M1078 A1 tachometer diagram

[Xengineguy]

Ok I’ve spent hours looking for a specific diagram for tach hook up. I don’t really want to tap the alternator.
Where can I get the signal from to crank sensor ? Inside the cab??? This is an 2001 A1.
The A0 seems pretty straightforward.
Thanks. Mike

 

[Ronmar]

The A1 crank sensor is an ECU sensor. The tach signal/RPM data is on the J1939 bus on the A1… I don’t think you can use the tach AC signal as the LBCD uses that signal…

Are you trying to add a tach? Google “J1939 tach”

 

[Xengineguy]

The A1 crank sensor is an ECU sensor. The tach signal/RPM data is on the J1939 bus on the A1… I don’t think you can use the tach AC signal as the LBCD uses that signal…

Are you trying to add a tach? Google “J1939 tach”

Yes trying to add tach, should have purchased the j1939. I don’t have any connections to the ac post on the alternator?

 

[Ronmar]

Yes trying to add tach, should have purchased the j1939. I don’t have any connections to the ac post on the alternator?

There is an excite terminal and an AC terminal on the alternator regulator. But this signal is used by the LBCD For its operation. Not sure if it is even in a format that would properly drive an AC or alternator tach, it was designed to feed the LBCD, both components built be C.E.Neihoff… in earlier versions that terminal was a F- terminal to monitor the field drive signal… no telling what Neihoff put on it to feed the LBCD…

 

[GeneralDisorder]

Yes trying to add tach, should have purchased the j1939. I don’t have any connections to the ac post on the alternator?

The alternator AC post is a voltage regulator output that indicates alternator load to the LBCD. It is not a tach signal.

The ECM does provide a "complimentary" tachometer drive signal other than the J1587/J1939. On the C7 (I believe it should be the same on the 3126 - communication and power/ground definitely are the same as is the 70 pin chassis connector, etc) the tach is wired positive to pin 38, and negative to pin 39. These pins are likely not populated on your chassis 70 pin ECM connector so you will need to get the correct pins and pin them to get that output.

 

[Ronmar]

The alternator AC post is a voltage regulator output that indicates alternator load to the LBCD. It is not a tach signal.

The ECM does provide a "complimentary" tachometer drive signal other than the J1587/J1939. On the C7 (I believe it should be the same on the 3126 - communication and power/ground definitely are the same as is the 70 chassis connector, etc) the tach is wired positive to pin 38, and negative to pin 39.

On the earlier alts used on the A0 it WAS a field load output(labeled F-) to the Standardized Test Equipment (STE) plug To gauge alt load. later when they added the LBCD they reconfigured the regulator and re-named that terminal to “AC”. The LBCD bases its actions on alternator RPM. Below a certain RPM, it ignores the overload/undervolt symptoms. Thats the only way I see for the LBCD to get its alt RPM information…

 

[GeneralDisorder]

On the earlier alts used on the A0 it WAS a field load output(labeled F-) to the Standardized Test Equipment (STE) plug To gauge alt load. later when they added the LBCD they reconfigured the regulator and re-named that terminal to “AC”. The LBCD bases its actions on alternator RPM. Below a certain RPM, it ignores the overload/undervolt symptoms. Thats the only way I see for the LBCD to get its alt RPM information…

Ah - yes so the AC terminal basically outputs alternator RPM to the LBCD and it judges battery state (and thus infers load) based on voltage.

But as you pointed out it's a Niehoff spec and we don't know how many pulses per rotation - plus it's alternator RPM not engine RPM. Not really easily useful for a gauge.

 

[Ronmar]

Ah - yes so the AC terminal basically outputs alternator RPM to the LBCD and it judges battery state (and thus infers load) based on voltage.

But as you pointed out it's a Niehoff spec and we don't know how many pulses per rotation - plus it's alternator RPM not engine RPM. Not really easily useful for a gauge.

exactly, its a Neihoff signal…

 

[Xengineguy]

LBCD, low battery cut off??
I have an open ac terminal on the regulator?
Ok so todays monkey work. I took the ac terminal as a tach signal . Used a 1 amp glass fuse in-line and wired it to the tach.
Used my scanner hooked to the diagnostics plug and monitored rpm. I entered off set numbers in to the tach until the idle of the new tach matched the scanner . Raised engine rpm and checked at many points, they match perfectly! I don’t remember what the final
Number in the tach was but I could check if anyone needs it. Took a long test drive (Deer watching) and all seems good.

 

[Wingnut13]

Yeah! Now which tach and which settings?

 

[coachgeo]

Yeah! Now which tach and which settings?

same same.... wonder if my Hummer alt's regulator is like the A1, A0 or something entirely different?

 

[Ronmar]

LBCD, low battery cut off??
I have an open ac terminal on the regulator?
Ok so todays monkey work. I took the ac terminal as a tach signal . Used a 1 amp glass fuse in-line and wired it to the tach.
Used my scanner hooked to the diagnostics plug and monitored rpm. I entered off set numbers in to the tach until the idle of the new tach matched the scanner . Raised engine rpm and checked at many points, they match perfectly! I don’t remember what the final
Number in the tach was but I could check if anyone needs it. Took a long test drive (Deer watching) and all seems good.

LBCD Load and Battery Control Device. Its located back between the spare tire and the air filter. It has 4 large terminals, a cannon plug for other signal wires(like the AC input) and a status LED. Without the AC input from the regulator it will not function as intended. If you have dropped to 2 batteries @~100AH each, you don’t really need it anyway. Its sole purpose was to try and keep the alternator alive longer with a grossly oversized battery bank that was perpetually in a state of discharge… it will disconnect the batteries and put them on a low current trickle charge to save the alternator if it cannot maintain rated voltage above 1500 alternator RPM(AC signal provides this information) It will also light the charge failure light in the dash if the alternator cannot maintain rated output…

 

[Xengineguy]

LBCD Load and Battery Control Device. Its located back between the spare tire and the air filter. It has 4 large terminals, a cannon plug for other signal wires(like the AC input) and a status LED. Without the AC input from the regulator it will not function as intended. If you have dropped to 2 batteries @~100AH each, you don’t really need it anyway. Its sole purpose was to try and keep the alternator alive longer with a grossly oversized battery bank that was perpetually in a state of discharge… it will disconnect the batteries and put them on a low current trickle charge to save the alternator if it cannot maintain rated voltage above 1500 alternator RPM(AC signal provides this information) It will also light the charge failure light in the dash if the alternator cannot maintain rated output…

I will look to see if I have that?? I did see the polarity protection device. Got to check the lugs on that today make sure they are tight
And clean. This truck doesn’t have master shutoff switches or any kind of outside start switch. Will post the tach settings I used.

 

[Ronmar]

Ok, it sounds like you have what I call an A0.5… it has a 3126 engine and the A1 circuit board power panel, but the basic DC power wiring is the same as the A0. I suspect you have a simple polarity protection box back inboard of the spare(has only 4 large terminal studs and no other connectors).

still only needs 2 batteries in the box

 

[Xengineguy]

Ok, it sounds like you have what I call an A0.5… it has a 3126 engine and the A1 circuit board power panel, but the basic DC power wiring is the same as the A0. I suspect you have a simple polarity protection box back inboard of the spare(has only 4 large terminal studs and no other connectors).

still only needs 2 batteries in the box

Yes checked the truck today seems I only have the polarity protection device. Yes inboard of the spare.

 

[Xengineguy]

Yeah! Now which tach and which settings?

The tach settings were set at 15. The jury is still out though, I drove the truck today and the tach was fluctuating some
Almost like a bad ground? Funny thing when I turned on the dash lights it’s works good. Will keep you informed.

 

[Ronmar]

Well its a brushless alt so it gets its excitation in pulses, which means the alt rotor is physically loaded in pulses. At low electrical load, and perhaps depending on how tight the belt is, this might cause fluctuations in alt RPM compared to actual engine RPM and these are reflected in the tach display. When you loaded the alt, the excitation increased field duty cycle and the rotor RPM probably becomes more stable with that load…

A pulsed output from the ECU that General suggested shouldn’t do this…

 

[coachgeo]

Yes checked the truck today seems I only have the polarity protection device. Yes inboard of the spare.

that is also where the LBCD sits.... but if you have 100amp alt on an early A1 with none of the disconnect switches/solenoids.... it will be Polarity Protection Device anyway.

 

[Ronmar]

The A1 has a 100A alt and LBCD. They didnt typically fit the 260A untill the A1R. In his case he has one of the "Tween" trucks. 3126 engine, but no external switches or battery disconnects. Has the newer circuit board power dist panel and 100A alt(AC signal and LEDs) but no LBCD. I call these an A0.5...

 

[coachgeo]

The A1 has a 100A alt and LBCD. They didnt typically fit the 260A untill the A1R. In his case he has one of the "Tween" trucks. 3126 engine, but no external switches or battery disconnects. Has the newer circuit board power dist panel and 100A alt(AC signal and LEDs) but no LBCD. I call these an A0.5...

my bad....... intended to add disconnect switches as one one of things that typically mean it will have LBCD . Will fix post so not to confuse anyone