Good Sam Club Open Roads Forum: Tachometer has stopped working Safari Sahara Diesel
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 > Tachometer has stopped working Safari Sahara Diesel

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clayhubler

La Center, WA

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Posted: 06/11/18 09:13pm Link  |  Quote  |  Print  |  Notify Moderator

So I went to a DSU Peterbilt service center on the way home today. They couldn't find the part. The sensor is actually mounted at the rear of the transmission, just forward of the bell housing, so the guy told me he thought I should go to an allison service center, and also that it was probably an engine speed sensor, not a crankshaft position sensor. I don't really know the difference, but I'll be going by the allison shop on thursday.

clayhubler

La Center, WA

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Posted: 06/11/18 09:20pm Link  |  Quote  |  Print  |  Notify Moderator

sorry, double post

clayhubler

La Center, WA

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Posted: 06/14/18 06:00pm Link  |  Quote  |  Print  |  Notify Moderator

Here is so me info I should have included in the post from the beginning, but at the time I didnt think it was related.

The tach quit before I replaced the alternator on a trip last year. When I replaced the alternator, there is a wire that was hooked to the old one, but is not hooked up now. The guy that I got the alternator from was from a diesel electric repair shop, and he really seemed like he knew what he was doing. I had already tried several autoparts stores, and they hadn't come up with anything. This guy saw the alternator and recognized it immediately.

I asked about the wire, and he told me that it wasn't needed because the new alternator didn't need to be energized like the old one had, and thats what the wire was for. However, he said it could be hooked up with no harm anyways, so that's what I did. Keep in mind at this time the tach was already not working, so I didn't believe the two problems were related. After the swap, I continued my drive home with now issues, the wire still hooked up. When we got home the next day, I could hear a tick, tick, tick at the back of the coach. After some investigation, I realized it was the hour meter still counting off, even when the engine was off. Knowing that the alternator swap was the only thing that had changed since the correct operation of the hourmeter, I disconnected the aforementioned wire and the meter stopped ticking off.

The wire remains disconnected. I do not know if the hourmeter runs when engine is running or not, but I am not as concerned with that at the moment.

Sounds like Mountainair05 may be right, but I don't know if I have a leece neville or not.

I am pretty sure now that the tach and the alternator are involved with each other, an allison transmission specialist told me as much when I questioned him about the sensor that I suspected was the crankshaft position sensor, but was in fact the engine speed sensor. He said the tranny has nothing to do with the tach, so I am back to the alternator, but even if I hook up the wire again, the tach doesn't work. This alternator looked identical to the old one, with terminals in the same locations and everything.

wolfe10

Texas

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Posted: 06/14/18 06:51pm Link  |  Quote  |  Print  |  Notify Moderator

So, tell us what terminals the alternator has OR its model number.

All will have: B+ for positive alternator output to battery or battery isolator.

Ground-- some ground through metal to metal contact, some have a separate ground lug and wire.

Some will have a "sense wire" from chassis battery side of the battery isolator to the alternator sense terminal (small gauge wire).

Some will have an excitor wire which is hot only when ignition is on.

Some will have a tach out terminal.


Brett Wolfe
2003 Alpine 38'


FMCA Forum: www.community.fmca.com/index

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MountainAir05

New Mexico

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Posted: 06/14/18 09:01pm Link  |  Quote  |  Print  |  Notify Moderator

wolfe10 wrote:

So, tell us what terminals the alternator has OR its model number.

All will have: B+ for positive alternator output to battery or battery isolator.

Ground-- some ground through metal to metal contact, some have a separate ground lug and wire.

Some will have a "sense wire" from chassis battery side of the battery isolator to the alternator sense terminal (small gauge wire).

Some will have an excitor wire which is hot only when ignition is on.

Some will have a tach out terminal.


When I work this system many years ago it was the stator tap but I have no current working knowledge. Neighbor is gone for a while so can not talk to him and he has my old books.

clayhubler

La Center, WA

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Posted: 06/15/18 06:25pm Link  |  Quote  |  Print  |  Notify Moderator

Here are photos of the alternator model number and the terminals. It also has a ground terminal that is out of frame.

[image]
[image]Click For Full-Size Image.

[image]
[image]Click For Full-Size Image.
I ran a wire directly to the tach gauge from the R terminal and didn't get anything. Wondering if this tach is not wired internally to support the tach funtion?
Thank you to rv.net member 1492 for the photo posting help.

clayhubler

La Center, WA

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Posted: 06/16/18 09:48am Link  |  Quote  |  Print  |  Notify Moderator

Thanks to everybody who offered advice on this problem. I am happy to report that the issue is now resolved and everything is functioning properly.

The root cause of the tachometer failure was that I had simply hooked the tach wire to the wrong terminal on the alternator. Initially, after the reinstall of the alt, this didn't raise a red flag because the tach had already quit on me before the alt swap. I now know that this must have been when the first alt went bad.

I got a tip on another forum about looking into the battery isolator that set me on the right path. I became convinced that the bad isolator had killed my new alternator. But after testing the isolator with my multimeter, it proved good. After further investigation I realized that the terminal on the isolator that should run to the (+) on the chassis battery was in fact run to the (-).

This could only have happened when I changed the chassis batteries about a week ago, another fact that I left out of the conversation because I didn't think it was relevant.

So, to make a long story short, I initially hooked up the tach wire wrong, then later when I hooked it up correctly, it still didn't work because the alt was not functioning properly at that time because of the battery swap.

Thanks again to all who helped me out with this matter.
BTW, on my alternator, the correct terminal was marked (R)

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