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Alternator Voltage & Amperage (load) Testing

Last night noticed that my 91 Saab 900 at idle, with nothing on except for hazard lights, the fuel pump pitch noise was going up and down in perfect synch w/hazard lights on/off. Tried slightly revving engine but then could no longer hear fuel pump

My alt V output is steady/good on both a scope (w/nominal AC ripple) and DMM, but has not been load tested, which will likely do. No batt charging or other known electrical problems [(all wires clean and secure) but ignition probs possible].

Fuel pump fuse is 20A, with these type fuel pumps generally pull 6 - 10 A (IIRC). Doesn't seem it would tax the alt in such a way. All fuel pressures, with exception of dead-heading pump (didn't do), are good (including while driving/bucking). Have hesistation/bucking problem which am t-shooting - just curious about this one FP phenomenon

Anyone else ever notice this (or may just be an artifact)?

i.e., is it possible for an alternator to put out good/steady voltage throughout the RPM range, per DMM or scope, but yet have current/amperage out drops, at narrow RPM range(s), that only a load test will pick up?

Thank you, Lance

Bosch 80A alt, w/new/tested good V output Voltage Reg and brushes

Reply to
LanceM
Loading thread data ...

Bad grounds.

DS

Reply to
Daniel Stern Lighting

That is due to the loading of the lamps.

An output test of the alternator would be in order to check it.

It doesn't look like a fuel problem. However, you can use your scope with a current probe to check the amp draw pattern.

If the alternator puts out 'good/steady voltage throughout the RPM range,' No. But you still haven't taken alternator output into consideration. If the output is not satisfactory, then a volt drop test should be done on the hot and ground sides of the charging system.

Reply to
saeengineer

Or bad primary power feed from somewhere upstream of the distribution/fuse center but downstream of the alternator/battery/regulator. Either one would cause the same problems despite a perfectly good alternator and a perfectly solid and stable battery voltage.

Reply to
Steve

I agree with Steve, but restating, you say you are seeing a voltage drop at the fuel pump caused by the flashers. I'd say this means you have a connection or component (small wires) somewhere in the wiring that has a relatively high resistance and it is in a common circuit with the flashers and fuel pump (when both are on, the current increases as does the voltage drop across this other resistance). If you cannot measure a voltage drop at the battery with the flashers flashing, then I wouldn't suspect the alternator. I wouldn't worry about it too much, but check all connections that are common to the fuel pump and flashers...

Reply to
bobby

Yes, agreed, but given the make-model-year involved, my money's still on bad grounds.

DS

Reply to
Daniel J. Stern

...remembering that all circuits on the car are in common with each other via their respective ground legs.

DS

Reply to
Daniel J. Stern

Fuel pump voltage drop measures 0.185 V DC @ idle. With hazard lights on, oscillates between 0.182 and 0.189 V DC. Seems negligible. Fuel pump ground is (still) clean and secure. Wiring appears good upon physical inspection & continuity. Will ck current feed.

Will then ck hazard circuit feed and ground(s)

Pursuing mostly out of curiosity

Hope to get a low-current probe/clamp - seem to be a great tool w/many apps (I've been heartily encouraged before...).

Reply to
LanceM

...as far as it goes.

Run a length of 12ga wire from the fuel pump ground terminal to the battery negative terminal, and run a similar length of wire (14ga is fine) from the rear lamps' common ground point to the battery negative terminal.

Results may well surprise you.

DS

Reply to
Daniel J Stern

The fuel pump and rear brake lights have a common ground ("9"), as far as I can tell.

These are for a 90, but are the same for my 91 in the pertinent areas

http://68.18.135.99/electrical_system_folder/Electrical_900_89-90/124-127.jpg

http://68.18.135.99/electrical_system_folder/Electrical_900_89-90/032-33jpg

http://68.18.135.99/electrical_system_folder/lh24wiringnfmp.htm [323 isfuel pump]

Just run a single 12 AWG between G9 and the battery negative terminal, and do another fuel pump V-drop (which would decrease from initial measurement if ground is marginal/bad?) and/or listen to see if fuel pump pitch stabilizes with hazards on?

Reply to
LanceM

"using G201 as the ground"

might better read "using G201 as the _leg_ ground..."

Reply to
LanceM

May be just normal. Remembering when Hondas first hit the states, when one turned on the headlites, it put such a load on the engine, via the alternator, that the engine rpm would instantly drop--and vice-versa. I incorrectly predicted they were not long for this world--this world being the US. Man, was I wrong!!! s

Reply to
sdlomi

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