First Gen Factory Tach Question

[JDancoe]

I have two 1st Gen (1992) diesel trucks. On one I have an Isspro Tach with a mag pick-up I installed on the flywheel hsng. I also located the tach lead under the dash for the O. E. M. accessory tach, which I'm told by my dealer, has not been available for years.

I have a good quality Digital Multimeter that I can attach to that lead, and get a reading on the tach setting, that corresponds to the RPM reading on my Pro-Link 9000 DDR, which I use when making adjustments, checking codes, etc.

I have tried 3 different aftermarket tachs 1-Isspro, 1-VDO and 1-AC Delco that I have and none of them register when connected to the tach lead.



Does anyone know what type of signal the tach lead puts out, and who manufactured the OEM accessory tach, or if any aftermarket supplier makes a tach that is "Plug & Play" to the factory wire? Just curious. Thanks

 

[Philip]

The tach signal is a square wave. But it is only a 5 volt signal. That is why most people have problems with other brands of tachs. Most tachs will not read a 5 volt signal. They are looking for 9 volt plus signal.



Isspro and westach make tachs to work in the volt range Dodge gave us.



The Isspro doesn't have the molded plug to fit. They use female spade terminals to hook to the factory plug.

 

[JGrefenstette]

i have the isspro 4k rpm tach in mine. i ordered the wireing kit designed for our trucks by isspro. the harness in the truck has 4 leads. the harness the send with the tach has 5. the signal lead has two options for different applications. i connected the one the manual told me too. the tach did not work. well it kind of did, but it was just jumping around and not giving me a reading. i called isspro and they told me to switch the signal lead, and see if they did the kit wrong at the factory. i did and it worked. they said one of the leads has a filter the other doesnt. it works great though. if you want i can give you the part #'s for the tach and the wiring kit.

 

[JDancoe]

J, Yes, if you have the part no's. I would really appreciate having them. Thanks

 

[JGrefenstette]

The 4k rpm tach P/N : R8503

Wiring kit for pre-94 Dodge Cummins P/N : R8912-92



hope this helps (if i recal i got both for a good deal. USDieselParts.Com is fabulous, their where i got the tach and wiring kit and my other gauges... easy site to use and great prices. customer service is great too. )

 

[JGrefenstette]

i think in total it set me back right near $160 for the tach and harness. if you get the set up make SURE you get the R8912-92!!!! the "92" on the end is crutial. they offer a R8912 that is for pre-92 trucks. the R8912-92 is for 1992 and UP models.



4k Tach : $115. 41

Sensor Harness : $38. 54



best of luck my friend

 

[MMiller]

I think this post should be put up as a sticky, or in the FAQ for first genners. Seems it comes up a lot, and this is about the best answers to the tach question for a long time, that I've seen. Can we tag it up top??



Michael

 

[JGrefenstette]

i agree, if i could have found this info when i first started lookin for it, it would have saved a lot of time.

 

[Tippin Spotted Cow]

I had the same problem with the Isspro harness, only when I used the wrong lead the tach didn't move at all. There is a note that references the size of the tach as relative to which lead to use. It's wrong. Particularly the note for the smaller tach. Well, kinda. The guy at Isspro explained to me that if you have a newer 2" tach it uses the same lead as the 3", where as an older 2" uses the lead with the filter on it. That should be in the sticky also, because I spent a long long time in the cold retracing my wiring over... and over... and over. Oh, the way to tell the two tachs apart is the newer one uses two bulbs, and the older uses one.

 

[JGrefenstette]

when did they start using two bulbs??? mine has one. i bought it a few months ago. for anybody who knows, is ther any way to make it brighter? can i get a brighter bulb?

 

[Tippin Spotted Cow]

I would pop the bulb out and take it to the hardware store, or even better, an electrical warehouse. I don't really recall what type of bulb it was, but I would guess you can find it, along with higher wattage rating bulbs to fit the same socket.



If you look at certain websites they'll list "old style tach" and "new style tach". I got it from PDR around November and they didn't have any more of the old style.



I'd go with a different bulb first. I've found halogen replacements for standard flashlight bulbs for around $20, as opposed a new tach for $100+.

 

[JGrefenstette]

great. thanks ill see what i can find. i just put in isspro boost and egt gauges and their sooo bright i cant hardly see the tach any more.

 

[Tippin Spotted Cow]

Yeah, I have the opposite problem. My pryo and boost have one bulb, and my tach outshines them both.

 

[JGrefenstette]

thats strange, the pyro/boost on mine both have 2 bulbs, or maybe just the pyro. either way, they are super bright.

 

[mysteryman]

the harness that Isspro sells for connecting the tach to the vehicle computer is just a waste of money. There is large lump in the harness. It is a resistor. . when you use the wire with the resistor in it as it says to do in the instructions. The tach will normally not work. . When you bypass it the tach will work. . Therefore all you are paying for and or using in the kit is 4 coppers wires and a few crimp connectors ... 38. 00 is a bit high for just that... . radio shack could set you right up for a whole lot less.

i think in total it set me back right near $160 for the tach and harness. if you get the set up make SURE you get the R8912-92!!!! the "92" on the end is crutial. they offer a R8912 that is for pre-92 trucks. the R8912-92 is for 1992 and UP models.



4k Tach : $115. 41

Sensor Harness : $38. 54



best of luck my friend

 

[WJones]

I put a 3 3/8 programmable isspro tach (R5535M) in my 92 and I get nothing out of it. I used a female spade terminal to grab the grey wire from the tach harness then used the wires from the radio (radio is on an aux battery) for switched and constant 12v as well as the ground. I set the gauge for a variety of different pulse/rotation settings. I just replaced the crank sensor (ac works & battery charges) so I'm not sure what's going on. My gauges are all drawing power and ground from the same spot and the others work fine. My multimeter says it's getting power and it twitches when the key goes to run, I get a fraction of a volt from the grey wire when running and varies with rpm (I figured my meter might not be catching the pulses). Bad pcm perhaps? I'm stumped on this one, any ideas?

 

[dpuckett]

I had to use the wire with the resistor to get my tach to work in my 92. Has worked just fine in 3 or 4 different trucks. From what I remember, the EV series tachs (bubbled face) dont need the resistor; the flat faced ones do. So, MM, you are partially correct. My advice would be to hook up your wires without the harness. If it works, great, you saved $40. If not, you need the harness and resistor.



Is a resistor a resistor, or are there different levels of resistance? Most electrical and electronic stuff hits me mid forehead.



Daniel

 

[cerberusiam]

the harness that Isspro sells for connecting the tach to the vehicle computer is just a waste of money. There is large lump in the harness. It is a resistor. . when you use the wire with the resistor in it as it says to do in the instructions. The tach will normally not work. . When you bypass it the tach will work. . Therefore all you are paying for and or using in the kit is 4 coppers wires and a few crimp connectors ... 38. 00 is a bit high for just that... . radio shack could set you right up for a whole lot less.

According to Isspro its not a resistor its a noise filter. Most EVA's tachs are good enough they don't need the extra filtering for the tach signal, others need the harness to clean the signal up so the tach can read it.

 

[PToombs]

DP, there are different levels of resistance. The stripes on the resistor tell how much.

Now you've exhausted my knowledge on it!

 

[biggy238]

My brother has a 93 with a Tach already installed, but it is not functioning. I need to find out the manufacturer and how it is attached when I can make it home, but I wanted to go ahead and ask about the factory harness you all have mentioned.



Where is it located and what wire carries signal V+? I have a fluke DMM with rpm function so if you can point me toward the harness I can get the rest.