OBD2b - OBD1 Conversion & Tach doesn't work
 

I put a Hondata s300 in my 2001 GSR (installed in an OBD1 P28). Everything started up fine, but the tach did not work. The car was running, but the tach did not work at all. I pulled the Hondata unit out, threw in a stock OBD1 P72 and the same result occurred.

When the S300 was in the car, all the datalogging was working. RPM, Vac, etc. all were working on the display in the laptop. The gauge in my cluster just sat on 0. All other gauges worked: Temp, Fuel, Speedo. The tach just sat on 0. Pulled everything out, and put the stock OBD2b ECU back in, and everything worked fine.

The GS-R is SC'd and running the JRSC hack and FMU. It started throwing the code for the Atmospheric sensor. I picked up a Hondata S300 and was going to put the unit in and run a stock P72 map in it (still running the JRSC hack) until I can get the cash to take it to the tuner. Now, seems like I have other issues.

Any suggestions?!

http://images.honda-tech.com/set1//smile/emcrook.gif

Re: OBD2b - OBD1 Conversion & Tach doesn't work (C_Rock77)
 

I'd try another conversion harness. I see bad ones all the time.

Re: OBD2b - OBD1 Conversion & Tach doesn't work (C_Rock77)
 

Many people that copy those harnesses and whore them out don't remember to jump the tach wire. OBD2b harnesses need the OBD2b A19 wire jumpered with the OBD2b B13.....all going to OBD1 A21.

Re: OBD2b - OBD1 Conversion & Tach doesn't work (Blown90hatcH)
 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Blown90hatcH &raquo;</TD></TR><TR><TD CLASS="quote">Many people that copy those harnesses and whore them out don't remember to jump the tach wire. OBD2b harnesses need the OBD2b A19 wire jumpered with the OBD2b B13.....all going to OBD1 A21.</TD></TR></TABLE>

Think you're right....looks like that jumped pin that's a combination of wires from OBD2b B13 and OBD1 A21 is actually pinned into OBD2b A18, not 19. I wonder if it's simple enough that A18 & A19 are switched?

I can't figure out how to get these pins out of the plug...it's a one piece plug http://is.rely.net/1-92-30009-l-tEzP...BTGi0jOCgA.gif

Re: OBD2b - OBD1 Conversion & Tach doesn't work (Blown90hatcH)
 

if i used an obd2 engine harness with obd1 body harness would i have this problem??

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Blown90hatcH &raquo;</TD></TR><TR><TD CLASS="quote">Many people that copy those harnesses and whore them out don't remember to jump the tach wire. OBD2b harnesses need the OBD2b A19 wire jumpered with the OBD2b B13.....all going to OBD1 A21.</TD></TR></TABLE>

if so how could i fix it, as of now my speedo jumps a little bit when i hit the gas and my tach does not work at all!!!

Re: OBD2b - OBD1 Conversion & Tach doesn't work (C_Rock77)
 

when you perform a conversion can you still pass emissions?always wandered that.

Re: OBD2b - OBD1 Conversion & Tach doesn't work (tony1)
 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by tony1 &raquo;</TD></TR><TR><TD CLASS="quote">I'd try another conversion harness. I see bad ones all the time. </TD></TR></TABLE>

Tried that, Tony....any other ideas?

Re: OBD2b - OBD1 Conversion & Tach doesn't work (Blown90hatcH)
 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Blown90hatcH &raquo;</TD></TR><TR><TD CLASS="quote">Many people that copy those harnesses and whore them out don't remember to jump the tach wire. OBD2b harnesses need the OBD2b A19 wire jumpered with the OBD2b B13.....all going to OBD1 A21.</TD></TR></TABLE>

+2 the issue is in the conversion harness. Contact who ever you purchased it from.

Re: OBD2b - OBD1 Conversion & Tach doesn't work (mtber)
 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by mtber &raquo;</TD></TR><TR><TD CLASS="quote">

+2 the issue is in the conversion harness. Contact who ever you purchased it from.
</TD></TR></TABLE>

Well...you guys are both wrong. Contacted the manufacturer and it's not in the harness. The FIX can be done in the harness....the issue is something with the tachometer electronics. For people's future reference:

https://img167.imageshack.us/img167/...hfixgh7.th.jpg


Modified by C_Rock77 at 5:48 PM 4/17/2008

Re: OBD2b - OBD1 Conversion & Tach doesn't work (C_Rock77)
 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by C_Rock77 &raquo;</TD></TR><TR><TD CLASS="quote">

Well...you guys are both wrong. Contacted the manufacturer and it's not in the harness. The FIX can be done in the harness....the issue is something with the tachometer electronics. For people's future reference:

https://img167.imageshack.us/img167/...hfixgh7.th.jpg


Modified by C_Rock77 at 5:48 PM 4/17/2008</TD></TR></TABLE>

can anyone verify the trick?

Re: OBD2b - OBD1 Conversion & Tach doesn't work (JDM turbo DC)
 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JDM turbo DC &raquo;</TD></TR><TR><TD CLASS="quote">

can anyone verify the trick?</TD></TR></TABLE>

If the harness is not built properly....many cheapo egay copiers do this....then this fix can be used to get the tach to work properly.

Re: OBD2b - OBD1 Conversion & Tach doesn't work (Blown90hatcH)
 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Blown90hatcH &raquo;</TD></TR><TR><TD CLASS="quote">

If the harness is not built properly....many cheapo egay copiers do this....then this fix can be used to get the tach to work properly.</TD></TR></TABLE>

heres the weird thing in my situation, my tach didnt work when i ran the greddy blue box with the conversion harness. When i removed the greddy blue box, the tach worked with the same conversion harness. Now I have my ECU chipped and again, the tach is not working https://honda-tech.com/forums/images...s/emthdown.gif

It seems that the resistor trick might make sense in my situation but when i double checked where the resistor is tapping into, A25 is a 12volt source so i dont want to fry the A21 input. Thats why i wanted some verification, thanks.

Re: OBD2b - OBD1 Conversion & Tach doesn't work
 

Shouldn't a good conversion harness already have this resistor wired in? This tach problem is apparent with EVERY OBD2b to OBD1 conversion so it doesnt make sense that it would fall short on this function.

Re: OBD2b - OBD1 Conversion & Tach doesn't work (C_Rock77)
 

I CAN VERIFY THE TRICK! It worked for me! Wired in the Resistor and the Tach worked.

Re: OBD2b - OBD1 Conversion & Tach doesn't work
 

Can you take a pic of how you wired in your resistor for me please? I also put a 1k resister on A21 and A25 and my now tach moves but not past 1500. I think it may be because the A21 pin (ICM output) is split to both A19 (tach output) and B13 (ICM output). And now there is a resistor feeding 12v power to both the tach and ICM outputs.

Re: OBD2b - OBD1 Conversion & Tach doesn't work
 

Do some tachs just require more "juice" than others?

Maybe the output is enough to drive the ignitor but not enough to drive the cluster?

Maybe some OBD1 ECUs have a weaker output?

We'll get this figured out!

Re: OBD2b - OBD1 Conversion & Tach doesn't work
 

Actually no it is not. I have sold over 2000 of those harnesses and very few have ever had issues like this. You also have to remember, with the exception of one manufacturer left, all of these harnesses are built out of the country, in mass quantities by individuals that don't have a clue what they are being used for. Asking them to install a resistor into a harness will probably take the failure rate of the harnesses up by 1000%.

I sent you a PM. When it is at idle, does the tach match the idle speed?

Re: OBD2b - OBD1 Conversion & Tach doesn't work
 

Thanks Jason, I replied to your PM. With the resistor the idle is rougher than without the resistor. The ECU (Neptune) has always registered the rpm correctly. With the resistor the tach matches the ECU rpm. However when rev'd past 1500 the tach goes dead. Very strange.

What are your thoughts on this resistor "fix"? Is it way out in left field or does it sound logical?

Re: OBD2b - OBD1 Conversion & Tach doesn't work
 

I was womdering, can you do this resistor fix to a p72 obd2a ecu? Becuz the wires A21 and A25 from the obd2b ecu would be A11 and A20 on the p72 obd2a ecu. Should work right?

I got a obd2b (00' gsr) motor in my obd2a (96 gsr) shell. The engine harness for obd2b doesnt have the blue wire on the plug at the distributor and the harness from the car have the blue wire, which is for the rpm/tachometer. So my rpm doesnt work.

Re: OBD2b - OBD1 Conversion & Tach doesn't work
 

I too have this problem with my tach not working...it works fine up until about 6000 rpms and then it just dies out zero...can anyone post a pic of how they wired the resistor in the harness? (if I understand correctly)..

Re: OBD2b - OBD1 Conversion & Tach doesn't work
 

can anyone please post a pic of the resistor soldered in the pins? i need a reference!

Re: OBD2b - OBD1 Conversion & Tach doesn't work
 

Here is my adapter harness. I used a 1k resistor as instructed. This fix DID NOT WORK for me. I ended up running a seperate wire from the Dist into A19 (tach).

https://lh5.googleusercontent.com/_L...%20Adapter.jpg

Re: OBD2b - OBD1 Conversion & Tach doesn't work
 

Yea, i end up doin what you did too.

Re: OBD2b - OBD1 Conversion & Tach doesn't work
 

i have similar problem with tach. i have made auto to manual swapin my "00 d16y8 civic. it use obd2b harness, but after swap i use obd2b to obd2a jumper and obd2a ecu. and tach doesn't work....

Re: OBD2b - OBD1 Conversion & Tach doesn't work
 

Montis i did the samething and my rpm dont work i have a obd2 jump harness with a p28 it work fine on others car but mine it driving me crazy