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Author Topic: Inaccurate Dash Gauges  (Read 990 times)
Tenor
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« on: January 08, 2008, 06:37:39 AM »

I have read a number of references to the bus dash gauges not being reliable and I have experienced the same issue.  I am installing a Vanner equalizer in the chassis electrical to supply 12 volts for running lights (not currently hooked up due to new caps) radio, CB, fog lights etc. and thinking about 12v gauges.  What has anyone done?

I know some will ask why I did not mention changing the headlights.  My bus came with 8 extra 24 volt headlights in the box.  Once I burn through those, I'll change.
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Glenn Williams
Lansing, MI
www.threemenandatenor.com
1968 MCI 7 Ser. No. 7476 Unit No. 10056
8v71
4 speed Spicer
Len Silva
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« Reply #1 on: January 08, 2008, 07:00:02 AM »

The secret to accurate gauges is a good ground from the sender to the instrument and a voltage supply that does not drop when other loads are present.  I think most quality gauges are reasonably accurate with good wiring. I don't think 12 or 24 volts makes any difference.

Typically temperature gauges will rise and pressure gauges will fall if the battery and ground conditions are not ideal.  This is a good thing, it would be much more hazardous if that were reversed.

With the engine running and up to temp, note the gauge readings, then turn on electrical loads (lights, ac, ect.) and see if they change.  Anything more than a quick flicker indicates an electrical problem (and your bus may have been born that way).

Power for the instruments is often picked up from the run switch and subject to voltage drop from many loads.  I would consider a good battery supply from the main panel, fed through a run switch operated relay, to power the instruments.  Also make sure there is a good COPPER ground from the sending units, don't depend on the chassis ground.

FWIW,
Len
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buswarrior
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'75 MC8 8V71 HT740




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« Reply #2 on: January 08, 2008, 07:12:05 AM »

Bus dashboards are/were notoriously under maintained.

Why would they be maintained?

Some paid drivers will keep driving it until the alarms come on, and then some of them will keep driving until the engine seizes. So, gauges are a waste of time for this crowd.

Some other drivers will park it/bother the maintenance staff/fail to deliver the service to the customer, because the gauge reads funny, when nothing is wrong with the coach. So, gauges are a trigger for immediate lost income for this crowd.

Pay an hourly rated mechanic to chase what circuit?!?!?!?
When there are real problems to fix on the rest of the fleet?

Remember, like it or not, your bus was a prostitute for the bus company pimp.

It's job was to make as much money with as little problem in the shortest time frame, and if it became a problem, it was disposed of, with little ceremony, and replaced with a younger flashier model.

We busnuts are the charitable shelter running types, who see the good in this old, worn out tramp, and try to get her cleaned up a bit, and due to our good religious upbringing, can't imagine why she was treated so poorly...

As noted, good wiring, and GROUNDS, are your best friend when it comes to restoring the measurment circuits to functionality.

happy coaching!
buswarrior

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Tenor
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« Reply #3 on: January 08, 2008, 07:28:25 AM »

Great advice on the grounds.  I'll go through those.  My temp gauge seems to work but I need to check it against infra red testing.  The two stumper's are the oil pressure, which rockets to above 80psi and stays there and the speedo (an MCI unit with the funny box).  The speedo sender is not original according to pictures.  It does operate, but once you get rolling above an actual 5mph, the speedo also heads off the chart.  Any different thoughts than grounds here?  The reason I was thinking about 12 volt gauges was to replace the temp system and oil pressure system.
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Glenn Williams
Lansing, MI
www.threemenandatenor.com
1968 MCI 7 Ser. No. 7476 Unit No. 10056
8v71
4 speed Spicer
Hobie
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« Reply #4 on: January 08, 2008, 11:08:30 AM »

Well stated buswarrior!! Funny while very true.
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Len Silva
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« Reply #5 on: January 08, 2008, 11:11:02 AM »

Somehow I always knew she was a whore Wink
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HB of CJ
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« Reply #6 on: January 08, 2008, 03:16:16 PM »

Do/can dash gages simply wear out from old age?  Kinda like our bodies?  Anyway, perhaps a good, dedicated ground along with the properly sized hot wire powering the dash board may reduce inaccuracy.  But then again, maybe it won't.  Another good reason to visit your local, friendly, $inexpensive$ heavy truck wrecking yard and loading up on fairly new various dash gages and senders gaging things we never thought of and maybe do or don't need.  Not my fault here, I just ate a chip cookie and I can't handle sugar.  Smiley Smiley Smiley
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lyndon
1988 MC-9 DDC 6V92TA Fuller T-11605D
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« Reply #7 on: January 08, 2008, 07:08:11 PM »

A bad ground was my first thought when my tach started acting up. What seems odd is that it is perfectly stable and accurate from idle to about 1600, then drops off into wild oscillations below 1200 as the RPMs rise above that point. Is a bad ground likely in this case?

Don
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Don
1988 MC-9
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