinions?

evan

Booyah45828 · 10-11-2021, 02:39 PM

Are there dipswitches on the back of the unit? The potentiometer isn't meant to adjust several hundred rpm, but more of a fine tune device.

I'm thinking the dipswitches are wrong or something else is out of cal.

Is this the thomas gauge cluster or international one? You mentioned international, and they're not know for gauge reliability.

stlthe · 10-11-2021, 09:29 PM

i dont see any dip switches, i guess i would have to take the unit apart to find any. There appears to be a rubber seal on the back though, maybe that is where the dip switch is. i will have to pry it off and see.

The case says plantronics, and the face says teleflex. The cluster is Thomas.

thanks for the info on calibration and the dip switch lead. i will take a look.

Rucker · 10-11-2021, 10:21 PM

Quote:

Originally Posted by stlthe

i dont see any dip switches, i guess i would have to take the unit apart to find any. There appears to be a rubber seal on the back though, maybe that is where the dip switch is. i will have to pry it off and see.

The case says plantronics, and the face says teleflex. The cluster is Thomas.

thanks for the info on calibration and the dip switch lead. i will take a look.

I think there are some youtube videos that describe the tach wire in the harness, generally. You might be able to suss it out pretty easily from watching some of those.

cadillackid · 10-12-2021, 06:27 AM

that bus has a computer, read out the computer tach and compare it with your gauge unit.. and then you can match them up .. or you can run a bluefire and just use the computer reading as your tach.

the computer on the C7 is accurate as it uses the CPS signal to run the engine so it will be about as close as it gets..

Booyah45828 · 10-12-2021, 07:25 AM

Quote:

Originally Posted by stlthe

i dont see any dip switches, i guess i would have to take the unit apart to find any. There appears to be a rubber seal on the back though, maybe that is where the dip switch is. i will have to pry it off and see.

The case says plantronics, and the face says teleflex. The cluster is Thomas.

thanks for the info on calibration and the dip switch lead. i will take a look.

Teleflex made the tach. They make a lot of marine instruments.

VDO had the dipswitches, teleflex has a rotary switch I believe.

Do you have a picture of the back. It should have a rotary type switch to knock it down another level, and then use the calibration adjust to dial it in. At 2k rpm with the calibration adjustment in the middle, you should be within 250 rpm of accurate. If not, rotate the switch to a new spot and see where that puts you.

You dial it in at 2k rpm though fwiw. Doing it at a higher rpm makes the final adjustment a little more accurate.

stlthe · 10-13-2021, 12:01 AM

well i found the dipswitches, some are on and some off. i think the teleflex marine models used a dial and the others use switches. i know lots of instruments use dip switches, and that they have proprietary settings. i cant tell what the switches are for. tommorow i will take a pic of them.

that said, if plus/minus 250rpm is acceptable i am golden.

thanks for the advice.

ii have 2 bluefire units which i will use, but there was just something about a mostly full working conventional dash that appealed to me. There was a big hole where the tach was supposed to be.

at 750rpm (per ecu) i read about 850, 1000 reads around 1100 and 1200 reads about 1300. 2200 reads about 2075. I find this acceptable but not ideal. im going to try tinkering with the dip switches.

Booyah45828 · 10-13-2021, 09:22 AM

Quote:

Originally Posted by stlthe

well i found the dipswitches, some are on and some off. i think the teleflex marine models used a dial and the others use switches. i know lots of instruments use dip switches, and that they have proprietary settings. i cant tell what the switches are for. tommorow i will take a pic of them.

that said, if plus/minus 250rpm is acceptable i am golden.

thanks for the advice.

ii have 2 bluefire units which i will use, but there was just something about a mostly full working conventional dash that appealed to me. There was a big hole where the tach was supposed to be.

at 750rpm (per ecu) i read about 850, 1000 reads around 1100 and 1200 reads about 1300. 2200 reads about 2075. I find this acceptable but not ideal. im going to try tinkering with the dip switches.

No, I didn't say +/- 250 is acceptable. There's no reason you can't get this spot on, so anything but spot on is unacceptable IMO.

Plus or minus 250 rpm is the acceptable range for the dip switches, then you fine tune to perfect with the adjustment screw. To say another way, the dipswitches put you in the ballpark, the screw puts you into centerfield. There should be a users manual available that shows what switch range you need for pulse per revolution, these tach's were used in a whole host of different vehicles.

So find the manual, see what range you're at, lower it one range, and then try fine tuning with the screw again.

stlthe · 10-13-2021, 09:58 PM

I am inspired to get this spot-on, thanks for the prompting.

i found the dip switches and what i think is the manual online, https://www.veethree.com/wp-content/...10/S034_R6.pdf. I understand the nature of the settings, but need to determine the frequency of the alternator signal.

probably a dumb question, but how many poles does a delco remy 40si have? Also, is there a common pulley size? the delcoremy website says the ratio is 6.67. the formula to figure the dip settings is number of poles/2*pulley ratio*full scale rpm/60.

assuming 12 poles and 4000rpm tach with 6.67 alternator signal ratio, the full scale frequency is 2668. the manual gives dip settings for this which i will try tommorow.

i appreciate the support

evan

Booyah45828 · 10-14-2021, 09:36 AM

Quote:

Originally Posted by stlthe

I am inspired to get this spot-on, thanks for the prompting.

i found the dip switches and what i think is the manual online, https://www.veethree.com/wp-content/...10/S034_R6.pdf. I understand the nature of the settings, but need to determine the frequency of the alternator signal.

probably a dumb question, but how many poles does a delco remy 40si have? Also, is there a common pulley size? the delcoremy website says the ratio is 6.67. the formula to figure the dip settings is number of poles/2*pulley ratio*full scale rpm/60.

assuming 12 poles and 4000rpm tach with 6.67 alternator signal ratio, the full scale frequency is 2668. the manual gives dip settings for this which i will try tommorow.

i appreciate the support

evan

Write down the setting you currently have, before changing anything.

I'd see what setting you currently have, then drop it one less range, and see what that gets you. If it's still high, drop it another range.

Counting the poles in this picture here https://www.jittruckparts.com/40si-r...IaAhWNEALw_wcB You'd have 18 poles. To find your pulley ratio you'd have to measure the circumference of your crank and cam pulleys.

I'd honestly go with plan A though and drop it a frequency range followed with another test, before I'd do all that measuring and counting. You're not real far off from where you should be, so it doesn't really require a clean slate approach IMO.

stlthe · 10-14-2021, 08:44 PM

I middled out the calibration potentiometer and started from there. after just a few ranges down the list i found one i was satisfied with. i was tired of messing with it due to the bending over the dash holding the instrument cluster in one hand and adjusting the settings with the other. it is not perfect, but only off by maybe 25 rpm on the idle end and the same (opposite direction) at the top. i'm willing to call it a day and appreciate analog gauges and parallax view.

doing the adjustments at 2000 rpm was helpful.

Booyah45828 · 10-15-2021, 07:17 AM

Good Job!

nords · 10-21-2021, 01:46 PM

If anyone else wants cheap instrumentation for their bus, get an OBD2 wireless tester for about $25. Use an old tablet or phone and install torque lite. You can then assemble a virtual dashboard of gauges for most of what your engine has sensors for. As a bonus, you can see any fault codes your engine throws. No internet needed.

Booyah45828 · 10-21-2021, 02:08 PM

Quote:

Originally Posted by nords

If anyone else wants cheap instrumentation for their bus, get an OBD2 wireless tester for about $25. Use an old tablet or phone and install torque lite. You can then assemble a virtual dashboard of gauges for most of what your engine has sensors for. As a bonus, you can see any fault codes your engine throws. No internet needed.

School buses don't have an obd2 connector or follow obd2 protocol. So your OBD2 wireless tester for about $25 is completely useless on them. As would be your torque lite app.

You can get a bluefire adapter and do what you describe, but it isn't $25, and there are a handful of threads on here that describe doing just that. Here is one of them https://www.skoolie.net/forums/f49/f...ent-29192.html

nords · 10-21-2021, 07:43 PM

Quote:

Originally Posted by Booyah45828

School buses don't have an obd2 connector or follow obd2 protocol. So your OBD2 wireless tester for about $25 is completely useless on them. As would be your torque lite app.

All 5 of the short buses that I am familiar with (I own 3 of them) have OBD2 connectors and I scan them regularly. What you say may easily be true of full-sized buses - I have no experience with them.

cadillackid · 10-21-2021, 07:51 PM

Quote:

Originally Posted by nords

All 5 of the short buses that I am familiar with (I own 3 of them) have OBD2 connectors and I scan them regularly. What you say may easily be true of full-sized buses - I have no experience with them.

VAN-based busses are different. full chassis and FE / RE busses use the commercial Diagnostic protocols which are J1708 and J1939.. using round 6 or 9 pin connectors..

the cheapie OBD2 readers are often based on the ELM327 chipset.. technically an ELM327 chip can read J1939 (CAN BUS) data, however the adapter doesnt have the firmware to translate J1939 nor does torque pro understand the data even if it could be sent to the android..

your Van chassis bus has the same engine / trans / computer as the COnsumer Light duty vans (express / savannah / E-series etc)... thus those will have OBD2 connectors.

nords · 10-21-2021, 07:58 PM

Cadillackid,
Thanks, that was clear. One of the reasons why I like the short buses is that regular garages can handle them (if their bays are big enough). You are right, they are all van chassis. Parts are more readily available too. I wish school district bus garages would allow their mechanics to work after hours on outside buses. The only shops in my area that can handle a full-sized bus are the truck shops and they are about $200/hr and don't like to work with individuals.

Booyah45828 · 10-22-2021, 09:04 AM

Yup, you're in the caterpillar drivetrain section, medium duty, no obd2 here.

OBD2 was used on the van/cutaway chassis. You can love them, I don't. They're soooooo light duty compared to an actual medium duty chassis.

The only medium duty stuff that I'm aware of that had OBD2 on it was the chevy/gmc kodiak/topkick, and those could be had with a cat engine. Those units had both an obd2 connector and j1939 connector, and I believe the obd2 was for abs/chassis stuff only. The cat engines communicated on the j1939, at least that's how I communicated to them, I never tried to talk to the engine on the obd2 connector.

cadillackid · 10-22-2021, 09:38 AM

Booyah - I continue to learn... GMC is interesting.. I asked a biddy of mine to look for the diagnostic connector on his full size Bluebird bus with a GM chassis.. i think its an 01.. has a 454 V8 gasser and an AT545.. full chassis bus.. and an OBD2 connector as opposed to the usual J1939.. so apparently there a mixture out there!!

Booyah45828 · 10-22-2021, 09:58 AM

Quote:

Originally Posted by cadillackid

Booyah - I continue to learn... GMC is interesting.. I asked a biddy of mine to look for the diagnostic connector on his full size Bluebird bus with a GM chassis.. i think its an 01.. has a 454 V8 gasser and an AT545.. full chassis bus.. and an OBD2 connector as opposed to the usual J1939.. so apparently there a mixture out there!!

Cool deal!

So gm used the obd2 connector with the gas 454 in medium duty, interesting.

Does the bus have air brakes and a wabco or bendix abs setup? Or I'm surprised there isn't a 1939 or 1708.

The last gas bus we had was as you described. Gas, GMC bluebird w/ lift, but it was a 90 IIRC. I don't remember displacement or if it was even efi, I just remember it was a pooch, real low power and sucked fuel. It was retired well before it was worn out.

We service a fleet of mid 2000's topkick straight trucks powered by a combo of cats and duramaxs. All of the topkicks use a combo of 1939 and obd2. Duramaxs are diagnosed using snap on scanner and obd2, cats are diagnosed with cat et/usblink/j1939. Cats are 3126 and c7's, never tried to mix and match protocols on them. Next one in I'll try to remember to do that.

I wonder what the bluebirds with the propane ford/roush v10 uses? If it's obd2 or 1939.

You may also like: