M998 Hmmwv tach installed.

for those that dont know tachs dont normally come in the hmmwv and they are hard as heck to find and very pricey when you do.

I was fortunate enough to find one on epay for a hefty price. Install wasn't bad.

drill 4 hole in the front windshield frame. Install new center dash cover and wire into the fuel gauge and splice a wire on the back of the alternator.

Can you show me how you wired up the tach itself? I have a tach like that I want to put on my Gasser M35.

I dont think it is the same. but for the hmmwv I simply found a 24V switched source (The mil used the fuel gauge for it.)


Then you use the IGN trigger thats on the generator. you run it in line with the IGN wire on the generator.

What RPM can you run down the road, without causing damage?..
What I mean is, What is the top RPM to Cruise?...3K..?...
Kevin...

I found that this motor in the gm 1/2T pickups have a redline of 3600rpms. I usually run mine about 3k and that nets me about 60mph. I just cant bring myself to run it over that. it sounds like it wants to come out from under the hood.

That's a nice setup. I saw that on some M1114's a few years ago and really liked it. It's high enough that not being water proof is a non issue.

OK, so I purchased one.
Now, is there any last minute advice you can give me before I begin the install process.
Anything NOT covered in the convoluted instructions, jwaller ?
Thank you,
Pat in NJ

jwaller said:

for those that dont know tachs dont normally come in the hmmwv and they are hard as heck to find and very pricey when you do.

I was fortunate enough to find one on epay for a hefty price. Install wasn't bad.

drill 4 hole in the front windshield frame. Install new center dash cover and wire into the fuel gauge and splice a wire on the back of the alternator.

Click to expand...

RE: Re: M998 Hmmwv tach installed.

ya. some stuff. I didnt use the center dash piece that comes in the kit bc it deletes the air vent on drivers side and doesnt have the holes for the radio rack. I put a notch in the original one and all is well.

I didnt drill another hole in the firewall for the alt connection. I used one that was already there.
also the rearview mirror will have to be moved down.

also, what alternator do you have? the instructions didnt cover the one I have. the 100a external regulator.

any questions please ask. It pretty easy but the directions are less than helpful.

Re: RE: Re: M998 Hmmwv tach installed.

Thanks for the advice jwaller.
As I progress with the install I may have more questions.
Thank you,
Pat

jwaller said:

ya. some stuff. I didnt use the center dash piece that comes in the kit bc it deletes the air vent on drivers side and doesnt have the holes for the radio rack. I put a notch in the original one and all is well.

I didnt drill another hole in the firewall for the alt connection. I used one that was already there.
also the rearview mirror will have to be moved down.

also, what alternator do you have? the instructions didnt cover the one I have. the 100a external regulator.

any questions please ask. It pretty easy but the directions are less than helpful.

Click to expand...

RE: Re: RE: Re: M998 Hmmwv tach installed.

what engine do you have in your HMMWV that has a 3600rpm redline? 6.2L, 6.5L NA or 6.5L turbo? just curious. as I have the 6.2L in my m1008 and was wondering what the safe rpm limit was for them. I uslly dont go over 2700 rpms which lets me do only about 55. If it were safe to do 60 mph @ 3000 rpms and not kill the engine that would be great. It's hard to tell though. this is my first diesel and it always sounds like it's screaming to me. darn 4.56 gears.

RE: Re: RE: Re: M998 Hmmwv tach installed.

I have the std 6.2 NA. I run about 3k at 60mph and the motor seems to like it. this thing doesnt have any power until it comes up above 2500.

RE: Re: RE: Re: M998 Hmmwv tach installed.

86M10086.2,
The Army feels the HMMWV doesn't need to be turning more RPMS than 3000(mil tach limit). The engine is governed at its red line of 3600 with the DB2 injection pump but DDA states that 3000 was the max sustained RPM under load. Really even that high is a little hard on it but if you are on the highway I usually go 60-65 cruising with the 3L80 trans. Slower then that on today's roads is a pain. I know it would be happier at 55 but its just too slow.

The DDA 6.2L NA that was put into the HMMWV's was spec 'ed at 3900 to 4100 RPM's"High Idle" (basically meaning; transmission in neutral and pedal to the floor until the test tach reads between 3,900 & 4,100 without the engine going ballistic).

idle = 600-700 RPM
30 MPH = 1428 RPM
45 MPH = 2159 RPM
60 MPH = 2856 RPM
Max Sustained RPM rec. by DDA 3000 RPM = Approx 63 MPH.

PM sent

RE: Re: RE: Re: M998 Hmmwv tach installed.

The tires, must make the difference. If I could do 60 at 2800 in my M1008 I'd be very happy. but I guess even with the gear reduction units the 36/37 inch tires make a big difference. That is one of the reasons I looking for some for the M1008. For me 60 mph is @ 3000 rpm according to my tach. 55 is @ 2800. I've been told on diesel specific forums that these engines will go off the rev limiter all day long but thats retarded in my opinion. The truck seems happiest at 50 which is 25-2600 rpms. But anything over 100 miles at 50 becomes a rather lenghty trip. I'vehad the 6.2L up to 3000, but I don't know if it's me or not as this is my first diesel, but 3000 seems a litttle high to me for the 6.2L

hmmwv tachometer

I put a tachometer in my 1987 m998 6.2 recently. It is showing 2400 rpm @55mph. I have 36x12.5x16.5 tires on it. I also did not use a2 duct cover and ran the wiring outside heater duct using harness retainers it is out of the way and looks fine. I usually keep it at 55mph on highway over that it can get squirrely especially if road is rutted from heavy trucks.

My truck seems to be limited by the transmission temperature. Any extended driving past 70 MPH will cause a steady increase over 220 degrees. The engine is at around 3000 RPM and feels like it has more in her....

(6.5 TD and 4L80e)

actionpaintball said:

My truck seems to be limited by the transmission temperature. Any extended driving past 70 MPH will cause a steady increase over 220 degrees. The engine is at around 3000 RPM and feels like it has more in her....

(6.5 TD and 4L80e)

Click to expand...

the 80E shouldn't be running that kinda of temp. are you sure the TC is locking and your cooler is in good condition?

I just bought a tach just like the one you have in your HMMWV for a little over $3.00 from Ebay; the description indicted it was "new". I am hoping it will work in my M1008A1 with the 6.2 & 24 volt system. I may want to talk you out of a photo of where the wires go. I'll include a photo of the front and back. The seller has more of these by the way.

you got a deal but your is very different from mine. mine had a single connector that came out the back and then branched out in the truck and went to the alternator, fuel level gauge and ground.

I think this one just has separate wires for the hour gauge, then again I will have to see what I have once it gets here. The seller is in Wilmington NC and will probably list more of these on Ebay pretty soon. Even if I can't get it to work it will look nice somewhere in the truck!

FWIW I've seen a tach that looks very much like these in the M35A3's.

the one in the hmmwv also didn't have a provision for back lighting.