Goldhawg

iTrader: (7)

Have a speedhut tach, E38 ECM w/T56. I added a pinout to pin 48 of connector J1. I verified continuity from the connector to ecm to my tach. I added a 680 ohm resister (see pic; purple wire is to my tach, red wire is 12V ign keyed). I've tried in multiple settings for pulse (1/2, 2, 4) although it should work w/2 pulses per rev from what I've read. My tuner said he followed the instructions here:
http://www.hptuners.com/forum/showthread.php?t=36611
setting tach output on crank, and HIbRES 14 and LO RES 15. but I have no joy. When the engine starts I get a temporary uptick to 500 rpm and then it goes directly back down.

Any thoughts what I could have wrong, or better yet, how to isolate what my problem is?

Thx!

71403

Resistor looks like it's in the right place.

What tach? I tried two older Sun "Super Tach II"'s that just would not work. Then used a newer Autometer tach and it worked fine.

If you have access to a scope hang it on the tach line and see if you're getting pulses.

Goldhawg

iTrader: (7)

It's a speedhut tach; and they claim theirs reads much lower voltage than an autometer--they thought I might be able to pick up signal w/o the resister and 12V signal.

I don't have a scope or particularly know how to use it, but I have some electronic technicians that probably can. Is there anyway that HPTuners can tell anything; i.e., does it see output from each pin?

conv55

Not sure but I think you have to tune in some changes to the ecm so that the correct signal is sent out pin 48

85MikeTPI

iTrader: (9)

71403

Scope really is preferred but a digital meter that reads frequency it might be a "signs of life" test.

Thread you referenced has the frequencies for various hi/lo combinations.

conv55

sorry missed the last paragraph, THANKS MIKE

Goldhawg

iTrader: (7)

I drove the car yesterday to work for the first time. A couple of times during the drive I saw the tach spike up but it came back to zero immediately. Still haven't had a chance to verify any signal coming over wire w/a scope.

85MikeTPI

iTrader: (9)

I finally got my swap running and was able verify the mod works on the factory '92 vette tach when using the pull up resistor. I initially tried a 1K ohm pull up resistor but the tach is reading a little low. I need to experiment with some other values before soldering it all together.

Did you use a 680 ohm or 680,000(K) ohm pull up? 680k wouldn't do much and may be your problem.

Goldhawg

iTrader: (7)

It was 680 ohm. I fixed error above.

Haggar

The ECM doesn't have any voltage on the line, what its actually doing is switching on and off a ground on that pin. So without a resistor pullup, the line is just reading zero. The pull up resistor bring the line up to battery level, and then the ECM creates the pulses by grounding. 680 ohm is good, that is what we use in factory GM clusters for the tach pull up, when they have that circuit (I design OEM cluster electronics, including the gen 5 camaro).

I would 2nd the notion to try a different tach, if you can borrow one. Or get an oscilloscope on the line. Some nicer handheld multimeters can read out frequency, which you can also use. Get a reading at idle, then rev up, and see if the frequency reading increases proportionally.