Powering FSD / ECM with Relay?

[damork]

Has anyone installed a relay to supply power to the FSD / ECM instead of relying on the ignition switch to carry the load? I'm considering doing so as it works so well for lift pumps, lights, transmission power, etc.

[Cowracer]

IIRC

The ECM and FSD dont pull that much power to begin with.

Putting them on a relay probably wont buy you much, and potentially be another source of failure.

Tim

[bluuzman]

IIRC

The ECM and FSD dont pull that much power to begin with.

Putting them on a relay probably wont buy you much, and potentially be another source of failure.

Tim



Well based on what I read about the SOL-D fuel solenoid driver or whatever you might call that - a lot of posts on this page and more info on the product homepages, the driver pulls several amps and actually the operating voltage can drop to below 9V measured at the FSD. Using a relay bypassing all the original wiring and switches can be justified.

"Cowracer", you're correct: all additional connections, relays, electronics whatever we like to add on these trucks can become a problem later ;)

-art

[joed]

And the results seem somewhat interesting. A lttle background:

Ever since my IP was replaced under warranty about 30K miles ago, I've noticed it doesn't seem to idle as smoothly as it did with the older IP and my scanner showed long Fuel Sol. Closure Times, ~ 2.2 ms at idle.

After reading gmctd's findings of voltage drop affecting closure times (see the "TDC Offset exposed - or, who knows what evil lurks in the hearts and minds of men" thread), I decided to try supplying the PMD with 12V directly from the battery (through a switched relay). In doing so, the sol. clousre times are lower, now generally around 1.95 ms). Better, but still not as low as my friend's almost identical 98 truck, with original IP and fresh PMD (his reads around 1.85 ms).

It may just be my imagination, but my truck now does seem to idle smoother, and seems to have a little better lower throttle response.

Food for thought. Thanks. Joe.

[gmctd]

Measure the voltage at the Engine Shut Off solenoid connector pin B - that will be same voltage on FSD\PMD, and is supplied thru the FSOL\ECM-B fuse from the Ign Switch.

Should be +12-14vdc - if not, try disconnecting the FSD\PMD at the splice, then powering the headlite\equiv relay from the seperated FSOL wire.

FSD\PMD wire thru relay contacts thru fuse to + battery feed at power block or fuse center.

[joed]

Actually, I think that's the way I have it set up - my new relay is switched on by the original 12v supply to the FSD, and I have a fused lead to the relay directly from the pass side battery.

Before I did this I measured the voltage at the FSD\FSOL splice harness. I seemed to gain about 0.5V by wiring it direct. Not a drastic difference but it did seem to help the closure times. More than anything, it's probably the way my fuel solenoid is set up mechanically in the IP, and I guess there's not much I can do about that on while it's on the truck. And really I shoudn't have to, being a pretty new pump. Seems many of the newer IPs are set up this way out of the box, unfortunately...

Joe.

[gmctd]

As stated in an earlier thread, my 5288 was running 1.80's pw for several years, no change.

Upgrading to 5521 with same 34583 FSD, pw increased to mid 1.90's, no other alteration, with supply pulsing between +14v and +10v, incl longer start-to-run interval.

Direct jumper to battery reduced pw to 1.84ms, restored 1/2 crank revolution starting.

Problem appears to be in the FS, as the vehicle wiring harness didn't change, and the FSD was pulled from the functional 5288 and installed on the new 5521 - only electrical change was the Fuel Solenoid and associated FSD IP harness.

Several spare FSD modules indicated same pw.

The new harness has larger AWG silver-plated conductors, teflon insulation, with crimped and soldered terminals, so I do not consider that as likely suspect.