Crazy tach reading during cranking HEI 7-pin

[Nick1224]

AMC 360 GM-TBI, MSII v3 assembled as per GM 7-pin instructions in the manual.

The Jeep is up and running well finally after some capacitor woes in the power circuit. 10000 uF capacitor installed into the 12v feed helped clean things up a bit. The Jeep usually fires right up, but sometimes it's hesitant and "pumping" the throttle gets her to fire.

When cranking, tach signal jumps around 20-50k RPMs....then it starts and the tach signal sits right at 650 rpm without any spikes. No spikes at high RPM.

The GM HEI 7-pin is from the junkyard. 5v is fed off of the Vref on the TPS and through a relay to the module. (TPS does seem to have spikes now, where before on the 4-pin it seemed pretty stable...goes -1,0,0,1,-1,0,1.....)The module is grounded back to the common ground.

Any ideas?

[grippo]

What you are seeing with the very high rpms is electronic noise on the tach input that evidently goes away after cranking, and is not bad enough to prevent you from starting. You see high rpm when noise pulses arrive in between the real tach pulses. So the ECU thinks the crank is spinning at really high rpm. Make sure your ignition wires aren't routed near the VR input wires and maybe install a large audio capacitor across battery to ground.

[Nick1224]

Still having problems with this.

I moved the VR wires away from everything. Added the OEM cap to the HEI ground. I increased the max dwell time up to 8 (it uses a ford tfi coil) and that gets it to fire easier, but the signal is still crazy during cranking. I thought I might have the VR wires switched, but reversing them did not help.

The battery is a big red top. MS has a big cap across it's 12v. All the grounds are run back to the battery neg post. One other thing I noticed is a fluctuating TPS signal. It bounces from -1 to +2 at idle. The Vref 5v for the HEI comes from the TPS Vref and then through a relay during running not cranking.

I had MSI before this and a HEI 4 pin for tach signal with no problems during cranking.

Any other ideas? I've been stranded once because it couldn't get turned over.

[Matt Cramer]

The 8 ms of dwell in the TFI guide is not because TFI coils actually take 8 ms to charge; it's because using 8 ms dwell and 8 ms max spark duration will generate a square wave with a 50% duty cycle to control a circuit inside the TFI module. So you'll probably need to back down the dwell a little.

Could you post a data log of the tach spike along with your MSQ? This will give me more to go on.

[Nick1224]

Thanks Matt

So I ended up using the MSnE hei tach mod and that has pretty much all the tach noise cleared during cranking. I get one spike when I turn the key.
RPM
19867
0
0
0
0
147
289
366
564
688
701
693.....


Changed my max dwell to 6 for now. Is there a place where I can look up dwell settings for the TFI coil?

[Matt Cramer]

There's quite a few different TFI coils out there - is this a stock one or an aftermarket piece?

I would increase the Skip Pulses and see if that gets rid of the RPM spike on startup.

[Nick1224]

It's an MSD version.

I'll give an increase to skip count tomorrow and see if it gets rid of the spike. Thanks

[Matt Cramer]

6 may be a bit much - 3.4 ms will charge this coil to 6 amps, and 5.0 will charge it to 8.

[Nick1224]

I think I'm going to give up on the stock dizzy and just try and set up Ford EDIS. I have the EDIS8 module and all the harnesses. Just need to weld the 36-1 ring up and figure a way to get the sensor in the right position.

Sometimes it fires up perfectly without any noise and then sometimes it'll get all kinds of noise in it. I can't tune any cranking parameters because it's so hit or miss. I'm done with the stock dizzy

[Nick1224]

I have EDIS8 now and have a great tach signal and it fires up every time (knock on wood). Yay modern electronics!

I was able to open the spark plug gap up and then remove 10% fuel from idle. I'll have to see if I get any mileage improvements.