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Old 02-07-2021, 06:14 PM   #1
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2001 Monaco Diplomat tachometer not working

Just picked up our new to us 01 Diplomat. All the gauges work fine except for the tachometer. I've looked around on the forum and most posts I can find folks have other issues along with the tachometer not working, so none really seem like the problem I'm having. Any ideas what the problem could be in my case? The Dip runs fine, shifts fine (drove it 700 miles home), the tach just stays at zero. I should also say that the dash lights don't work but I'm hoping that's just a bad headlight/dash dimmer switch. Thanks in advance!
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Old 02-08-2021, 05:07 AM   #2
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I have a 1998 Holiday Rambler Imperial. In this MH the tachometer gets a signal from the alternator to determine the engine RPM. When I bought this MH the tachometer was not working but the PO said I really didn't need it since it had an automatic transmission. I now know better so today I decided to determine whether the tack was bad or there was a problem with the wiring. To make a long story short, it's the wiring. To make sure I ran a temporary wire from the alternator directly to the tach and everything worked fine. Rather than put on my Dick Tracy hat and find the break in the original wire I just ran a new permanent wire directly connecting the alternator and tach.
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Old 02-08-2021, 06:04 AM   #3
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Originally Posted by veraken View Post
I have a 1998 Holiday Rambler Imperial. In this MH the tachometer gets a signal from the alternator to determine the engine RPM. When I bought this MH the tachometer was not working but the PO said I really didn't need it since it had an automatic transmission. I now know better so today I decided to determine whether the tack was bad or there was a problem with the wiring. To make a long story short, it's the wiring. To make sure I ran a temporary wire from the alternator directly to the tach and everything worked fine. Rather than put on my Dick Tracy hat and find the break in the original wire I just ran a new permanent wire directly connecting the alternator and tach.
Just curious about the "now I know better"? Is there something I need to be worried about with the tach not working? Like I said it seemed run/drive fine on the way home & it'll probably not be going anywhere for a while as we make updates and fix stuff, just want to gain some more knowledge. Thanks!
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Old 02-08-2021, 09:07 AM   #4
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On that vintage rig the alternator provides the rpm signal. Check for a wire at the alternator not connected. Instead of replacing the alternator it’s recommended to rebuild as newer alternators do not have the connection for the rpms. Someone may have replaced instead of rebuilding the alternator.
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Old 02-08-2021, 09:42 AM   #5
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Originally Posted by RvplusE View Post
Just curious about the "now I know better"? Is there something I need to be worried about with the tach not working? Like I said it seemed run/drive fine on the way home & it'll probably not be going anywhere for a while as we make updates and fix stuff, just want to gain some more knowledge. Thanks!
If you do any mountain driving you're gonna want that tach working to find out when you need to upshift or downshift to control engine speed for best climbing or descending performance. If going downhill in a lower gear you need to know beforehand if the tranny is going to upshift automatically regardless of the gear you have selected to protect the engine from overspeeding.
When I bought my '05 Knight the tach either didn't work at all or when it did it would zip around all over the dial. I drove it from AZ to CO that way. The speedometer and the engine coolant gauges weren't quite right either.
One of the first things I bought for the coach was a ScanGaugeD. Money well spent!
After quite a few more miles the tach slowly repaired itself but I still have a good backup if it ever becomes unreliable again. I still rely on the ScanGaugeD for a lot of other things though.
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Old 02-08-2021, 09:46 AM   #6
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If you do any mountain driving you're gonna want that tach working to find out when you need to upshift or downshift to control engine speed for best climbing or descending performance. If going downhill in a lower gear you need to know beforehand if the tranny is going to upshift automatically regardless of the gear you have selected to protect the engine from overspeeding.
When I bought my '05 Knight the tach either didn't work at all or when it did it would zip around all over the dial. I drove it from AZ to CO that way. The speedometer and the engine coolant gauges weren't quite right either.
One of the first things I bought for the coach was a ScanGaugeD. Money well spent!
After quite a few more miles the tach slowly repaired itself but I still have a good backup if it ever becomes unreliable again. I still rely on the ScanGaugeD for a lot of other things though.
Good to know. This is my first diesel and first class A, so it's all new to me! Thanks for the info!
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Old 02-08-2021, 09:51 AM   #7
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On that vintage rig the alternator provides the rpm signal. Check for a wire at the alternator not connected. Instead of replacing the alternator itís recommended to rebuild as newer alternators do not have the connection for the rpms. Someone may have replaced instead of rebuilding the alternator.
Crawled underneath and took a look at the alternator, hard to tell if it's been replaced since it'd probably be just as cruddy either way, but it looks like all the wires are attached. Guessing that small one in the middle is probably the signal wire for the tach? I have yet to pull out the wiring diagram (I loathe electrical problems :/) Click image for larger version

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Old 02-08-2021, 11:12 AM   #8
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As mentioned very important tach is working, over revving motor can be very expensive, my tach wires to alternator.
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Old 02-08-2021, 12:40 PM   #9
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Thanks for the advice/info so far. So, another question: in the event I can't get the tach working by checking/running a new wire.... Can I just rely on the rpm readout on a ScanGauge D? (assuming it provides rpm info, seems like it does). I verified I had the connector under the dash and went ahead and ordered one.
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Old 02-08-2021, 02:11 PM   #10
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It will upshift before it overspeeds, that's part of the transmission programming.

Most owners stick it in D, switch on the engine mounted brake and go. That's all you need to do on normal highways.

That way they can watch where they're going.

If you overspeed in high gear, your going over 100 MPH.
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Old 02-08-2021, 03:06 PM   #11
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Originally Posted by RvplusE View Post
Thanks for the advice/info so far. So, another question: in the event I can't get the tach working by checking/running a new wire.... Can I just rely on the rpm readout on a ScanGauge D? (assuming it provides rpm info, seems like it does). I verified I had the connector under the dash and went ahead and ordered one.
If you stay away from mountain driving until you get enough experience driving your coach/engine/transmission combination and can tell by its sound when your engine is lugging, cruising or turning too fast you'll be fine. The many engine parameters you can read from the ScanGaugeD can help you learn what power setting/engine speed combinations to use for best performance or economy in different situations.
If you list your coach's info on a signature page we can help you with better info and recommendations. Meanwhile, here is some power info on a Cummins ISC 350.Click image for larger version

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Old 02-08-2021, 04:17 PM   #12
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If you stay away from mountain driving until you get enough experience driving your coach/engine/transmission combination and can tell by its sound when your engine is lugging, cruising or turning too fast you'll be fine. The many engine parameters you can read from the ScanGaugeD can help you learn what power setting/engine speed combinations to use for best performance or economy in different situations.
If you list your coach's info on a signature page we can help you with better info and recommendations. Meanwhile, here is some power info on a Cummins ISC 350.Attachment 317514
Thanks. I've updated the RV info in my profile. Every car I've ever driven has been a manual, so I'm no stranger to using the engine to slow down on steep grades to save the brakes, from what I can tell I'll be doing pretty much the same thing with the rv just with a push button transmission & the benefit of an exhaust brake instead. Hopefully I can get the tach working or just use the ScanGauge since I'll definitely want to be able to monitor my rpms. Like you said, just gotta get used to it
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Old 02-08-2021, 05:14 PM   #13
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Thanks. I've updated the RV info in my profile. Every car I've ever driven has been a manual, so I'm no stranger to using the engine to slow down on steep grades to save the brakes, from what I can tell I'll be doing pretty much the same thing with the rv just with a push button transmission & the benefit of an exhaust brake instead. Hopefully I can get the tach working or just use the ScanGauge since I'll definitely want to be able to monitor my rpms. Like you said, just gotta get used to it
You're welcome.
I'm on a phone app so I can't see all the information available on the website. I hope you included all the engine and transmission year, make, model, displacement, etc. so folks don't have to go-a-googlin' to find the correct info to help you with probs.
Don't forget the engines and tranny's in coaches these days will do things they're programmed to do which may or may not be exactly what you wanted done or expect them to do. Research, read and keep asking them questions.
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Old 02-15-2021, 12:51 PM   #14
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Just thought I'd post a quick update: picked up a ScanGaugeD & it looks like I'm getting the RPMs through that successfully, so even if I can't get the tach working I'll at least have something to go by Click image for larger version

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